PHYSICS
Topics
SpaceMotors and Generators
Ideas to Implementation
From Quanta to Quarks (Option)
Space
Title: 16 October 2007 Physics
Date: 16 October 2007 5:30 PM
Category: School
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Started the topic Space. Read the syllabus a bit.
'Mass is how much of you is there while weight is force applied by a body'
F=ma
F=mg (for gravity)
Work:
W=Fs
W=mgh
∆Ep is change in Gravitational Potential Energy.
∆Ep = mgh
________________________________________________________
As you get closer to earth, r gets smaller. ∆Ep is minimal, F = mg
As you get infinitely far, r gets infinite, ∆Ep is maximum, F = 0
Ep = -G m1 m2
r
Title: 17 October 2007 Physics
Date: 17 October 2007 7:09 PM
Category: School
Tags:
Calculations involving potential energy and force with gravity. Some exercises.
Title: 18 October 2007 Physics
Date: 18 October 2007 8:47 PM
Category: School
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Projectile Motion
ux = vx
(Initial sideways velocity is the same as final sideways velocity on a parabolic motion)
uy = -vy (uy2= xy2)(only if returning to same height)
v = u +at
(vx = ux + ax t)
(vy = uy + gt)
∆x = ux t
And there is another one. Look in your book.
Title: 19 October 2007 Physics
Date: 19 October 2007 11:28 AM
Category: School
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More on parabolic motions. Did a sheet with lots of movement problems on it.
Title: 24 October 2007 Physics
Date: 24 October 2007 7:55 PM
Category: School
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History of Parabolic Motion and mankind's understanding
Aristotle: Was wrong. Objects always fall straight down UNLESS there is a force pushing it along (he thought the vortex behind the object caused by air movement)
Early Warfare: Involved spears etc.
Canons: more accurate info required to shoot one well.
Buridan and pupil Oresme: Objects travell horizontally until gravity could take hold.
Galileo: 1. Experiments with ramps:
t2 ∝ d OR t ∝ √d
2. Final velocity and acceleration are independent of mass
3. Deduced projectile motion would be a parabola
Title: 25 October 2007 Physics
Date: 25 October 2007 8:17 PM
Category: School
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How to find the escape velocity on Earth/Planet:
Get kinetic energy formula (1). Get Gravitational Potential Energy formula (2).
= GmEm
RE
E above is Earth.
Ek + EG = 0 (3)
Rearrange.
Ek = - EG
Substitute (1) and (2) into (3)
v = √2GME
RE
Launching Rockets
- As close to the equator as possible to get 460 ms-1
- From an east coast (debris)
- Towards the east to not fight Earth spin.
Orbits: Speed, Time + Distance
Gravity by force formula.
F = G m1 m2 (1)
r2
Force on a body m2 in a gravitational field
F = m2v2 (2)
r
Force on an object in circular motion
2πr = 0
v = d/t = (2πr)/t (3)
where t is time for 1 revolution.
G m1 m2 = m2v2 (4)
r2 r
(Gm1)/r = v2 (5)
Plug (3) into (5)
(Gm1)/4π2 = r3/T2
Title: 30 October 2007 Physics
Date: 30 October 2007 6:05 PM
Category: School
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How to work out the maximum range for a projectile for a gicen initial velocity.
Eventually proved the best agle to launch something at is 45 degrees if you want max distance. See book.
Compare qualitatively Geo-Stationary and Low Earth Orbits
| LEO | Geo-Stationary | 1-2 hours | 24 hours | 200-500km | ~36000 km above surface | air drag | no air drag | spy satellites weather | communications | anywhere | must orbit equator | ||||||
Title: 5 November 2007 Physics
Date: 5 November 2007 6:23 PM
Category: School
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Orbital velocity is the speed an object takes to travel around an orbital path.
v2 = (Gm)/r
m is the mass of the thing you are orbiting around.
9.2.2 k)
- Early space stations eg Sky Lab is 200-450 km up
- ISS @ 822 km
- Hubble @ 600
- Landsat @ 705
9.2.2 l) + m) Re-entry is tricky
Too steep: burn up, g forces too high
Too shallow: bounce off atmosphere
Optimum angle: 5.5 - 7.5 degrees.
Space shuttles use heat insulating tiles
1500-2000 degrees C
Ionisation of air causes loss of communication.
Title: 13 November 2007 Physics
Date: 13 November 2007 6:59 PM
Category: School
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Relativity. Oh no.
Title: 14 November 2007 Physics
Date: 14 November 2007 6:59 PM
Category: School
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(Before this there was something about waves out of phase and mirrors. I didn't write it down)
Michelson-Morley Experiment
Before Einstein there was Newton.
People knew light was a wave so where was the medium?
There was an aether wind that flowed across the planet. This should slow down waves that travel against it instead of sideways through it.
It didnt.
Therefore there was no aether.
Title: 16 November 2007 Physics
Date: 16 November 2007 7:36 PM
Category: School
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The Theory of Special Relativity
The speed of light is always constant.
It doesn't matter if you (an observer) happen to be going at 0.99c
It will always move away/towards you at c
This is true fo inertial observers (you are not accelerating)
Time Dialation
From the point of view of an observer time goes slower for objects moving at relativistic speeds
Title: 31 January 2008 Physics
Date: 31 January 2008 6:29 PM
Category: School
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Prac: Regarding Projectile motion
Air cannon shoots a tennis ball.
AIM: determine initial velocity, final velocity, max height, range, tim of flight.
Uses light gates to measure the velocity at the start
Had to calculate everything from initial velocity
Motors and Generators
Title: 4 February 2008 Physics
Date: 4 February 2008 2:21 PM
Category: School
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MOTORS AND GENERATORS
An experiment to show the motor effect.
