Transport on Track Flashcards
How do AC induction motors work?
An alternating current flows through a static part. This creates a changing magnetic flux which induces and EMF and therefore a current in copper bars rotating around the stationary part.
Newton’s second law of motion:
F = ma
Impulse
Change in momentum
Force x time
Work done =
Energy transferred
Force x distance moved in direction of force
When can we use Flemings Left Hand Rule?
For the motor effect; when something moves or a force is exerted
Flemings left hand rule
thuMb - Movement
First finger - Field (north to south)
seCond finger - Current (positive to negative)
What factors affect the size of the force?
- the strength of the magnet
- the size of the current
- the length of the wire
Force is proportional to…
… the current in the wire, the length of the wire within the magnetic field, the strength of the magnetic field
F = BIl
What is ‘B’?
Magnetic flux density - the strength of the field measure in Tesla’s
Field lines closed together higher magnetic flux density
Magnetic field
A region where a magnet has an effect
What experiment is used to show the motor effect?
Witches cradle the ‘trapeze/ swing’ is passed through a magnet and will kick outwards. Or you could place a magnet on a set of scales and put a current carrying wire through it the scales will either ‘lose’ mass or ‘gain’ mass
The size of the force varies with angle between field and current. How does this change the equation?
F = BIlsinx
How do we represent a magnetic field on a 2D piece of paper?
Into the page, away from you - you’d see a cross in a circle
Out of the page, towards you - a dot in a circle
Think of an arrow: arrowhead or feathers
Torque
Forces acting to produce a turning effect
Simple electric motor. How does a force kicking a wire into/out of a magnet drive a train further and go continuously?
When a wire is wound into a coil and the current switched on, the two faces have magnetic poles. Suspend a coil in a magnetic field and the pull of the attraction between opposite poles will turn the coil. The turning will stop once the coil faces opposite poles and is repelled by like poles so the current will reverse meaning the coil is always attracted to opposite poles and will keep turning.
Armature
Many coils orientated in a full 360^ about the axis of rotation
Eddy currents can be used…
…for braking
What are eddy currents?
Currents circulating within the body of a conductor
How does eddy current breaking work?
In trams there are large coils fitted to the bae of the carriage close to the tram rails. In sudden braking current is diverted from the motor circuit into the coils turning them into strong electromagnets moving just above the metal tracks. The resulting eddy currents in the tracks provide magnetic fields to generate forces that will oppose the motion of the track brake magnet attached to the tram.
Does it matter whether the magnets are moving and the conductor is fixed or the magnets are fixed and the conductor is moving?
No the braking effect will be the same
When an EMF is produced using magnetism this is called…
…electromagnetic induction: the EMF and current are an induced EMF and an induced current
The size of the induced EMF is proportional to…
… the strength of the magnetic field
The rate at which the wire/ coil or magnet is moved
The number of turns of the coil
What will happen to the induced EMF if the direction of movement or the magnet is reversed?
The EMF will change direction
How is a current induced?
When a conductor moves through a magnetic field it will experience a changing magnetic flux (linkage). Due to this an EMF will be induced and therefore a current
Magnetic Flux
‘Amount of field’ in an area
Flux (phi) (Weber’s/ Wb) = flux density (B) x area (A)
Magnetic flux linkage
The magnetic flux x number of turns of wire
N x phi
How can we work out the direction of the induced current?
Using Flemings right hand rule
Lenz’s law
‘The direction of the current induced in a conductor by moving it relative to a magnetic field is such that it’s own field opposes the motion’
Flemings right hand rule
thuMb - Motion/ movement
First finger - Field
seCond finger - Current
How do we know which of Flemings rules to use?
rIght hand for Induction
Types of braking
- air braking
- regenerative braking
- rheostatic braking
- track braking
- parking brake
How do train lines work?
