Fields (5): Electromagnetic Induction Flashcards
What is the phenomenon called where an electric current is generated in a wire by moving it through a magnetic field?
Electromagnetic induction
Describe how electromagnetic induction works
- A wire is moved across a magnetic field between two magnets
- This induces an electromotive force, emf, into the wire
- If this wire is part of a complete circuit then th induced emf forces electrons round the circuit -> a current is read
How can the induced emf in electromagnetic induction be increased?
- Moving the wire faster
- Using a stronger magnet
- Making the wire into a coil and pushing the magnet in or out
In what scenario will an emf not be induced into a wire that moves through a magnetic field?
If the wire is parallel to the field lines as it moves through the field
Will a current always be induced when a magnet is moved relative to a conductor?
No, the conductor has to be part of a complete circuit in order for a current to be induced
Does the emf induced in a wire act in the same way as the emf of a battery?
No - the emf of a battery is constant whereas the emf induced in a wire becomes zero when the relative motion between the magnet and the wire ceases
What does an electric current do when it is induced in a wire?
The electric current transfers energy from the source of the emf to the other components in the circuit
Why will a spinning/moving magnet stop, even in the absence of other resistive forces, when it is in a magnet-coil system?
- The spinning magnet is doing work on the electrons in the wire/coil
- As an emf is induced, current flows in the coil
- The current transfers energy from the source of emf to the other components
- Therefore, work must be done to keep the magnet spinning
Energy transferred from the coil/wire to the components in its circuit is equal to…?
The work done on the magnet (inducing the emf) to keep it spinning
What does a wire contain?
Electrons
When electrons move through a magnetic field what do they experience?
A force
What rule do you need to use for generators?
Flemings right hand rule
Thumb - motion of conductor
First finger - field
Second finger - Current
What rule to you need to use for the motor effect?
Flemings left hand rule
What is the name of a wire that has been strung into loops?
A solenoid
What rule can be used to determine the direction of a magnetic field/direction of current in a solenoid?
Right hand grip rule
State Lenz’s Law
The direction of the induced current is always such as to oppose the change that causes the current
What is the conservation law that explains Lenz’s law?
Conservation of energy
What happens when the north pole of a magnet is push into a solenoid?
- A current is induced into the coils of the solenoid
- As you have a current flowing through a wire a magnetic field is produced
- The magnetic field produced is one that opposes the motion of the magnet
- Therefore the end of the solenoid closest to the north pole of the magnet becomes a north pole itself
NB - if this wasn’t the case then the coil would pull the north pole in even faster which would generate a stronger magnetic field in the wire, pulling the north pole in even faster and etc.
What happens when the north pole of a bar magnet is brought out of a solenoid?
- A current is induced into the coils of the solenoid in the opposite direction to when the north pole entered the solenoid
- A magnetic field is produced in the opposite direction and is one that opposes the motion of the magnet
- Therefore the end of the solenoid closest to the north pole of the magnet becomes a south pole
State Faradays Law of electromagnetic induction
The induced emf in a circuit is equal to the rate of change of flux linkage through the circuit
Describe what happens when a conducting poll vertical across metal rails is forced right, in a magnetic field going into the page?
Electromagnetic induction
- Relative movement of a conductor in a magnetic field
- Induces a current upwards
Lenzs Law - the direction of the induced current is always such that it opposes that change that caused the current
Motor effect
- Current flowing through the wire in a magnetic field causes a force on the wire to the left, F = BIl
Due to this opposing force being produced, an equal but opposite force must be applied to the conducting poll to keep it moving in the field
What is the equation for the work done by the applied force to move a conducting rod across a magnetic field?
W = Fd
= BIl(delta)s
What is the equation for the charge transfer across a conducting rod being moved across a magnetic field in time, t?
Q = I(delta)t
What is the equation for the induced emf in a conductor be forced across a magnetic field, with work done, W, and charge transfer across, Q? How do you derive it?
induced emf = W/Q
= BIl(delta)s / I(delta)t
= Bl(delta)s / (delta)t
l(delta)s = A, the area swept out by the conductor in time (delta)t
induced emf = BA/(delta)t
How would you describe magnetic flux?
The area swept out by a conductor in a magnetic field with flux density B
What is the equation for magnetic flux?
magnetic flux, theta = BA
It what scenario only is the equation for magnetic flux true?
When the magnetic field is normal/perpendicular to A
When is magnetic flux linkage used?
When a coil is sweeping across a magnetic field
What is the equation for magnetic flux linkage?
N(theta) = NBA
where:
- N is the number of turns of the coil
- B is the magnetic field perpendicular to area A
State the unit of magnetic flux
Weber, Wb
= Tm^2
Describe flux density using theta = BA?
Flux density is the flux per unit area, passing at right angles through the area
If a field is not at right angles to the direction of motion of the conductor, what do you have to do
- The normal is drawn -> the direction of motion of the conductor
- Need to resolve the magnetic field to find the perpendicular component of the magnetic field
- The hypotenuse is = B
- Need to think about resolving all the field lines
- Resolve in the direction of motion of the conductor
If the magnetic field is not perpendicular to the line of motion of the conductor/coil, what is the equation used for magnetic flux?
theta = BAcos(theta)
NB - thetas are not the same
If the magnetic field is not perpendicular to the line of motion of the conductor/coil, what is the equation used for magnetic flux linkage?
N(theta) = BANcos(theta)
When a coil is perpendicular to the magnetic field, the flux linkage = …?
N(theta) = BAN
When a coil is turned through 180 degrees to the magnetic field, the flux linkage = …?
- BAN
When a coil is parallel to the magnetic field, the flux linkage = …?
= 0