Define magnetic flux .

Magnetic flux is the product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.

What is electromagnetic induction ?

Electromagnetic induction is the process of inducing an e.m.f. in a conductor when there is relative movement between a charge and a magnetic field, causing the conductor to cut through magnetic field lines.

What is the unit of magnetic flux, and what symbol represents it?

Magnetic flux is represented by the symbol Φ and measured in Webers (Wb) .

True or False? Magnetic flux is maximum when θ = 90°, where θ is the angle between the field lines and the normal to the area.

False. Magnetic flux is maximum when θ = 0°, since cos(0°) = 1; at θ = 90° the flux is zero because the field lines are parallel to the plane of the area.

Name three changes that can induce an e.m.f. in a circuit due to a changing magnetic flux linkage.

A changing magnetic flux density, B A changing cross-sectional area, A A change in angle, θ

Magnetic flux is a measure of the number of magnetic field .......... passing through a given area.

Magnetic flux is a measure of the number of magnetic field lines passing through a given area.

Define magnetic flux linkage .

Magnetic flux linkage is the product of the magnetic flux and the number of turns of a coil.

True or False? Magnetic flux linkage and magnetic flux are measured in the same units.

False. Magnetic flux is measured in Webers (Wb) , whereas magnetic flux linkage is measured in Weber turns (Wb turns) , since it also depends on the number of turns, N .

Magnetic flux linkage is commonly used for .......... , which are made of N turns of wire.

Magnetic flux linkage is commonly used for solenoids , which are made of N turns of wire.

What two situations cause electromagnetic induction to occur?

A conductor moves relative to a magnetic field A magnetic field varies relative to a conductor

Describe the process by which an e.m.f. is induced when a conductor cuts through magnetic field lines.

The free electrons in the conductor experience a magnetic force , doing work as charges become separated . This mechanical work is transferred to the charges as electric potential energy , creating a potential difference (induced e.m.f.) between the ends of the conductor.

In the magnet-and-coil experiment, what does the voltmeter show when the bar magnet is stationary, and why?

The voltmeter shows a zero reading , because the rate of change of magnetic flux is zero , so no e.m.f. is induced.

What happens to the induced e.m.f. when the bar magnet is withdrawn from the coil compared with when it enters?

The induced e.m.f. reverses in direction (opposite sign), because the direction of the current changes as the magnet's direction changes.

Name three factors that increase the magnitude of the e.m.f. induced when a magnet moves through a coil.

Moving the magnet faster through the coil Adding more turns to the coil Increasing the strength of the bar magnet

Name three factors that increase the magnitude of the e.m.f. induced when a wire moves through a magnetic field.

Increasing the length of the wire in the field Moving the wire faster between the magnets Increasing the strength of the magnets

True or False? In the wire-through-magnets experiment, a current must first be flowing through the wire in order to induce an e.m.f.

False. There is no current flowing through the wire to start with; the e.m.f. is induced purely by the wire moving through the magnetic field.

When a bar magnet moves through a coil, its magnetic field lines ' .......... ' the coil, generating a change in magnetic flux.

When a bar magnet moves through a coil, its magnetic field lines ' cut through ' the coil, generating a change in magnetic flux.

Define Faraday's law of electromagnetic induction.

Faraday's law states that the magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.

Lenz's law states that the induced e.m.f. acts in such a direction as to produce effects which oppose the change causing it .

What apparatus is needed to demonstrate Lenz's law experimentally?

A bar magnet A coil of wire A sensitive ammeter Note: a cell is not required

In the Lenz's law demonstration, what happens to the deflection on the ammeter if the bar magnet's direction is reversed?

The ammeter shows an opposite deflection , since the direction of the induced current reverses.

True or False? When a bar magnet is pushed into a coil, the induced magnetic field in the coil attracts the magnet towards it.

False. By Lenz's law , the induced field repels the bar magnet, opposing the change (the magnet's motion) that created it.

If a d.c. power supply is replaced with an a.c. supply, the induced e.m.f. will alternate with the same .......... as the supply.

If a d.c. power supply is replaced with an a.c. supply, the induced e.m.f. will alternate with the same frequency as the supply.

