# 7.3.1 Electromagnetic (EM) Induction

Higher Tier Only

### The Generator Effect

• The generator effect is the opposite of the motor effect
• Instead of using electricity to create motion, motion is being used to create electricity
• The generator effect is defined as:

When a potential difference is induced across a conductor which is experiencing a change in an external magnetic field

• The generator effect occurs when a wire cuts through the magnetic fields lines

#### Generating Potential Difference

• A potential difference will be induced in the conductor if there is relative movement between the conductor and the magnetic field
• Moving the electrical conductor in a fixed magnetic field
• When a conductor (such as a wire) is moved through a magnetic field, the wire cuts through the fields lines
• This induces a potential difference in the wire Moving an electrical conductor in a magnetic field to induce a potential difference

• Moving the magnetic field relative to a fixed conductor
• As the magnet moved through the coil, the field lines cut through the turns on the coil
• This induces a potential difference in the coil When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference

• A sensitive voltmeter can be used to measure the size of the induced potential difference
• If the conductor is part of a complete circuit then a current is induced in the conductor

#### Factors Affecting the Induced Potential Difference

• The size of the induced potential difference is determined by:
• The speed at which the wire, coil or magnet is moved
• The number of turns on the coils of wire
• The size of the coils
• The strength of the magnetic field
• The direction of the induced potential difference is determined by:
• The orientation of the poles of the magnet

1. The speed at which the wire, coil or magnet is moved:

• Increasing the speed will increase the rate at which the magnetic field lines are cut
• This will increase the induced potential difference

2. The number of turns on the coils in the wire:

• Increasing the number of turns on the coils in the wire will increase the potential difference induced
• This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines

3. The size of the coils:

• Increasing the area of the coils will increase the potential difference induced
• This is because there will be more wire to cut through the magnetic field lines

4. The strength of the magnetic field:

• Increasing the strength of the magnetic field will increase the potential difference induced

5. The orientation of the poles of the magnet:

• Reversing the direction in which the wire, coil or magnet is moved

#### Worked Example

A coil of wire is connected to a sensitive voltmeter. When a magnet is pushed into the coil the needle on the voltmeter will deflect to the right as shown in the diagram below. What will happen to the pointer on the voltmeter 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

• C is correct because there the magnet is stationary
• This means there is no relative movement between the coil and the magnetic field, therefore there are no magnetic field lines being cut
• If the magnetic field lines are not being cut then there will not be a potential difference induced
• AB & D are incorrect because a deflection on the voltmeter 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

#### Exam Tip

When discussing factors affecting the induced potential difference:

• Make sure you state:
• 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
Higher Tier Only

### Lenz's Law

• Lenz Law states:

The direction of an induced potential difference always opposes the change that produces it

• This means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving

#### Demonstrating Lenz’s Law

• If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole
• Explanation
• Due to the generator effect, a potential difference will be induced in the coil
• The induced potential difference always opposes the change that produces it
• The coil will apply a force to oppose the magnet being pushed into the coil
• Therefore, the end of the coil closest to the magnet will become a north pole
• This means it will repel the north pole of the magnet Magnet being pushed into a coil of wire

• If a magnet is now pulled away from the coil of wire then the end of the coil closest to the magnet will become a south pole
• Explanation:
• Due to the generator effect, a potential difference will be induced in the coil
• The induced potential difference always opposes the change that produces it
• The coil will apply a force to oppose the magnet being pulled away from the coil
• Therefore, the end of the coil closest to the magnet will become a south pole
This means it will attract the north pole of the magnet Magnet being pulled away from a coil of wire ### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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