Get a wire that will have a current running through it. Put magnet on one side of the wire and a magnet of opposite pole on the other side. Put the current through the wire. The wire should wobble back a forth a bit.
This is because the current generates a magnetic field which interacts with the other field.
Right hand screw rule
Get your right hand. Point the thumb in the direction of conventional current (the opposite way that the electrons move: (electrons move from - to +))
Curl your fingers around the wire. This is the direction of the magnetic field
Right hand palm rule
Point thumb in the direction of conventional current. Point fingers straight but in the direction of the magnetic field. The force on the conductor comes out of the palm
Effect on the magnitude of the force on a current carrying conductor
The force on a current carrying conductor with a length of l carrying a current of I in a magnetic field is:
F=BIlsinθ
Theta is the angle between the current and magnetic field. B is magnetic field intensity in Teslas (T)
Title: 5 February 2008 Physics
Date: 5 February 2008 2:30 PM
Category: School
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Conventional current runs from positive to negative. Real current (electrons) flow the other way.
Magnetic forces between parallel conductors
There are two wires running parallel to each other. The both carry a current and therefore induce a magnetic field.
When the current runs the same direction.
Using the right hand rule we can see that the magnetic field come towards each other in the opposite directions. This makes the wires attract to each other.
If the current is traveling in different directions to each other, the magnetic fields will be traveling the same way and the wires repel.
The formula for this stuff is:
F = k (I1I2)/d
l
Where
F is the force on the current carrying conductors (VECTOR)
l is the length of parallel wiring
k is a constant (2 x10-7)
d is the distance between the parallel wires
I1 and I2 are the currents in each wire
Title: 6 February 2008 Physics
Date: 6 February 2008 2:37 PM
Category: School
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Torque
The turning effect of a force
τ = Fp
τ is Tau, the symbol or torque.
F is the force
p is the perpendicular distance from the turning point to the line of action of the force (see diagram) in m
What is the motor effect due to?
It is the force acting on the current carrying conductor in a magnetic field
Motor
AB and CD experience a force of up and down respectively.
Magnitude of the force does not change as the plane rotates. BC and AD experience a force of nothing.
Title: 11 February 2008 Physics
Date: 11 February 2008 4:48 PM
Category: School
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Referring back to 6 Feb 08
Torque is produced in the plane. When the plane is at 90 degrees to how it was ie the plane is straight up[ and down/perpendicular to the magnetic field, the torque and force is zero. This is because the wires are trying to go up and down but they cant because they are attached to the plane which is 90 degrees.
Due to its momentum, it will swing a bit past 90 and then back to 90 because after 90 degrees the forces push it to that 90 degree point.
A DC electric motor
A current carrying loop will rotate but not revolve ie it wont go completely around.
If the current was reversed at the right time however, it would continue to rotate past 90 degrees.
A DC motor is made from a commutator, armature and field magnet
Title: 13 February 2008 Physics
Date: 13 February 2008 4:58 PM
Category: School
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Commutator
I metal ring that reverses the current. It reverses the current every half turn. There is a ring split in two and each half is insulated from each other. Brushes made of some kind of conductor brush on the ring, one on each half. The ring is attached to the plane of wires and rotates with it. When it rotates past the 90 degree point, the brushes are in contact with the other half of the ring and the current goes the other way around.
Armature
The coil of wires is wrapped around a thing called an armature. It is a soft iron core. Torque I increased because of it. The armature is magnetised by the coil around it and is affected by the field magnets.
Field Magnet
The magnet(s) providing the magnetic field that the coil is in. Can also be an electromagnet.
Torque and such
to calculate torque:
τ = BIAn
Where: B is magnetic field strength in Teslas
I is amperage in the wire (Amps)
A is the area of the plane that the wire is (m squared)
n is the number of turns of the wire in the plane
A galvanometer
Measures current in a DC circuit. It is kind of like a motor without a commutator except it is held back by a spring so the higher the current, the more it resists the spring and the bigger the reading it shows.
Loudspeaker
Contains a circular magnet with a pole in the centre and a pole as the circumference.
A coil is wrapped around the middle pole. This is hooked up to an amplifier which gives of a current according to a sound wave. It is AC.
The coil bounces back and forth because of the magnetic field of the magnets.
The coil is attached to a cone which move the air like the sound wave.
Title: 14 February 2008 Physics
Date: 14 February 2008 5:06 PM
Category: School
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HERE IS AN EVEN BETTER EQUATION LOL
T = BIAcosø
T = torque
B = Magnetic field strength (Teslas)
I = Current (A)
A = Area of plane (m squared)
ø = angle between plane of wiring and plane of magnetic field
cosø where ø = 90 is 0 (therefore no torque)
Supplying magnetic field
You can make a magnetic field an electromagnet or a permanent magnet.
Title: 15 February 2008 Physics
Date: 15 February 2008 5:09 PM
Category: School
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Michael Faraday
1830: Hans Christian Oersted and Andre Marie Ampere discovered magnetic field is produced by current.
Faraday imagined that the opposite could happen. He did it in 1831
He attached 2 sliding wires to a rotating copper disk between the poles of a horseshoe magnet. He induced a current by a moving magnetic field. The only thing before him that did DC was a battery.
FARADAY WAS NOT THE FIRST TO DO THIS. He documented it though.
Electromagnetic induction: The process of making a current from magnetic field.
An experiment: Get a coil and hook it up to an ammeter. Move a bar magnet through the middle of the coil. Tada! You just generated electricity
Title: 21 February 2008 Physics
Date: 21 February 2008 6:55 PM
Category: School
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Magnetic Field Strength
Consider a magnet where field lines are arranged north to south and the lines are really close at either end.
