Electromagnetic induction

Question 1

What is electromagnetic induction?

Answer 1

When an e.m.f is induced in a closed circuit conductor due to it moving through a magnetic field.

Question 2

Give examples of conductors that can exhibit electromagnetic induction.

Answer 2

A flat coil or a solenoid.

Question 3

When does electromagnetic induction occur?

Answer 3

When a conductor cuts through magnetic field lines.

Question 4

What determines the amount of e.m.f induced?

Answer 4

The magnetic flux.

Question 5

What is magnetic flux?

Answer 5

The total magnetic field that passes through a given area.

Question 6

When is magnetic flux a maximum?

Answer 6

When magnetic field lines are perpendicular to the plane of the area.

Question 7

When is magnetic flux zero?

Answer 7

When magnetic field lines are parallel to the plane of the area.

Question 8

Define magnetic flux.

Answer 8

Product of magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.

Question 9

What is the symbol for magnetic flux?

Answer 9

Φ (phi).

Question 10

What is the unit for magnetic flux?

Answer 10

Weber (Wb).

Question 11

State the equation for magnetic flux.

Answer 11

Φ = B × A

Question 12

What do the variables in Φ = B × A represent?

Answer 12

Φ = magnetic flux (Wb), B = magnetic flux density (T), A = area (m²).

Question 13

Draw and explain the graph of magnetic flux vs angle for a rotating window.

Answer 13

It is provided in the document.

Question 14

What is magnetic flux linkage?

Answer 14

The product of the magnetic flux and the number of turns of the coil.

Question 15

State the equation for magnetic flux linkage.

Answer 15

Flux linkage = ΦN = BAN

Question 16

What are the units of magnetic flux linkage?

Answer 16

Weber-turns (Wb turns).

Question 17

State the equation for flux through a rotating rectangular coil.

Answer 17

Φ = B × A × cos(θ)

Question 18

What do the variables in Φ = B × A × cos(θ) represent?

Answer 18

θ = angle between magnetic field and the normal to the area.

Question 19

When is magnetic flux maximum in a rotating coil?

Answer 19

When θ = 0°, i.e., field is perpendicular to the plane.

Question 20

When is magnetic flux zero in a rotating coil?

Answer 20

When θ = 90°, i.e., field is parallel to the plane.

Question 21

What causes an induced e.m.f in a circuit?

Answer 21

A changing magnetic flux linkage.

Question 22

State the modified flux linkage equation for rotating coil with N turns.

Answer 22

ΦN = BAN cos(θ)

Question 23

Define electromagnetic induction in principle.

Answer 23

Process in which an e.m.f is induced due to changes in magnetic flux.

Question 24

State two ways electromagnetic induction occurs.

Answer 24

1) Conductor cuts through magnetic field. 2) Direction of magnetic field through coil changes.

Question 25

List uses of electromagnetic induction.

Answer 25

Generators and transformers.

Question 26

Describe the setup for moving magnet through a coil to induce e.m.f.

Answer 26

A bar magnet moved through a coil connected to a voltmeter.

Question 27

State the effect of increasing speed of the magnet.

Answer 27

Increases the magnitude of the induced e.m.f.

Question 28

Draw and explain the graph of e.m.f vs time when moving magnet through coil.

Answer 28

It is provided in the document.

Question 29

Describe what happens when moving a wire through a magnetic field.

Answer 29

An e.m.f is induced if it cuts through field lines.

Question 30

State factors that increase induced e.m.f in a wire.

Answer 30

Increase length, speed, or magnetic field strength.

Question 31

State Faraday’s Law.

Answer 31

Induced e.m.f is proportional to the rate of change of magnetic flux linkage.

Question 32

State Lenz’s Law.

Answer 32

Induced e.m.f opposes the change causing it.

Question 33

Give the equation for Faraday’s Law.

Answer 33

ε = -N × ΔΦ / Δt

Question 34

What do the variables in Faraday's law equation represent?

Answer 34

ε = induced e.m.f (V), N = number of turns, ΔΦ = change in flux (Wb), Δt = time (s).

Question 35

State the e.m.f equation for a straight conductor moving through a magnetic field.

Answer 35

ε = B × L × v

Question 36

What variables are used in ε = B × L × v?

Answer 36

B = magnetic flux density (T), L = length (m), v = velocity (m/s).

