Paper 3 Transformers Answers

Q1 Power Transmission

1. $I=P/V=60000/2000=30\mathrm{A}$. [2]
2. $P_{loss}=I^{2}R=30^2(8.0)=7.2\times 10^3\mathrm{W}$. [2]
3. $I=60000/20000=3.0\mathrm{A}$; $P_{loss}=3.0^2(8.0)=72\mathrm{W}$. [3]
4. For fixed transmitted power, higher voltage gives lower current; cable heating depends on $I^2$, so losses fall greatly. [2]

Q2 Transformer Losses

1. Heating in coil resistance; reduce with thick, low-resistance copper wires. [2]
2. Circulating currents induced in the iron core; reduce using insulated laminations. [2]
3. Energy lost in repeated magnetisation/demagnetisation; reduce using soft iron/low-hysteresis material. [2]
4. Some input energy is dissipated in these losses and by flux leakage. [1]