transformers - electromagnetic induction

Question 1

why do we use alternating currents

Answer 1

so there is a change in flux linkage in the secondary coil

Question 2

explain how an alternating current is induced in the secondary coil

Answer 2

• alternating pd across the primary coil therefore there is an alternating current across the primary coil
• alternating magnetic field around the primary coil
• therefore, the flux linkage is alternating in the secondary coil
• therefore there is an induced emf in the secondary coil and therefore, there is an alternating current across the secondary coil

Question 3

what is a step up transformer

Answer 3

• Vs > Vp
• Ns > Np
• increases the voltage by having more turns on the secondary coil than the primary

Question 4

what is a step down transformer

Answer 4

• Vs < Vp
• Ns < Np
• reduce the voltage by having fewer turns on the secondary coil

Question 5

why do we want to use transformers

Answer 5

• step up in power stations so we have a high voltage and low current for transmission power - this reduces energy losses in transmission lines as P = I^2R
• step down transformer is then used at the sub-station at our homes/factories

Question 6

why is a soft iron core used

Answer 6

• to direct the magnetic field from the primary coil to the secondary coil
• soft iron is easily magnetised and demagnetised so there is less energy dissipated due to hysteresis

Question 7

what could be done to reduce the eddy currents in the core

Answer 7

• laminate the core: thin layers of iron separated by layers of insulator
• the insulator has a high resistivity and the thin layers of iron also mean higher resistance

Question 8

what equation can we use if transformer is 100%

Answer 8

IpVp = IsVs

Question 9

why might the transformer not be 100% efficient

Answer 9

• energy dissipated in the wires (heat) due to resistance
• eddy currents in the iron core (energy is dissipated as heat)
• there are energy losses due to heating as the core is magnetised and demagnetised (this is hysteresis)

Question 10

what is the step up voltage in the national grid for transmission

Answer 10

around 400 000 v

Question 11

what is the step down voltage for use at home

Answer 11

230 v