Magnetic flux (ø) (phi) is the density of magnetic field lines in a given area. The unit of magnetic flux is the Webber (Wb, 1 Wb = 1 T.m2)
When we think of magnetic field strength magnetic flux is also used and is represented by a B.
Magnetic Flux density and surface area
ø = B.A
Where A=area in m2
Experiment
Aim: What affects magnitude of current produced? Change the distance between coil and magnet, magnet strength and relative motion of magnet to conductor.
(A) Hook up a micro-ammetre to a coil. Place a plastic ruler so that the edge is running along the coil. Run a magnet along the other edge. Flip the ruler so the face is on the coil. Run the magnet on the other face.
(B) 2 magnets of the varying strength are moved in the same way at the same distance from the coil.
(C)Vary the motions of the same magnet at the same distance from the coil.
Title: 25 February 2008 Physics
Date: 25 February 2008 4:42 PM
Category: School
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For a current to be generated in a conductor, the magnetic field must be changing.
Frequency of current is dictated by the speed of the magnet moving.
The phase of the alternating current is reversed if you use a different pole of the magnet.
The number of turns in a coil changes the magnitude of the current.
Rate of change of Magnetic Flux
Voltage produced by a battery or magnetic field is called emf (electromotive force).
It represents energy per unit charge available. IT IS NOT REALLY A FORCE
When there is motion between conductor in a circuit and magnetic field, the conductor cuts the flux.
You can increase flux by decreasing the distance between magnet and conductor, increasing magnet strength, increasing speed of magnet’s motion and making the angle between motion of the conductor and magnetic field lines closest to 90 degrees.
The above increases in flux cause more current and more voltage/potential difference/emf
emf = rate of change of flux in a circuit
∆ø = emf
∆t
This means change in flux over change in time equals emf.
Title: 28 February 2008 Physics
Date: 28 February 2008 3:41 PM
Category: School
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Faraday’s Law of Magnetic Induction is that one from the other day.
∆ø = emf
∆t
When a circuit involves lots of loops cutting the same flux then
N ∆ø = emf
∆t
N being the number of loops through.
Conservation of energy
Energy cannot be created or destroyed but can be converted from different forms of energy.
Consider a magnet and coil. Energy is transferred to the coil to produce a current flow in it. This energy is equal to the work done on the coil in moving it around. So when a magnet is moved into a coil, the work done on the magnet is equal to the current that is generated in it.
Lenz’s Law
The direction of induced current is such that it tends to oppose the motion or change to which it is due.
Back emf
If you have a motor, why doesn’t it rev all the way around infinitely? It is because when you put current through the coil, it moves around because of the external magnetic field. But this is a coil moving in a magnetic field, cutting flux. So an emf is induced in the OPPOSITE DIRECTION of the current that was there in the first place. This is too keep the conservation of energy rule true.
Title: 29 February 2008 Physics
Date: 29 February 2008 7:32 PM
Category: School
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The emf of input voltage take away the back emf = net voltage
Back emf limits the current in a coil so the motor doesn’t rev out to infinity. Revving out is impossible because of Lenz’s Law
Say input voltage is 240V at the beginning and the coil has constant resistance of 10 ohms. Current is 24 amps.
Now there is about 235V net voltage so 0.5 amps.
To avoid huge motors blowing out fuses etc., start up resistors are added to the circuit to reduce the initial voltage of the motor. They are turned off once the back emf increases.
When the load (a bit like the resistance experienced by the motor) is increased, back emf is reduced. This means there is more current being drawn. The motor SELF ADJUSTS.
Title: 3 March 2008 Physics
Date: 3 March 2008 8:28 PM
Category: School
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Eddy Currents
You have a lump of metal. It is in a magnetic field. The magnetic field changes. A CURRENT IS INDUCED IN THE METAL. This is called an eddy current because it looks like a circle.
If a pendulum with a lump of metal on it swings between two poles of a magnet, it will be slowed down. When moving, it is cutting flux and an eddy is created in the metal. This current induces a magnetic field which opposes the original one according to Lenz’s law so the pendulum slows.
Lenz’s law states that eddies will always produce a field that opposes the flux that created it.
Induction Cooktops
You have an electromagnet under a ceramic plate. The magnetic field it produces is changing rapidly. If you put a metal pot on it, eddies would be formed in the bottom of the pot. When electrons move around in a metal, they cause heat if it is a resistant metal. These pots heat up and heat whatever is in it. This is an Inductive Cooktop. It doesn't loose heat from escaping gases.
Induction Furnace
A container wrapped in an electromagnet holds a metal. The electromagnet is activated and the metal melts. This is because eddies are formed and the metal gets really hot.
Title: 4 March 2008 Physics
Date: 4 March 2008 7:59 PM
Category: School
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A trick to determine the flow of eddies.
When a pendulum moves towards the magnet: on the north side of the electromagnet, the side of the pendulum facing the north pole will become north because it wants to repel the magnet. Draw arrows on the pointy bits of the N so the eddies go in that direction. It’s the same with an S.
Title: 5 March 2008 Physics
Date: 5 March 2008 8:39 PM
Category: School
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Main bits of a generator:
Rotor: An axel that the wire is attached to. A force is applied on it to make it turn. Rotates in a magnetic field.
Armature: On which the wire is wrapped around. Made of iron.
Coil: The wire wrapped around the armature is coiled to get more current.
Stator: The fixed part of the generator, holds the field magnet in place.
Brushes: Brush on the split ring commutator (DC) / Slip rings (AC) to conduct the current to the terminals.
Magnetic field can be applied by a permanent or electromagnet.
Title: 6 March 2008 Physics
Date: 6 March 2008 8:43 PM
Category: School
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Differences: AC and DC
DC generators are called dynamos sometimes
When using a split ring commutator, the generator is a DC one and the current produced is pulsating.
