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.

State the angle θ between the field lines and the normal for which the magnetic flux through a coil is (a) maximum and (b) minimum.

(a) Maximum at θ = 0°, when the field lines are perpendicular to the plane of the area. (b) Minimum (zero) at θ = 90°, when the field lines are parallel to the plane of the area.

True or False? Magnetic flux is at a maximum when the magnetic field lines are parallel to the plane of the coil.

False. Magnetic flux is a maximum when the field lines are perpendicular to the plane of the coil; it is a minimum (zero) when the field lines are parallel to the plane.

What is the independent variable in the experiment investigating magnetic flux (linkage)?

The angle, θ , between the normal to the search coil and the magnetic field lines.

Name three control variables in the experiment investigating magnetic flux linkage.

Area of the search coil, A ; number of turns , N , on both coils; magnetic field strength , B ; frequency of the power supply, f .

Define magnetic flux linkage .

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

How does flux linkage relate to the magnetic flux through a single turn of a coil with N turns?

Flux linkage is N times the magnetic flux through a single turn: flux linkage = NΦ .

True or False? Magnetic flux linkage is measured in webers.

False. Magnetic flux linkage is measured in weber turns (Wb turns) ; webers (Wb) is the unit of magnetic flux alone.

Define Faraday's law .

The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage .

The induced e.m.f. is set up in a direction to produce effects that oppose the change causing it.

What equipment is used to experimentally verify Lenz's law?

A bar magnet , a coil of wire , and a sensitive ammeter .

A north pole of a bar magnet is pushed into a coil. What pole is induced at the near face of the coil, and why?

A north pole is induced, because two north poles repel each other, opposing the incoming magnet (Lenz's law).

The direction of an induced current can be found using the .......... rule, where the fingers curl in the direction of the current and the thumb points along the field from north to south.

The direction of an induced current can be found using the right hand grip rule, where the fingers curl in the direction of the current and the thumb points along the field from north to south.

True or False? Lenz's law states that the induced e.m.f. acts to enhance the change in flux linkage that produces it.

False. Lenz's law states the induced e.m.f. acts to oppose the change in flux linkage causing it.

What physical quantity does the gradient of a graph of magnetic flux linkage against time represent?

The magnitude of the induced e.m.f.

The e.m.f. induced in a rotating coil is a maximum when the plane of the coil is .......... to the magnetic field lines.

The e.m.f. induced in a rotating coil is a maximum when the plane of the coil is parallel to the magnetic field lines.

At what angle θ between the field and the normal to the coil is the flux linkage a maximum, and what is the e.m.f. at this point?

θ = 0° ; the flux linkage is a maximum and the e.m.f. is zero .

True or False? The induced e.m.f. and the flux linkage reach their maximum values at the same angle θ .

False. The e.m.f. and flux linkage are 90° out of phase : flux linkage is maximum when the e.m.f. is zero, and the e.m.f. is maximum when the flux linkage is zero.

An alternator is a generator that converts mechanical energy into electrical energy in the form of alternating current , using a coil connected to slip rings .

A dynamo is a direct-current generator; it is the same as an alternator but uses a split-ring commutator instead of slip rings.

What components allow an alternator's coil to maintain continuous contact with the external circuit while spinning?

Slip rings (commutator rings) and brushes .

How does a split-ring commutator keep the current from a dynamo flowing in the same direction?

It swaps the connections between the coil and the brushes every half turn, each time the coil is perpendicular to the magnetic field.

An alternator uses two .......... (or commutator rings) and brushes to maintain contact between the rotating coil and the external circuit.

An alternator uses two slip rings (or commutator rings) and brushes to maintain contact between the rotating coil and the external circuit.

True or False? The output current from a dynamo periodically reverses between positive and negative values, just like an alternator.

False. A dynamo's output stays one polarity (always positive or always negative), varying from zero to a maximum twice per rotation; only the alternator's output alternates in direction.

A device that changes high alternating voltage at low current to low alternating voltage at high current , and vice versa.

Distinguish between a step-up and a step-down transformer in terms of number of turns.

Step-up : N s > N p (secondary voltage larger). Step-down : N p > N s (secondary voltage smaller).

