Eduqas Physics for A Level Year 1 & AS: Study and Rev Guide

Tom’s answer (a) e A e e n (no. of free electrons per unit volume) charge e − drift velocity, v e I = nAve for current velocity = I nAe n = I Ave = I A nev = I e Ane (b) v = I nAe = - - ´ ´ ´ ´ ´ 6 16 29 2.0 1.7 10 1.0 10 1.0 10 7 – incorrect substitution = 1.176 × 10 - 7 m s - 1 7 – no e.c.f. (c) Free electrons in the wire knock into 3 ions in the metal lattice 3 they block the way and slow electrons down giving resistance. (d) R= V I . P = VI so R = 2 I P = 4 0.1 = 40 W ρ - ´ ´ = = 6 40 1.7 10 2.5 RA l 3 (e.c.f.) = 2.72 × 10 - 5 W m 3 (e) smaller 3 same 3 smaller 7 Examiner commentary (a) Good diagram – excessive labelling as the quantities are defined in the question stem. Tom clearly has not learnt the derivation. He has just played with the equation. (b) Tom has made a mistake with his substitution – the value for e is wrong. He has not helped himself by not keeping the quantities on the bottom line of the fraction in the same order as that in the algebraic equation. The second mark cannot be given following an incorrect substitution. (c) Tom has given a satisfactory answer here. (d) Tom has made a mistake in finding R . His use of this incorrect value was credited on the e.c.f. principle and therefore so was his incorrect final answer. (e) The resistivity of a metal does not depend upon its shape – only the composition. Tom gains 7 out of 15 marks. 177 Questions and answers