The graph of current as a function of time looks like y = |sinx|
That means it’s a sine graph but it bounces off the x-axis and never goes bellow it. Everything bellow the x-axis is reflected up.
Current flows in 1 direction.
AC generators are called alternators.
When a slip ring is used, it must be an AC generator.
The current looks like a sine graph. emf is maximum when the arms of the coil are cutting the magnetic field at right angles.
Title: 10 March 2008 Physics
Date: 10 March 2008 8:27 PM
Category: School
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Advantages and Disadvantages of AC and DC generators:
DC:
Split ring commutator has narrow gaps as the splits. This increases wear on the brushes.
Even if there was no split they would wear out eventually.
Bits of metal can get stuck in the gap and short out.
The coil must be big and heavy if you want lots of current. This wears the bearings and other bits out in general.
Lots of current can cause arcs between gaps of the commutator. Causes radio noise.
DC is generally used in low current applications.
If there are a few separate coils, the commutator can be set up to pick up each coil. This is done by putting in multiple split instead of just 2. This keeps the voltage (virtually) constant. AC can’t do this without lots of equipment.
AC:
Less ware on the brushes due to the continuous smooth surface of the slip ring. They still need replacing though.
No shorts or arcs in between slip rings.
Less maintenance, more reliable.
AC generators are used in power stations. The overcome the need to move huge coils around.
They have stationary coils around a magnet in the middle that rotates. The coils are now the stator and the magnet is on the armature.
The rotor is a DC supplied electromagnet.
Title: 12 March 2008 Physics
Date: 12 March 2008 12:42 PM
Category: School
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Resistance = (resistivity * length)/area
Power stations are a long way away from people.
Power loss = I2R (loss is in watts)
We have to transmit electricity at low current and therefore high voltages.
Energy losses when being generated
Friction in bearings.
Eddy currents in iron cores of transformers and armatures.
Voltage is stepped up in a transformer to reduce energy loss in transmission.
Transmission lines
Those big power line holders can get struck by lightning so there is a high up line called the continuous earth line. It carries lightning strike energy and fault energy away to the earth.
There are insulators on the power lines to stop high voltage arcing to the power line holder.
Title: 13 March 2008 Physics
Date: 13 March 2008 12:47 PM
Category: School
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Transformers
Transformers change the voltage of AC ONLY
Used in TV’s, cordless phone charges, everywhere.
240V comes from the power point in Aust.
Industrial supply is 415V
Transformers are an iron square in a ring shape. On one side of the ring is a coil, and on the other side, a coil with less or more turns. The coils are coiled around the ring.
In a step up transformer, voltages are stepped up. In a step down transformer, the opposite.
In a step up, the primary coil (where the input current is put), there are less coils than the secondary coil (the output current comes out here).
When voltage is increased, current is decreased and vice versa.
Society and electricity
Electricity brought less unskilled jobs and more unemployment.
It made the richer live in the outer suburbs while the poor live inner city.
Global warming and fossil fuel issues.
Title: 14 March 2008 Physics
Date: 14 March 2008 12:53 PM
Category: School
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Prac Observation of a transformer
SAFETY
Never use 240V
Don’t hook up the transformer until you are finished setting up.
We have a step down transformer.
Primary coil: 240V input, 2-5 amps, 540 turns
Secondary coil: Max: 20V, Max: 50 turns.
We get a crapload of current out of the secondary coil.
In a transformer, the more coils in the secondary coil, the more voltage you will get and the less current you will get.
The opposite is true.
Vp = np
Vs ns
Title: 17 March 2008 Physics
Date: 17 March 2008 8:33 PM
Category: School
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Voltage Transformations
ENERGY CANNOT BE CREATED NOR DESTROYED.
(Power)
P = VI
Where
P is power in watts
V is voltage in volts
I is current in amps
So
Power In Primary Coil = Power In Secondary Coil
VpIp = VsIs
Vp = np = Is
Vs ns Ip
Title: 18 March 2008 Physics
Date: 18 March 2008 5:34 PM
Category: School
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When real transformers are used, they heat up and loose power. From eddy currents forming.
This is overcome by making the iron core into several layers separated by an insulator. There are more eddy currents, but tiny ones.
Or user ferrites: iron oxides + other metals that don't conduct electricity well but they do conduct magnetic flux.
Transformers and Substations
Transformers step up the voltage for transmission because the more current, the more energy loss from heat along the way.
If you don't use transformers then:
Lots of power loss, towns need to be closer to power stations, more pollution due to more power stations.
No small appliances that run off less/more than 240V. These appliances are easier to make, safer and cheaper.
From a power station the voltage is stepped up for transmission, stepped down at a distribution station, stepped down at a regional substation, stepped down at a city sub-station and stepped down at the box transformer on the poles. This is to stop lines arcing in city/busy areas.
Title: 19 March 2008 Physics
Date: 19 March 2008 6:47 PM
Category: School
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There are two different types of AC motor
Universal motor.
It runs off AC and DC. The circuit wire powers the electromagnet AND the commutator (which is situated IN the armature)
That means if DC, the electromagnet stays the same direction and the current in the wire changes.
If it’s AC then when the current in the armature changes, so does the electromagnet.
Induction motors.
They have a stator and a rotor.
Its a bit like a transformer.
The stator is the primary coil and the rotor is the secondary coil.
The stator has current running through it, induces a thing like an eddy current in the rotor, which in turn induces a magnetic field that turns the rotor.
3-phase induction:
There are six coils that make up the stator: 3 pairs that lie opposite each other. They use 3-phase electricity, usually only in industry supply.