By conduction: when a train is over a part of the train line the current will be conducted along the rail and through the train from this we know a train is on that part of track. When there’s no train there no current will flow so that part of the track is empty
Problems using conduction on train tracks
If the tracks get wet it will conduct all the time or if leaves cover the track no current will flow even if there is a train there. Good electrical contact is needed
How does a train driver know to control the speed?
There will be screens periodically along the track at the end of each block. If all is clear there will just be a number or If there’s a target speed for the end of the block. If a signal is flashing it means there will be a new signal at the beginning of the next block so the driver has warning of change.
Capacitors
Components that can store electrical charge
When does a capacitor stop charging?
When the pd of the capacitor equals the of of the cell
What is capacitance??
The amount of charge stored per unit potential difference across the plates
What happens when a capacitor is charged?
One of the plates is positive and one of the plates is negative
Time constant
Resistance x capacitance
Or the time taken for 37% of the initial voltage or charge to remain
Change in momentum is related to…
… the impulse
For all collisions the impact force on the two or more bodies is…
…equal and opposite
Conservation of linear momentum:
The total momentum before equals the total momentum after provided no external forces act on the system
Elastic collisions
Kinetic energy and momentum is conserved
Inelastic collision
Momentum is conserved but there is a net loss of kinetic energy
How do trains know where other trains are?
Trains move into blocks, only 1 train is allowed in each block and there must be a block between them. To ensure this a circuit is set up and signals sent to a control room.
Solenoid
Coil of wire
What two things do you need for electromagnetic induction?
Magnets and wires
What is Faraday’s law
The size of the induced EMF is proportional to the rate of (change) of magnetic flux linkage
Dynamo
Magnet that turns in a coil of wire (induces current, charges up)
Retardation force
Slowing down force
How can you make a magnet?
By stroking a metal with a magnet and around the field lines; this will line up all of the little magnets, put inside a coil of wire with d.c current and the metal will come out as a magnet
Eddy pendulum
- 2 magnets on a string, closer the magnets are of stronger the magnets the greater the braking effect if a pendulum passes through the magnet
- if solid aluminium will stop almost immediately
- if it has slats in it will continue normally for quite a while
Neon bulb - what happens?
When the switch is closed nothing happens, when the switch opens the bulb flashes (it flashes when the p.d is 200V)
Neon bulb - explained
- when the switch is closed current flows
- this makes a magnetic field around the coil of wire
- open the switch, the field disappears, from this the magnetic flux changes in a short time
- this induces a large EMF so a flash of light is seen from the neon bulb
Explain how a magnet moving up and down through a coil of wire induces a current and the current time graphs produced
- as the magnet moves through the coil of wire, the flux linkage changes which induces a potential difference and therefore a current
- when moving closer to the coil the EM field strength will increase so flux linkage will be greater and induced EMF will be bigger
- the negative peak is greater than the positive peak so a greater EMF was induced due to a bigger change in flux linkage or a shorter time interval. In this case it’s a shorter time interval as we know it’s going faster due to acceleration from gravity
Explain a capacitor charging
When the flying lead is connected, charge (electrons) flow round the circuit gathering on one side of the capacitor. This makes the plate negatively charged and repels the electrons on the other side of the gap - on the other plate. Because the electrons on the other side are repelled and flow round towards the already negative plate this plate becomes positively charged
Conventional current
Positive to negative (electrons flow the opposite way)
How fast is the charging process? Why?
There’s a very quick, very big initial charge however as the charge builds up the electrostatic forces between the electrons, gets bigger, which repel each other so the charging slows
Discharge process
Great initial discharge, all the electrons repel each other, they have electric fields and feel a greater force of repulsion, when the flying lead disconnected and connected by cell the electrons are free to flow as they wish - away from each other
Where are capacitors used?
- in timing circuits mostly
- car indicators
- crossings (level and pedestrian)
- microwave quick start
What graph does a capacitor make when charging and discharging?
An exponential
Momentum equation
Momentum = mass x velocity
Is momentum a vector or scalar?
A vector quantity so it has a magnitude and direction
