| Date | November 2016 | Marks available | 1 | Reference code | 16N.3.SL.TZ0.4 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 0 |
| Command term | Define | Question number | 4 | Adapted from | N/A |
Question
An electron X is moving parallel to a current-carrying wire. The positive ions and the wire are fixed in the reference frame of the laboratory. The drift speed of the free electrons in the wire is the same as the speed of the external electron X.
Define frame of reference.
In the reference frame of the laboratory the force on X is magnetic.
(i) State the nature of the force acting on X in this reference frame where X is at rest.
(ii) Explain how this force arises.
Markscheme
a coordinate system
OR
a system of clocks and measures providing time and position relative to an observer
OWTTE
i
electric
OR
electrostatic
ii
«as the positive ions are moving with respect to the charge,» there is a length contraction
therefore the charge density on ions is larger than on electrons
so net positive charge on wire attracts X
For candidates who clearly interpret the question to mean that X is now at rest in the Earth frame accept this alternative MS for bii
the magnetic force on a charge exists only if the charge is moving
an electric force on X , if stationary, only exists if it is in an electric field
no electric field exists in the Earth frame due to the wire
and look back at b i, and award mark for there is no electric or magnetic force on X
