Electromagnetic Induction (WJEC GCSE Physics): Revision Note
Electromagnetic Induction
Inducing a Current in a Circuit
The process of generating a current in a conductor is called induction
There are two methods of inducing a current in a conductor:
Moving a conductor within a fixed magnetic field
Placing a conductor in a changing magnetic field
The direction of the induced current is determined by the orientation of the poles of the magnet
Reversing the poles reverses the current's direction
The direction of the current is obtained using Fleming's Left Hand Rule
Moving the Electrical Conductor
When a conductor (such as a wire) is moved perpendicular to the direction of the field lines of a magnetic field (which are fixed) the wire cuts through the field lines
A potential difference is produced in the wire which induces (creates) the current
An ammeter detects the current in the circuit
Moving an Electrical Conductor in a Magnetic Field
Moving an electrical conductor in a magnetic field to induce a potential difference and a current
The size of the current induced can be increased by:
Increasing the strength of the magnetic field (getting a bigger magnet)
Increasing the speed of movement of the conductor
Increasing the length of wire (or amount of conductor) within the magnetic field
Moving the Magnetic Field
As the magnet moves through a fixed coil, the field lines cut through the turns on the coil
This generates a potential difference in the coil and induces a current
Moving the Magnetic Field Relative to the Conductor
When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference
The size of the induced current can be increased by:
Increasing the strength of the magnetic field (getting a bigger magnet)
Increasing the speed of movement of the magnet within the coil
Increasing the number of turns in the coils of wire
Increasing the size of the coils
The Operation of an A.C. Generator
Electromagnetic induction is often referred to as the generator effect and is the opposite of the motor effect
Instead of using electricity to create motion, motion is being used to create electricity
In the motor effect, there is already a current in the conductor which experiences a force
In the generator effect, there is no initial current in the conductor but one is induced (created) when it moves through a magnetic field
A generator generates an alternating current (a.c)
Worked Example
A coil of wire is connected to an ammeter. When a magnet is pushed into the coil the needle on the ammeter will deflect to the right as shown in the diagram below.
What will happen to the pointer on the ammeter when the magnet is stationary in the centre of the coil?
A. The needle will deflect to the left
B. The needle will deflect to the right
C. There will be no deflection of the needle
D. The needle will deflect to the left and then to the right
Answer: C
C is correct because there the magnet is stationary
This means there is no relative movement between the coil and the magnetic field, therefore no magnetic field lines are being cut
If the magnetic field lines are not being cut then there will not be a current induced
A, B & D are incorrect because a deflection on the ammeter would indicate that a potential difference has been induced
This could only happen if there was relative movement between the coil and the magnetic field
Examiner Tips and Tricks
When discussing factors affecting the induced current:
Make sure you state:
“Add more turns to the coil” instead of “Add more coils”
This is because these statements do not mean the same thing
Likewise, when referring to the magnet, use the phrase:
“A stronger magnet instead of “A bigger magnet”
This is because larger magnets are not necessarily stronger
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