There are 3 pairs, so as each phase kicks in, one of the pairs of coils turns on and the magnetic field rotates. This keeps going so there is a continual rotating field that motes the rotor.
Title: 20 March 2008 Physics
Date: 20 March 2008 3:07 PM
Category: School
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So you have those 6 electromagnets in 3 pairs. The changing magnetic field causes a current to be induced in the bars of the squirrel cage. The current is in a magnetic field so it experiences a force. The force makes the cage spin around, ‘chasing’ the moving magnetic field.
The force on one of the bars is in the direction of the movement of the magnetic field induced by the current in the bar.
PRAC Demonstrating AC Induction
You have a bar magnet on the end of a drill. The drill spins. Above the spinning magnet is a copper disk. The disk spins in the same direction as the magnet.
There is a current induced in the disk which induces a magnetic field which opposes the magnet that is spinning.
Slip is the difference (in speed) between the squirrel cage and the magnetic field rotation.
The squirrel cage never moves at the same speed as the magnetic field, it is always slipping behind.
Title: 26 March 2008 Physics
Date: 26 March 2008 9:23 PM
Category: School
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shitshitshitshitshit exams next week
Ideas to Implementation
Title: 27 March 2008 Physics
Date: 27 March 2008 8:17 PM
Category: School
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IDEAS TO IMPLEMENTATION
Shared electrons:
Metals have low electronegativities while non metals have higher electronegativity.
Metals consist of a big crystal lattice made of positive metal ions, surrounded by a sea of delocalised electrons.
These electrons move randomly unless acted upon by an electric field, where they move randomly in a net direction.
Elements have energy bands; layers where electrons reside.
The highest band is the valence band (the outer shell of the element). In a conductor, this band is only partially filled by electrons, unlike insulators which are completely filled.
In a conductor, the conduction band includes the valence band so electrons are free to move.
In a semi-conductor, the conduction band is separated to the valence band by a small energy gap.
In an insulator, the conduction band is separated by a large energy gap.
Title: 28 March 2008 Physics
Date: 28 March 2008 12:38 PM
Category: School
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Electrons and holes.
You heat a semi-conductor. An electron from the valence shell pops up into the conduction band. IT LEAVES A HOLE IN THE CONDUCTION BAND.
This hole, in a way, is positive. The atom the electron just popped out of now has one electron less than it should. An electron from a nearby atom decides to leave and pop into the hole left by the first electron. This electron leaves a hole, which is filled by another electron which itself leaves a hole which is fill-...
So the movement of electrons to fill holes is equivalent to the electric current (in semi-conductor)
Title: 30 April 2008 Physics
Date: 30 April 2008 8:35 PM
Category: School
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Conductors: lots of free electrons
Semiconductors: fewer electrons but if you increase the temperature or light it differently or apply a different voltage, you can get more electrons.
Insulators: No free electrons.
Title: 1 May 2008 Physics
Date: 1 May 2008 8:37 PM
Category: School
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SEMICONDUCTOR:
Germanium was easy to refine so it was used a lot as a semiconductor back in the day.
It is rare, expensive and heats up too much. When it heats up, it lets more current through, burning up other electrical components.
Silicon:
Second most abundant element in the crust.
Cheap
Doped easily
When impurities are added to semiconductors they can change the way the conductor acts. Doped semi conductors are called extrinsic. The opposite is intrinsic.
P-type semi conductors have more Positive holes. They are doped with group 3 elements.
N-type have more Negative electrons. They are doped with group 5 elements.
Title: 5 May 2008 Physics
Date: 5 May 2008 10:05 PM
Category: School
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Thermionic vs. Solid State
Thermionic devices get hot and need a separate heating circuit. They take time to heat up and work.
Solid State work immediately and don’t need heating.
Thermionic requires a vacuum so it is packaged in a vacuum tube and is delicate. Big and clunky.
Solid State things aren’t. They make devices smaller.
Thermionic wears out often, needs replacing.
Solid State doesn’t.
Thermionic devices used in musical amplifiers produce a unique sound that Solid State can’t.
Title: 6 May 2008 Physics
Date: 6 May 2008 7:25 PM
Category: School
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Watching electricity pass through a vacuum tube.
The negative end of the battery is connected to the cathode of the tube. It has to have relatively high potential to get electricity to flow.
*check this observation* As pressure in the tube increases (more gas in the tube) the colour changes from purpleish to red.
Title: 7 May 2008 Physics
Date: 7 May 2008 7:19 PM
Category: School
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Cathode Rays:
End of the 19th Century. People don't understand cathode rays. It is wave or particle?
Early experiments suggested both. Hertz did an experiment but got the wrong result: that it was not affected by an electric field. The error was caused by gases in the vacuum tube being ionised and canceling out the electric field.
J.J Thompson got it right. He showed they were deflected.
Hertz showed the rays penetrated thin metal without damaging it. He thought they were EM waves.
Magnetic fields and electric fields affect cathode rays.
If an object is placed in the way of a cathode ray, a shadow will appear behind it. Therefore cathode rays travel straight. Also, radiation emerges at right angles to the cathode.
They are deflected by magnetic and electric fields. They are deflected in such a way that suggested they were negatively charged particles.
Title: 8 May 2008 Physics
Date: 8 May 2008 8:52 PM
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Production of fluorescence: Some objects fluoresce when cathode rays fall on them.
Cathode rays have mass. If you put a wheel in a cathode ray tube, it will spin (experience a force). Heat is produced when rays are stopped.
They are deflected by magnetic fields.
They penetrate thin objects like aluminium foil or gold leaf. This is because there are large gaps between atoms.
Roentgen deduced that electrons at the end of the glass tube were excited and gave off X-Rays when a screen fluoresced at the end of a tube.