The core of a transformer is made from .......... because it can be easily magnetised and demagnetised.

The core of a transformer is made from soft iron because it can be easily magnetised and demagnetised.

Why are transformers used in the National Grid?

To step up the voltage (reducing the current) for transmission, which reduces power loss ( P = I 2 R ) in the cables and increases efficiency.

Why are transformer cores often laminated (built from glued layers rather than a single block of iron)?

To reduce heating caused by eddy currents forming in the core.

True or False? Real transformers are 100% efficient, just like the ideal case assumed in calculations.

False. Real transformers are around 99% efficient ; some energy is lost to resistance in the wires and eddy currents in the core, though 100% efficiency is assumed for calculations.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsElectromagnetic Induction

Electromagnetic Induction (OCR A Level Physics): Flashcards

Exam code: H556

1/42

0Still learning

Know0

FrontMagnetic Flux
Define magnetic flux.

Was this flashcard helpful?

Cards in this collection (42)

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.
State the equation for magnetic flux when the field is perpendicular to the area, defining each symbol.
$Φ = B A$
Φ = magnetic flux (Wb), B = magnetic flux density (T), A = cross-sectional area (m²).
In $Φ = B A \cos θ$ , θ is the angle between the magnetic field lines and the .......... to the plane of the area.
In $Φ = B A \cos θ$ , θ is the angle between the magnetic field lines and the normal to the plane of the area.
What is the unit of magnetic flux?
The Weber (Wb).
State the angle θ between the field lines and the normal for which the magnetic flux through a coil is (a) maximum and (b) minimum.
(a) Maximum at θ = 0°, when the field lines are perpendicular to the plane of the area.
(b) Minimum (zero) at θ = 90°, when the field lines are parallel to the plane of the area.
True or False?
Magnetic flux is at a maximum when the magnetic field lines are parallel to the plane of the coil.
False.
Magnetic flux is a maximum when the field lines are perpendicular to the plane of the coil; it is a minimum (zero) when the field lines are parallel to the plane.
What is the independent variable in the experiment investigating magnetic flux (linkage)?
The angle, θ, between the normal to the search coil and the magnetic field lines.
Name three control variables in the experiment investigating magnetic flux linkage.
Area of the search coil, A; number of turns, N, on both coils; magnetic field strength, B; frequency of the power supply, f.
Define magnetic flux linkage.
Magnetic flux linkage is the product of the magnetic flux and the number of turns of the coil.
State the equation for magnetic flux linkage when the coil is perpendicular to the field.
$Φ N = B A N$
Where Φ = magnetic flux (Wb), N = number of turns, B = magnetic flux density (T), A = cross-sectional area (m²).
What is the unit of magnetic flux linkage?
Weber turns (Wb turns).
When the field is not perpendicular to the coil, flux linkage is found using $Φ N = B A N \cos θ$ , where θ is the angle between the field lines and the .......... to the plane of the coil.
When the field is not perpendicular to the coil, flux linkage is found using $Φ N = B A N \cos θ$ , where θ is the angle between the field lines and the normal to the plane of the coil.
How does flux linkage relate to the magnetic flux through a single turn of a coil with N turns?
Flux linkage is N times the magnetic flux through a single turn: flux linkage = NΦ.
True or False?
Magnetic flux linkage is measured in webers.
False.
Magnetic flux linkage is measured in weber turns (Wb turns); webers (Wb) is the unit of magnetic flux alone.
Define Faraday's law.
The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
State Faraday's law as an equation, defining each symbol.
$ϵ = \frac{Δ (N Φ)}{Δ t}$
ε = induced e.m.f. (V), Δ(NΦ) = change in flux linkage (Wb turns), Δ*t* = time interval (s).
Define Lenz's law.
The induced e.m.f. is set up in a direction to produce effects that oppose the change causing it.
What equipment is used to experimentally verify Lenz's law?
A bar magnet, a coil of wire, and a sensitive ammeter.
A north pole of a bar magnet is pushed into a coil. What pole is induced at the near face of the coil, and why?
A north pole is induced, because two north poles repel each other, opposing the incoming magnet (Lenz's law).
The direction of an induced current can be found using the .......... rule, where the fingers curl in the direction of the current and the thumb points along the field from north to south.
