Topic 12 - Electromagnetic induction

Question 1

How can an emf be induced?

Answer 1

By relative motion between a conductor and a magnetic field

Question 2

Which factors affect the size of the emf induced?

Answer 2

1. The speed of the wire
2. The strength of the magnetic field
3. The length of the wire in the magnetic field

Question 3

Explain how an emf is induced in a conductor moving in a magnetic field

Answer 3

The magnetic force pushes the free electrons on the conductor to one side, so that side becomes negative. At the same time the other side is left with protons and becomes positive. This charge separation produces a potential difference (emf) across the ends of the conductor

Question 4

What is required in order an emf to be induced due to relative motion between a conductor and a magnetic field?

Answer 4

The motion needs to be across the magnetic field

(a motion parallel will not induce an emf)

Question 5

What are four ways to increase the emf induced?

Answer 5

1. Faster motion
2. Using a magnet with a greater magnetic field strength
3. Wrapping a wire into a coil with many turns
4. Using magnetic fields and coils with larger areas

Question 6

What reverses the emf and current?

Answer 6

Reversing the motion or the magnetic field.

Question 7

Derive the formula for the emf induced in a straight conductor moving in a magnetic field

Answer 7

Question 8

What is magnetic flux?

Answer 8

The product of the area, A, and the component of the magnetic field strength perpendicular to that area, Bcosθ

Φ = BAcosθ

Question 9

What is the magnetic flux if the magnetic field is perpendicular to the area?

Answer 9

Because cosθ = 1,

Φ = BA

Question 10

What is the relationship between field strength, area, and direction and magnetic flux?

Answer 10

1. There is greater flux through B than A because the field strength is greater over the same area (more lines)
2. There is more flux through C than D because the area is greater for the same field strength
3. There is more flux through E than F because the field is perpendicular to the area

Question 11

What is magnetic flux linkage?

Answer 11

The product of magnetic flux and the number of turns in the circuit (no symbol):

Magnetic flux linkage = NΦ

N = number of turns in a circuit

Question 12

How is an emf induced by a time-changing magnetic flux?

Answer 12

(see picture)

• Changes in the current in one circuit affect another, separate circuit
• When the switch of A is closed, a current starts to flow around circuit A and this sets up a magnetic field around it
• The magnetic field passes through circuit B
• The sudden change of magnetic field induces an emf and a current
• Only lasts for a moment because the current in A becomes constant after a while
• When the switch is turned off, there is an induced emf/current for a moment in the opposite direction

Question 13

What are the three ways in which an emf can be induced?

Answer 13

1. Part of a circuit can be moved through a stationary magnetic field
2. A magnetic field (bar magnet) can be moved through a circuit
3. A changing current in one circuit can produce a changing magnetic field which spreads out and passes through another circuit

Question 14

What is Faraday’s law?

Answer 14

The magnitude of an induced emf is equal to the rate of change of magnetic flux linkage:

Question 15

What is Lenz’s law?

Answer 15

The direction of an induced emf is such that it will oppose the change that produced it (negative sign in Faraday’s equation)

Question 16

Describe the emf in a coil rotating at constant speed within a uniform magnetic field

Answer 16

The induced emf is sinusoidal

Question 17

Explain the function of an AC generator

Answer 17

• The coil of wire rotates in the magnetic field due to an external force
• The flux linkage of the coil changes with time and induces an emf
• The work done rotating the coil generates electrical energy
• Carbon brushes are in contact with slip rings so that induced current can flow into an external circuit

Question 18

What are the amplitudes of emf when the coil in an AC generator is at different positions?

Answer 18

1. The plane of the coil is parallel to the magnetic field, WX and YZ are cutting across the field at the fastest rate → maximum emf
2. WX and YZ are moving parallel to the magnetic field → no emf
3. Sides are moving in the opposite direction to 1. → reverse maximum emf

Question 19

What is the effect of changing the frequency of an AC generator on the emf induced?

Answer 19

Slower frequency → smaller rate of change of magnetic flux through it → smaller emf

Halving the frequency will halve the rate of change and therefore halve the induced emf (time period is doubled)

Question 20

What is root mean square (rms) value?

Answer 20

The rms value given to an alternating current (or voltage) is the same as the value of a direct current (or voltage) that would dissipate power in an ohmic resistor at the same rate

Question 21

What is the maximum power generated in an AC circuit?

Answer 21

• The power varies but always remains positive because V and I are either both positive or both negative at all times

Question 22

What is the average power in an AC circuit?

Answer 22

Question 23

What are effective values of the current and voltage?

Answer 23

The values that give the average power, not the average values, but are less than the maximum values:

(also known as rms value)

Question 24

What is the resistance in an AC circuit?

Answer 24

Question 25

How does a transformer work?

Answer 25

An alternating current in the primary coil creates an alternating magnetic flux which then passes through the iron core to the secondary coil and induces an emf

Question 26

What is the relationship between the output voltage and the number of turns on the secondary coil?

Answer 26

The greater the number of turns on the secondary coil, the greater the output voltage

Question 27

What is the difference between a step-up and a step-down transformer?

Answer 27

A step-up increases the voltage whereas a step-down reduce the voltage

Question 28

What is an ideal transformer?

Answer 28

A transformer that is 100% efficient: power into primary coil equals power out of secondary coil:

Question 29

Why is power lost in transmission lines?

Answer 29

Some of the electrical energy is always transformed to thermal energy due to resistance

Question 30

How can the power lost be reduced in transmission lines?

Answer 30

1. Keeping the resistance of the cables and the currrent through them as low as possible 2. Metal chosen for the conductor needs to have a reasonably low resisitivity

Question 31

How is power lost in transformers?

Answer 31

1. The wires of the coils have resistance, so power is generated when currents flow through them 2. Currents are induced in the core of the transformer → resistive heating 3. Energy is transferred in the core as it is repeatedly magnetised and demagnetised 4. All of the magnetic flux created in the primary coil will not pass through the secondary coil (leakage of magnetic flux)

Question 32

How can the enery loss be reduced in transformers?

Answer 32

1. Using thick wires of a metal with low resistivity (copper) 2. Cores can be made with layers of a ferromagnetic material to reduce eddy currents 3. Larger transformers are usually the most efficient (98%)

Question 33

Why are high-voltage step-up and step-down transformers used in the transmission of electricity?

Answer 33

- Decreasing current decreases the power and thermal energy lost - If the current is to be reduced, the voltage must be increased by the same factor - Voltages are stepped up as high as possible - Later on the voltage has to be stepped down again to a value that is safe for use - The need to transform voltages explains why alternating currents are used (only ac can induce electricity)

Question 34

What is meant by extra-low-frequency fields?

Answer 34

Electric fields created by low-frequency sources, such as home appliances and power lines

Question 35

Are the electromagnetic fields around power lines a health risk?

Answer 35

- Human bodies are conductors of electricity - Very small currents may be electromagnetically induced - Due to low frequencies, the rate of change of magnetic flux is relatively small → any induced currents will be very small - Due to large wavelength, relatively little electromagnetic energy spreads from the circuits

Question 36

What are the arguments on the possible health risks involved in living and working near high-voltage power lines?

Answer 36

- Most scientists believe that the effects are negligible - No evidence of any significant health risk, such as damage to genetic material - The magnetic fields due to electrical devices is much smaller than the Earth's magnetic field strength - Speculations about fields causing leukaemia, other cancers, depression, heart disease - It is possible that there is a health risk but no proof of it - In order to create health risks there would need to be higher currents, higher frequencies, or a longer time in the fields - The energy carried by electromagnetic photons is believed to be too small to cause harmful reactions