Title: 12 May 2008 Physics
Date: 12 May 2008 8:08 PM
Category: School
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Moving charged particles in a magnetic field experience a force.
Parallel plates have a uniform electric field between them.
F = qvBsinø
where
F is the force acting on a particle in newtons
q is the charge of the particle in coulombs
v is the velocity of the particle in m/s
ø is the angle between direction of charge and magnetic field
Use the palm rule: fingers in the direction of magnetic field, thumb in direction of CONVENTIONAL current and out of the palm comes the direction of force on the particle (whatever charge it is).
If you have a positive charge, field lines come out of it, for negative charge it is the opposite.
The direction of an electric field is the direction that a positive charge moves. Likes repulse, opposites attract.
Title: 14 May 2008 Physics
Date: 14 May 2008 1:58 PM
Category: School
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Parallel plate capacitors.
E = F/q
E is the electric field in N/Coulomb
F is the force on the particle
q is the charge of the particle in Coulombs
E = v/d
d is the distance between plates (m)
v is the potential difference (volts)
So:
F=qE
F=qvBsinø
E=v/d
Title: 15 May 2008 Physics
Date: 15 May 2008 1:45 PM
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Thompsons Experiment:
JJ Thompson tried to establish what cathode rays were.
He put it through a magnetic field and then an electric field. They acted like negative particles.
Then he passes it through both fields at the same time in a way that the forces on the particles would cancel out.
v=E/B (using F=qvB and F = qE)
He calculated charge to mass ratio (q/m):
1. Measured the radius of the arc that it caused when passed through a magnetic field.
F=qvB and F=(mv2)/r
q/m = v/Br
2.
Title: 18 May 2008 Physics
Date: 18 May 2008 1:58 PM
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Application of Cathode Rays:
Cathode ray tubes have a heated cathode that emits electrons thermionically.
Electrodes focus the beam and accelerate it. Plates change the direction of the ray. It is projected onto a fluorescent screen.
TV’s use it: different coloured phosphors are used to represent the different colours. They are deflected by deflecting coils. Paint emits light when it is hit by electrons.
Cathode Ray Oscilloscopes use it to display voltages. The vertical displacement of the ray is caused by parallel plates.
Electron microscopes use the property that particles can act like waves. A specimen is bombarded by electrons. It uses electrostatic or magnetic ‘lenses’
Title: 19 May 2008 Physics
Date: 19 May 2008 7:20 PM
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In 1864 James Maxwell predicted light was an e.m. wave and that there was a spectrum of e.m. waves and that their speed was constant. Hertz in 1886 experimented. He created a rapidly oscillating field using an induction coil that caused a spark between a gap in the wire. He put another loop near the sparking one and this loop sparked too but there was no power source for it.
He showed that light had the same properties of E.M waves.
He calculated c to be 3x108 ms-1
The spark was brighter when exposed to light.
Title: 20 May 2008 Physics
Date: 20 May 2008 10:46 PM
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The photoelectric effect:
Light shining on certain metals removes electrons more easily from its surface.
Allows for solar cells and breathalisers and burglar alarms.
Hertz measured c (speed of light) by reflecting light of metal sheets and measuring the interference.
Get an induction coil and put it near a radio. When you use the coil the radio will receive interference showing that there are radio waves caused by a spark jumping the gap.
Title: 21 May 2008 Physics
Date: 21 May 2008 10:58 PM
Category: School
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Planck.
The interaction of light with matter is hard to explain with classical physics.
Heating a poker, it gets hot, then red hot, then white hot then blue hot.
A black body is a furnace kind of thing that absorbs all radiation that hits it.
It emits radiation at different temperatures.
Energy in the black body makes it hotter. The hotter it is, the more intense the radiation emitted.
According to classical physics, once the black body got hot enough, it would start to emit an infinite amount of radiation. This is wrong (called the ‘ultraviolet catastrophe’).
Max Planck of the 1900’s had an idea. The atoms in the black body emit energy in packets. It’s like a bucket with holes. If you fill past the holes, water will continually be dumped. This is the same with matter, atoms will emit energy constantly but in small packets so you can’t get infinite radiation. This is quantum physics.
Title: 22 May 2008 Physics
Date: 22 May 2008 11:08 PM
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E = hf
where
E = energy in Joules
h = Planck constant (6.63 x 10 -39 J s)
f = frequency in hertz
Einstein:
Einstein agreed with Planck. He applied quantum physics to the photoelectric effect and got a Nobel Prize for it.
If a quantum (a packet of energy) was more energetic, the faster an electron would travel after being released from a metal.
He discovered metals hold electrons with different strengths and the energy required to remove an electron is the work function.
E = hfo + 1/2 mv2
The photoelectric effect applies to solar cells. They are made of silicon semiconductors. Light hits a n type semiconductor, releasing electrons with are used to make a current. The electrons return to the p type semiconductor.
Intense light removes MORE electrons but higher frequency energy releases them at more speed.
From Quanta to Quarks
Title: 11 June 2008 Physics
Date: 11 June 2008 8:51 PM
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The first nuclear reactor was made under a squash court by Fermi. Later the patents for it were given to the US government.
He realised impurities in the uranium would cause extra neutron capture and therefore prevent fission.
He encased his uranium in graphite bricks because they were one of the more pure things at the time.
He created a self sustaining reaction for 28 minutes.
Fermi bombarded elements and discovered transuranic elements. His work colleagues discovered bombardment worked better on wood surfaces than marble.
Also, slow neutron bombardment is better than fast.
Title: 12 June 2008 Physics
Date: 12 June 2008 8:56 PM
Category: School
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Mass defect continued
Situation 2
Splitting deuterium into proton and neutron gives more mass than you started with.
