18. Transformers

Question 1

How does an induction cooker work?

Answer 1

• use alternating magnetic fields to induce currents in saucepan, which heats saucepan
• little energy transferred to thermal store of surroundings, making it v efficient

Question 2

Describe the structure of a transformer

Answer 2

1. Alternating current in primary coil produces continually changing magnetic field within iron core
2. Iron core guides magnetic field into secondary coil
3. Field lines continually moving because magnetic field is changing strength
4. Potential difference is induced across secondary coil because secondary coil cuts through field lines
5. If secondary coil is attached to a circuit then current will flow

Question 3

What is a transformer used for?

Answer 3

• transforming alternating voltage from a larger/smaller value to a smaller/larger value

Question 4

What does a step up transformer do to voltage current and power output, and why

Answer 4

• more turns in secondary coil
• increases voltage
• decreases current
• no effect on output power (because P=IV)

Question 5

What does a step down transformer do to voltage, current and power?

Answer 5

• fewer turns in secondary coil
• decreases voltage
• increases current
• power stays same

Question 6

What happens when voltage is increased using step up transformer?

Answer 6

Because power = current x voltage, current decreases

Question 7

What is the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer?

Answer 7

Vp/Vs = Np/Ns

P= primary
S= secondary
N= number of turns

Question 8

What is the relationship between input and output power when a transformer is 100% efficient?

Answer 8

• input power (IpVp) = output power (IsVs)

Question 9

How do you calculate efficiency of transformer?

Answer 9

Power in secondary/power in primary x 100

Question 10

Why do transformers not work with direct current?

Answer 10

In direct current, magnetic field would be constant so no field lines would be cut and there would be no potential difference generated

Question 11

How is energy wasted in transformers (3 ways)

Answer 11

1. Heat is produced in coils, since current is flowing through them
2. Magnetic field in core produces current which also causes heating
3. Magnetic field may leak out of core if there is an air gap between primary and secondary

Question 12

How do you make transformers efficient?

Answer 12

• heating in wires -> use low resistance wires
• eddy currents in iron core -> laminate iron core
• leaking magnetic field -> use an iron core to direct current through coils of wire

Question 13

How are transformers used on an industrial scale?

Answer 13

• used to increase voltage of electricity supply after it has left power station -> lower current, less heating of wires, less energy wasted
• voltage reduced at sub stations before reachers consumer, as this is safer