Paper 3 Current Electricity Fundamentals Answers

Q1 I-V characteristic and power

1. It is not ohmic because current is not directly proportional to p.d.; $V/I$ changes as p.d. increases. [3]
2. $R=V/I=2.0/0.38=5.26\Omega$. [2]
3. $P=VI=4.0(0.62)=2.48\mathrm{W}$. [2]
4. Higher temperature makes metal ions vibrate more strongly, increasing collisions with electrons and reducing mobility, so resistance increases. [3]

Q2 Drift speed

1. $v_d=I/(nqA)=1.20/(8.5\times 10^{28}\times 1.60\times 10^{-19}\times 3.0\times 10^{-7})=2.94\times 10^{-4}\mathrm{m}{\mathrm{s}}^{-1}$. [3]
2. The electric field is established around the circuit rapidly, causing electrons throughout the circuit to start drifting; individual electrons still have a small net drift speed. [3]
3. Electron drift is opposite to conventional current. [1]