MASS DEFECT * 931.5 = BINDING ENERGY
The amount of energy converted to mass would be the same as the mass defect. This is called binding energy.
If you add the total binding energy of a certain nucleus to that nucleus then its nucleus will bust up into nucleons.
Deuterium’s binding energy is 2.234 MeV
When splitting uranium the binding energy is required which splits the nucleus. The nucleons form elements with higher binding energies, giving off energy as they do this. This energy is greater than the binding energy of the uranium. Chain reaction.
Title: 13 June 2008 Physics
Date: 13 June 2008 12:54 PM
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Controlled and uncontrolled nuclear reactions:
Uncontrolled: each neutron released in a Uranium atom split is used to split another Uranium atom. Because there are 3 neutrons in every Uranium split, the number of atoms being split rapidly increases. Boom.
Controlled: Only one neutron from each U atom split is used to split another atom, creating a sustained and controlled reaction.
Focus point 4:
The Fission Reactor:
The atomic bomb was understood before the reactor. Uncontrolled fission is easier.
Fissile: a nucleus that might undergo fission.
Critical mass: the mass of fissionable material that will create a sustained reaction.
Super critical:
Moderator: in which the material is held to slow down neutrons: slower neutrons are more effective.
Coolant: carries heat away to heat water which turns turbines to generate electricity.
Neutrons are highly penetrative. Control rods are made of cadmium or boron and absorb a certain number of neutrons to sustain reaction.
Neutron scattering: when a neutron interacts with matter it could be scattered, absorbed or captured.
Title: 16 June 2008 Physics
Date: 16 June 2008 1:01 PM
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Neutrons penetrate easily. They are neutral. During scattering, collisions between matter could be elastic (momentum and energy conserved) or inelastic (only momentum conserved)
Neutrons have wavelike properties. (wave-particle duality)
When bombarding stuff with neutrons, the scatter according to their wavelength. This scattering helps to determine the structure of certain matter.
X-Rays scatter off electrons but neutrons scatter off protons because they have similar mass. They can also deal with magnetic structures, organic structures. But it requires a nuclear reactor.
Particle accelerators:
Particle accelerators input energy into nuclei. The accelerator makes particles go fast to bombard the nuclei with stuff.
Title: 17 June 2008 Physics
Date: 17 June 2008 1:07 PM
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First particle accelerators:
John D Crockroft and Ernest T.S. Walton in 1932 made a proton go at 770 keV
Van der Graff generators went up to 1.5 MeV
Linear accelerators such as the Stanford Linear Accelerator Center (SLAC) is 3km long. Made of long evacuated tubes. There are gaps between the tubes where the particle accelerates.
Cyclotron:
Its like two big semicircles forming a circle (with a gap between them) that alternate in voltage. The particle moves in a spiral and accelerates in the space between the semicircles (‘dees’).
Synchrotron:
Particles are kept on a constant radius circle. Radio frequencies accelerate the particles by causing an electric field.
After collision due to the conservation of momentum, particles that get hit usually fly away so you cant measure them. So they get two particles moving in opposite directions so that they don't move when they collide.
Title: 18 June 2008 Physics
Date: 18 June 2008 1:13 PM
Category: School
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MODERN CLASSIFICATION OF MATTER
This is pretty dumb.
Matter is made of:
Quarks, Leptons, Bosons.
Quarks have charges that are factions of the charge on electrons. i.e. -1/3 of e or +2/3 of e
Quarks are pointlike, they have no measurable size or components.
There are six types (flavours) of quarks (from lightest to heaviest):
up (u;+2/3)
down (d;-1/3)
strange (s;-1/3)
charm (c;+2/3)
bottom (b;-1/3)
top (t;+2/3)
They are grouped in pairs above.
The last 4 don’t exist in normal matter, they were made in particle accelerators.
They also have what is referred to as a colour charge: red, blue or green.
Antiquarks have colour charges antired antiblue or antigreen.
Different colours attract, like colours repel.
Hadrons are made up of quarks and there are two types: Baryons and Mesons.
Quarks never exist by themselves, they combine to form hadrons. Hadrons have no net colour charge
Baryons are protons and neutrons. They are made of 3 quarks (qqq)
A proton is made of 2 up and one down quark. (uud) Net charge (if you ad it up) is +e (positive electron charge)
Neutron is made of 1 up and 2 down (udd) Net charge is 0
Mesons are made of 1 quark and one antiquark (qq(bar))
A meson could be a pion (π-)
Pions are made of an antiup quark and a down quark (u(bar)d)
Title: 19 June 2008 Physics
Date: 19 June 2008 1:23 PM
Category: School
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Here comes some more dumb.
Leptons:
Have minute mass. Six flavours (symbol;charge):
electron (e-;-1)
electron-neutrino (ve;0)
muon (µ-;-1)
muon-neutrino (vµ;0)
tau (τ-;-1)
tau-neutrino (vτ;0)
Each charged lepton has a neutral neutrino equivalent. Each lepton has an antilepton too. Leptons can exist in combination with quarks or on their own. There are 3 generations of all these particles:
1st generation: quarks: up down
leptons: electron electron-neutrino
(normal matter)
2nd generation: quarks: charm strange
leptons: muon muon-neutrino
(unstable and decay into 1st gen)
3rd Generation: quarks: top bottom
leptons: tau tau-neutrino
(unstable and decay into 1st gen)
Here ends the dumb.
FOCUS POINT 1
Rutherford’s model of the atom.
Rutherford observed alpha particles interacting with gold foil. Most particles went through but some bounced off or were deflected. So he concluded there was a heavy nucleus where the collision occurred.
He then thought that the nucleus had lots of tiny positive charges in it. It had equal number of negative charges orbiting the nucleus.