A LevelPhysicsCambridge (CIE)Flashcards20. Magnetic FieldsElectromagnetic Induction

Electromagnetic Induction (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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Cards in this collection (30)

Define magnetic flux.
Magnetic flux is the product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.
What is electromagnetic induction?
Electromagnetic induction is the process of inducing an e.m.f. in a conductor when there is relative movement between a charge and a magnetic field, causing the conductor to cut through magnetic field lines.
What is the equation for magnetic flux when the field is perpendicular to the area?
$Φ = B A$
What is the equation for magnetic flux when the field lines are at an angle θ to the normal of the area?
$Φ = B A \cos (θ)$ , where θ is the angle between the magnetic field lines and the line perpendicular (normal) to the plane of the area.
What is the unit of magnetic flux, and what symbol represents it?
Magnetic flux is represented by the symbol Φ and measured in Webers (Wb).
True or False?
Magnetic flux is maximum when θ = 90°, where θ is the angle between the field lines and the normal to the area.
False.
Magnetic flux is maximum when θ = 0°, since cos(0°) = 1; at θ = 90° the flux is zero because the field lines are parallel to the plane of the area.
Name three changes that can induce an e.m.f. in a circuit due to a changing magnetic flux linkage.
- A changing magnetic flux density, B
- A changing cross-sectional area, A
- A change in angle, θ
Magnetic flux is a measure of the number of magnetic field .......... passing through a given area.
Magnetic flux is a measure of the number of magnetic field lines passing through a given area.
Define magnetic flux linkage.
Magnetic flux linkage is the product of the magnetic flux and the number of turns of a coil.
What is the equation for magnetic flux linkage when the field is perpendicular to the coil?
$N Φ = B A N$
What is the equation for magnetic flux linkage when the magnetic field lines are at an angle θ to the normal of the coil?
$N Φ = B A N \cos (θ)$
What are the units of magnetic flux linkage?
Weber turns (Wb turns)
True or False?
Magnetic flux linkage and magnetic flux are measured in the same units.
False.
Magnetic flux is measured in Webers (Wb), whereas magnetic flux linkage is measured in Weber turns (Wb turns), since it also depends on the number of turns, N.
Magnetic flux linkage is commonly used for .........., which are made of N turns of wire.
Magnetic flux linkage is commonly used for solenoids, which are made of N turns of wire.
What two situations cause electromagnetic induction to occur?
- A conductor moves relative to a magnetic field
- A magnetic field varies relative to a conductor
Describe the process by which an e.m.f. is induced when a conductor cuts through magnetic field lines.
The free electrons in the conductor experience a magnetic force, doing work as charges become separated. This mechanical work is transferred to the charges as electric potential energy, creating a potential difference (induced e.m.f.) between the ends of the conductor.
In the magnet-and-coil experiment, what does the voltmeter show when the bar magnet is stationary, and why?
The voltmeter shows a zero reading, because the rate of change of magnetic flux is zero, so no e.m.f. is induced.
What happens to the induced e.m.f. when the bar magnet is withdrawn from the coil compared with when it enters?
The induced e.m.f. reverses in direction (opposite sign), because the direction of the current changes as the magnet's direction changes.
Name three factors that increase the magnitude of the e.m.f. induced when a magnet moves through a coil.
- Moving the magnet faster through the coil
- Adding more turns to the coil
- Increasing the strength of the bar magnet
Name three factors that increase the magnitude of the e.m.f. induced when a wire moves through a magnetic field.
- Increasing the length of the wire in the field
- Moving the wire faster between the magnets
- Increasing the strength of the magnets
True or False?
In the wire-through-magnets experiment, a current must first be flowing through the wire in order to induce an e.m.f.
False.
There is no current flowing through the wire to start with; the e.m.f. is induced purely by the wire moving through the magnetic field.
When a bar magnet moves through a coil, its magnetic field lines '..........' the coil, generating a change in magnetic flux.
When a bar magnet moves through a coil, its magnetic field lines 'cut through' the coil, generating a change in magnetic flux.
Define Faraday's law of electromagnetic induction.
Faraday's law states that the magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
What is the equation for Faraday's law?
$ϵ = N \frac{Δ Φ}{Δ t}$ , where ε is the induced e.m.f., N is the number of turns, ΔΦ is the change in magnetic flux and Δt is the time interval.
Define Lenz's law.
Lenz's law states that the induced e.m.f. acts in such a direction as to produce effects which oppose the change causing it.
How is Lenz's law incorporated into Faraday's law equation, and why?
A minus sign is added: $ϵ = - N \frac{Δ Φ}{Δ t}$ , showing that the induced e.m.f. (and the magnetic field it produces) acts in the opposite direction to the change that caused it.
What apparatus is needed to demonstrate Lenz's law experimentally?
- A bar magnet
- A coil of wire
- A sensitive ammeter
Note: a cell is not required
In the Lenz's law demonstration, what happens to the deflection on the ammeter if the bar magnet's direction is reversed?
The ammeter shows an opposite deflection, since the direction of the induced current reverses.
True or False?
When a bar magnet is pushed into a coil, the induced magnetic field in the coil attracts the magnet towards it.
False.
By Lenz's law, the induced field repels the bar magnet, opposing the change (the magnet's motion) that created it.
If a d.c. power supply is replaced with an a.c. supply, the induced e.m.f. will alternate with the same .......... as the supply.
If a d.c. power supply is replaced with an a.c. supply, the induced e.m.f. will alternate with the same frequency as the supply.

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