Topic 13: Electromagnetic Indtroduction

Question 1

13.1P Explain how to produce an electric current by the relative movement of a magnet and a conductor on a small scale in the laboratory

Answer 1

• Spin a coil of wire in between two permanent magnets which will cause a current to flow in the wire.
• Passing a wire through a field will also show a deflection in an ammeter.

Question 2

13.1P Explain how to produce an electric current by the
relative movement of a magnet and a conductor
b in the large-scale generation of electrical energy

Answer 2

• In a thermal power station, water heats up and evaporates into steam.
• Steam is put under pressure and forces into a turbine, which begins to rotate.
• The turbine is a coil of wire in a strong magnetic field, which generates current.

Question 3

13.2 Recall the factors that affect the size and direction of an induced potential difference, and describe how the magnetic field produced opposes the original change

Answer 3

• Number of coils of wire
• Speed of rotation
• Magnetic field strength

Question 4

13.2 Recall the factors that affect the direction of an induced potential difference

Answer 4

• Changing direction of magnetic field/magnet
• Changing polarity of magnet

Question 5

13.2 Describe the correlation between the original change and the magnetic field produced

Answer 5

The magnetic field produced opposes the original change

• When a current is induced in a wire, that current produces its own magnetic field
• the magnetic field produced by an induced current always acts against the change that made it.

Question 6

13.3P Explain how electromagnetic induction is used in alternators to generate current which alternates in direction (a.c.)

Answer 6

Every half-turn, the current switches direction, as the wire will be in the opposite orientation from the starting position.
- Produces alternating current

Question 7

13.3P Explain how electromagnetic induction is used in dynamos to generate direct current (d.c.)

Answer 7

swaps the connections every half-turn of the coil to keep the current flowing in the same direction.
- This generates direct current

Question 8

13.4P Explain the action of the microphone in converting the pressure variations in sound waves into variations in current in electrical circuits, and the reverse effect as used in loudspeakers and headphones

Answer 8

• Microphones convert the pressure variations in sound waves into variations in current in electrical circuits.
• Sound waves cause variations in air pressure
• The pressure variations make a diaphragm vibrate.
• The diaphragm moves a coil of wire backwards and forwards.
• The coil moves relative to a permanent magnet, so a potential difference is induced in the coil.
• The coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit.
• The movement of the coil (and so the generated current) depends on the properties of the sound waves (louder sounds make the diaphragm move further)

Question 9

13.5 Explain how an alternating current in one circuit can induce a current in another circuit in a transformer

Answer 9

An alternating current creates a changing magnetic field, which cuts through the secondary coil, which induces a (alternating) current in the secondary coil.

Question 10

13.6 Recall that a transformer can change the size of an alternating voltage

Answer 10

A transformer can change the size of an alternating voltage.

Question 11

3.7P Use the turns ratio equation for transformers to
calculate either the missing voltage or the missing
number of turns

Answer 11

(potential difference across primary coil ÷ potential difference across secondary coil) = (number of turns in primary coil ÷ number of turns in secondary coil)

Question 12

13.8 Explain why, in the national grid, electrical energy is transferred at high voltages from power stations, and then transferred at lower voltages in each locality for domestic uses as it improves the efficiency by reducing heat loss in transmission lines

Answer 12

Electrical energy is transferred at high voltages as there either needs to be a high voltage or high current. A high current causes power to be lost due to resistive heating.

The energy is then transferred at a lower voltage at each locality for domestic use.

It reduces heat loss in transmission lines.

Question 13

13.9 Explain where and why step-up and step-down transformers are used in the transmission of electricity in the national grid

Answer 13

Step=up transformers increase the voltage and keep the current low.

Step-down transformers decrease the voltage to be safe, usable levels at the consumer’s end.

Question 14

13.10 Use the power equation (for transformers with 100% efficiency)

Answer 14

Power in = Power out

IpVp = IsVs

Where:
Ip = current in the primary coil (A)
Vp = voltage in the primary coil (V)
Is = output current from the secondary coil (A)
Vs = output voltage from the secondary coil (V)

Question 15

13.4P Explain the action of loudspeakers and headphones

Answer 15

• Loudspeakers convert variations in an electrical current into sound waves.
• A current in the coil creates a magnetic field.
• The magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
• When the current is reversed, the force acts in the opposite direction.
• These movements make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave.

Question 16

13.11P Explain the advantages of power transmission in high voltage cables, using the equations in 10.29, 10.31,
13.7P and 13.10

Answer 16

It is more efficient.

• power = energy ÷ time
• power = current x potential difference
• power = current² x resistance²
• potential difference across primary coil ÷ potential difference across secondary coil = number of turns in primary coil ÷ number of turns in secondary coil
• potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil