Paper 2 Transformers Answers

Q1 Ideal Transformer Ratios

1. $V_s/V_p=N_s/N_p=2400/400=6.0$, so $V_s=72.0\mathrm{V}$. [2]
2. Ideal power: $V_p{I}_p=V_s{I}_s$, so $I_p=72.0(0.30)/12.0=1.80\mathrm{A}$. [2]
3. Step-up transformer. [1]

Q2 Transformer Operation

1. Energy is transferred magnetically by changing flux in the core, not by charge passing from primary to secondary. [2]
2. AC produces continuously changing magnetic flux linkage; steady dc gives no continuous induced emf after switching. [2]
3. It links/concentrates magnetic flux between coils. [1]

Q3 Efficiency

1. $P_p=230(0.80)=184\mathrm{W}$. [1]
2. $P_s=12.0(13.5)=162\mathrm{W}$. [1]
3. $\eta =162/184\times 100\%=88.0\%$. [2]