Question 37

State the flux linkage equation for a rotating coil using angular velocity.

Answer 37

NΦ = BAN cos(ωt)

Question 38

State the e.m.f equation using angular velocity.

Answer 38

ε = BANω sin(ωt)

Question 39

What does ω represent?

Answer 39

Angular speed (rad/s).

Question 40

What is the phase difference between flux linkage and induced e.m.f in a rotating coil?

Answer 40

They are 90° out of phase.

Question 41

Draw and explain the sinusoidal graph of e.m.f and flux linkage.

Answer 41

It is provided in the document.

Question 42

What is an alternating current (a.c)?

Answer 42

A current which periodically varies between a positive to a negative value with time.

Question 43

What does it mean that the direction of an alternating current varies every half cycle?

Answer 43

It means the direction of flow of current changes after every half cycle.

Question 44

Describe the variation of current or p.d. with time for a.c.

Answer 44

It can be described as a sine curve (sinusoidal).

Question 45

What motion do electrons undergo in a wire carrying a.c?

Answer 45

They move back and forth with simple harmonic motion.

Question 46

State the relationship between time period T and frequency f for a.c.

Answer 46

f = 1 / T

Question 47

Define peak current (I0) or peak voltage (V0).

Answer 47

The maximum value of the alternating current or voltage.

Question 48

How can peak current or voltage be determined?

Answer 48

From the amplitude of a current-time or voltage-time graph.

Question 49

How is peak voltage V0 calculated from peak-to-peak voltage?

Answer 49

V0 = peak-to-peak voltage ÷ 2

Question 50

Draw and explain a graph of alternating current against time showing the time period, peak current and peak-to-peak current.

Answer 50

It is provided in the document.

Question 51

Define rms current (Irms).

Answer 51

The square root of the mean of the squares of all the values of the current in one cycle, or the equivalent direct current that produces the same power.

Question 52

State the equation for rms current.

Answer 52

Irms = I0 / √2

Question 53

Define rms voltage (Vrms).

Answer 53

The square root of the mean of the squares of all the values of the voltage in one cycle, or the equivalent dc voltage that produces the same power.

Question 54

State the equation for rms voltage.

Answer 54

Vrms = V0 / √2

Question 55

What is the relationship between rms and peak values?

Answer 55

Rms value is approximately 0.707 × the peak value.

Question 56

What is the average power of a supply?

Answer 56

Average power = Irms × Vrms

Question 57

Explain why mains electricity is supplied as a.c.

Answer 57

Because it allows the use of transformers to reduce energy losses in transmission.

Question 58

What is the UK mains electricity specification?

Answer 58

230 V at 50 Hz (rms value).

Question 59

Which devices use mains a.c. directly?

Answer 59

Lamps, heaters, cookers, and devices with large electric motors.

Question 60

Which devices convert a.c. to d.c. using transformers?

Answer 60

Televisions, computers, games consoles.

Question 61

What is a Cathode-Ray Oscilloscope (CRO) used for?

Answer 61

To display, measure and analyse waveforms of electrical circuits.

Question 62

Draw and explain a diagram of an oscilloscope showing wavelength and time-base setting.

Answer 62

It is provided in the document.

Question 63

What do the x-axis and y-axis represent on a CRO?

Answer 63

X-axis: time, Y-axis: voltage (y-gain).

Question 64

State the formula to calculate frequency from time period.

Answer 64

f = 1 / T

Question 65

What is the transformer equation?

Answer 65

Vs / Vp = Ns / Np

Question 66

What is the ideal transformer power equation?

Answer 66

Ip × Vp = Is × Vs

Question 67

What is the efficiency formula for a transformer?

Answer 67

Efficiency = Power out / Power in

Question 68

How can eddy currents be reduced?

Answer 68

By laminating the core with insulated layers.

Question 69

What causes inefficiencies in transformers?

Answer 69

Resistance in wires, eddy currents, magnetic hysteresis, poor insulation.

Question 70

How can energy loss be reduced in transformers?

Answer 70

Laminated cores, thick wires, soft iron, linking all magnetic flux.

Question 71

What equation is used for power loss in transmission lines?

Answer 71

P = I²R

Question 72

Explain how step-up and step-down transformers reduce energy loss.

Answer 72

Step-up increases voltage and decreases current, reducing heat loss; step-down adjusts voltage for home use.