| Date | May 2014 | Marks available | 5 | Reference code | 14M.2.SL.TZ1.3 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Question number | 3 | Adapted from | N/A |
Question
This question is about electric and magnetic fields.
A proton travelling to the right with horizontal speed 1.6×104ms–1 enters a uniform electric field of strength E. The electric field has magnitude 2.0×103NC–1 and is directed downwards.
Calculate the magnitude of the electric force acting on the proton when it is in the electric field.
A uniform magnetic field is applied in the same region as the electric field. A second proton enters the field region with the same velocity as the proton in (a). This second proton continues to move horizontally.
(i) Determine the magnitude and direction of the magnetic field.
(ii) An alpha particle enters the field region at the same point as the second proton, moving with the same velocity. Explain whether or not the alpha particle will move in a straight line.
Markscheme
=3.2×10-16(N);
(i) \(\left( {F = qvB \Rightarrow } \right)B = \frac{F}{{qv}}\) or \(\left( {Eq = qvB \Rightarrow } \right)B = \frac{E}{v}\);
\(\left( { = \frac{{3.2 \times {{10}^{ - 16}}}}{{1.6 \times {{10}^{ - 19}} \times 1.6 \times {{10}^4}}}} \right) = 0.13\) or 0.125(T);
directed into the page / OWTTE;
(ii) both electric and magnetic forces double / both forces increase by the same factor / both forces scale with q/charges and cancel;
so straight line followed; (only award if first mark awarded)
or
straight line followed if qE = qvB ⇒E v=B;
E, v and B constant (so straight line followed);
Examiners report
This calculation was successfully done by the majority of candidates.
bi) The magnitude of the magnetic field was often successfully calculated, but few candidates were able to identify the direction. Most thought that it was in the opposite direction to the electric field, presumably confusing it with magnetic force.
bii) Many thought that it would carry on in a straight line but this was often based on spurious reasoning.