He thought that the electrons should be emitting EM radiation because they were accelerating as they moved around the nucleus. He thought that the atoms were unstable because of this.
Title: 23 June 2008 Physics
Date: 23 June 2008 11:17 AM
Category: School
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Bohr.
Took Rutherfords image of the atom and used Planck’s Quantum theory and put them together. Got the 1922 Nobel Physics Prize. After lots of experimenting, he concluded that if the electron REMAINED in the same orbit it would not emit energy. If an electron changes its orbit position it will absorb/emit energy. Electrons are either in one shell or another, not between.
Spectroscopy: study of light spectrum emitted by elements.
Radiation occurs when electrons jump from higher to lower orbits. Energy is radiated as photons.
Bohr calculated the wavelengths that would be emitted by certain elements. He compared this to the spectrum of light emitted by Hydrogen and they matched exactly.
The hydrogen spectrum is called the Balmer series.
Title: 26 June 2008 Physics
Date: 26 June 2008 11:23 AM
Category: School
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Prac: observe the hydrogen spectrum.
λ = (dsinø)/n
Use a hydrogen tube to observe light emitted by hydrogen.
Title: 27 June 2008 Physics
Date: 27 June 2008 11:26 AM
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Bohr’s Postulates:
Electrons in atoms exist in stationary states where they are stable for some reason. A complete change in stationary state must be achieved to make a permanent change in motion.
Electrons don’t emit radiation within stationary states.
hf = E1-E2
h is Planck’s constant
E1 and E2 are the energies of initial and final states.
f is frequency I guess.
An electron in a stable state has angular momentum.
L=mvr
L is angular momentum
m is mass of point
v is velocity of point mass
r is the radius of the circle which the point mass orbits.
Quantisation condition: angular momentum is an integral of h/2π
Title: 23 July 2008 Physics
Date: 23 July 2008 3:48 PM
Category: School
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Planks contribution to Quantum Physics
He used the idea of discreet packets of energy and explained why radiation emitted by a black body was the way it was.
Bohr and Einstein liked this idea. Bohr used it in the model of the atom and Einstein used it in the photoelectric effect.
This equation explains the wavelength emitted by H when an electron jumps from ni shell to nf shell
1/λ = RH(1/nf2 - 1/ni2)
R is Rhydberg’s constant = 1.097 x 107 m-1
nf and ni are the energy shells (lower and higher respectively)
Calculating radius:
rn = n2r1
rn = radius of integer shell
n = integer shell no.
r1 = radius of ground state (n=1)
Title: 24 July 2008 Physics
Date: 24 July 2008 3:55 PM
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Bohr model of the atom only applies to H and ions with a single electron.
It could not explain the behaviours of atoms with more.
When observing spectral lines, some where more intense. Why?
Spectral lines are actually made up of hyper fine lines.
Argh im confused.
de Broglie’s wave particle duality was important.
He thought no one had proved light was a wave yet. So he thought why not be a particle AS WELL.
Usually physicists develop a mathematical solution then test it. This guy did it the other way.
His PHD brought about the age of quantum mechanics.
λ = h/mv
where:
λ is wavelength (m)
h = Planck’s constant (6.626 x 10-34)
mv = mass x velocity or momentum in kg m s-1
wavelength is a wave property and mv is a particle property.
Title: 25 July 2008 Physics
Date: 25 July 2008 4:16 PM
Category: School
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Diffraction and Interference.
Diffraction: when waves go through a small opening that is small compared to the size of the wavefront then kind of bend out so it looks like a semicircle from the opening.
Interference: When waves interact constructively or destructively.
Title: 28 July 2008 Physics
Date: 28 July 2008 7:24 PM
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de Broglie’s PhD was only accepted because Einstein liked it. It was proved later.
Davisson and Germer in 1923 got some nickel that had been oxidised and burnt of the oxide. They overdid it and annealed it (the lattice became uniform)
They put a beam of electrons through it and found diffraction and interference in the electron receiver on the other side.
This is how they proved de Broglie’s ideas.
Title: 29 July 2008 Physics
Date: 29 July 2008 11:49 AM
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Confirmation of de Broglie’s proposal by Davisson and Germer.
de Broglie’s PhD was accepted but it was kinda questionable.
Germer and Davisson proved it when they burnt some oxide off of some nickel a bit too much and annealed the nickel lattice. This made the lattice uniform. They pointed a beam of electrons at it and scattered them throughout the lattice.On the electron detector on the other side they detected interference patterns much like you would get with light waves. This proved particles had wavelike properties. PhD CONFIRMED!
The concept of particles as wavelike allowed people to understand why electrons could orbit the nucleus stably. If an integral number of wavelengths can fit into anorbit length, standing waves can be possible: no energy was lost.
Bohr’s first postulate: nλ = 2πr
λ = h/p = h/mv
nh/mv = 2πr
Bohr’s third postulate: mvr = nh/2π
Title: 30 July 2008 Physics
Date: 30 July 2008 11:58 AM
Category: School
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Heisenberg, Pauli and Atomic Theory.
Heisenberg was impressed by Bohr’s lectures but openly didn't agree with some of his statements. Bohr was impressed by Heisenberg’s understanding and challenging of his statements. Heisenberg thought Bohr didn’t understand why everything was happening as he explained, so he set out to develop a mathematical model for quantum mechanics.
Pauli applied this model to hydrogen and got Balmer’s equations and Rhydberg’s constant.
There are three quantum numbers
n is the principal quantum number
l is angular momentum number
m is magnetic quantum number.
The last number had to do with electron spin and was introduced by Pauli.
No electron in an atom will have the same three number combination. (Pauli’s exclusion principal)
Heisenberg said uncertainty is inherent to quantum physics.
If you know position accurately you can’t know momentum accurately and vice versa.