The direction of an induced current can be found using the right hand grip rule, where the fingers curl in the direction of the current and the thumb points along the field from north to south.
True or False?
Lenz's law states that the induced e.m.f. acts to enhance the change in flux linkage that produces it.
False.
Lenz's law states the induced e.m.f. acts to oppose the change in flux linkage causing it.
Write Faraday's law including the negative sign that represents Lenz's law.
$ϵ = - \frac{Δ (N Φ)}{Δ t}$
What does the negative sign in $ϵ = - \frac{Δ (N Φ)}{Δ t}$ represent?
Lenz's law: the induced e.m.f. is set up in the opposite direction to oppose the changing flux linkage causing it.
What physical quantity does the gradient of a graph of magnetic flux linkage against time represent?
The magnitude of the induced e.m.f.
State the equation for the e.m.f. induced in a coil of N turns rotating at angular velocity ω in a magnetic field of flux density B and area A.
$ϵ = B A N ω \sin (θ)$
The e.m.f. induced in a rotating coil is a maximum when the plane of the coil is .......... to the magnetic field lines.
The e.m.f. induced in a rotating coil is a maximum when the plane of the coil is parallel to the magnetic field lines.
At what angle θ between the field and the normal to the coil is the flux linkage a maximum, and what is the e.m.f. at this point?
θ = 0°; the flux linkage is a maximum and the e.m.f. is zero.
True or False?
The induced e.m.f. and the flux linkage reach their maximum values at the same angle θ.
False.
The e.m.f. and flux linkage are 90° out of phase: flux linkage is maximum when the e.m.f. is zero, and the e.m.f. is maximum when the flux linkage is zero.
Define alternator.
An alternator is a generator that converts mechanical energy into electrical energy in the form of alternating current, using a coil connected to slip rings.
Define dynamo.
A dynamo is a direct-current generator; it is the same as an alternator but uses a split-ring commutator instead of slip rings.
What components allow an alternator's coil to maintain continuous contact with the external circuit while spinning?
Slip rings (commutator rings) and brushes.
How does a split-ring commutator keep the current from a dynamo flowing in the same direction?
It swaps the connections between the coil and the brushes every half turn, each time the coil is perpendicular to the magnetic field.
An alternator uses two .......... (or commutator rings) and brushes to maintain contact between the rotating coil and the external circuit.
An alternator uses two slip rings (or commutator rings) and brushes to maintain contact between the rotating coil and the external circuit.
True or False?
The output current from a dynamo periodically reverses between positive and negative values, just like an alternator.
False.
A dynamo's output stays one polarity (always positive or always negative), varying from zero to a maximum twice per rotation; only the alternator's output alternates in direction.
Define transformer.
A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa.
State the transformer equation relating voltage and number of turns.
$\frac{V_{p}}{V_{s}} = \frac{N_{p}}{N_{s}}$
For an ideal transformer, how does the transformer equation extend to include current?
$\frac{V_{p}}{V_{s}} = \frac{N_{p}}{N_{s}} = \frac{I_{s}}{I_{p}}$
Distinguish between a step-up and a step-down transformer in terms of number of turns.
Step-up: N_s > N_p (secondary voltage larger). Step-down: N_p > N_s (secondary voltage smaller).
The core of a transformer is made from .......... because it can be easily magnetised and demagnetised.
The core of a transformer is made from soft iron because it can be easily magnetised and demagnetised.
Why are transformers used in the National Grid?
To step up the voltage (reducing the current) for transmission, which reduces power loss (P = I²R) in the cables and increases efficiency.
Why are transformer cores often laminated (built from glued layers rather than a single block of iron)?
To reduce heating caused by eddy currents forming in the core.
True or False?
Real transformers are 100% efficient, just like the ideal case assumed in calculations.
False.
Real transformers are around 99% efficient; some energy is lost to resistance in the wires and eddy currents in the core, though 100% efficiency is assumed for calculations.

Sign up to unlock flashcards

By signing up you agree to our Terms and Privacy Policy

Previous:Magnetic FieldsNext:The Nuclear Atom