A-level Physics (Advancing Physics)/Cloud Chambers and Mass Spectrometers/Worked Solutions

Charge of electron = -1.6 x 10−19C

Mass of electron = 9.11 x 10−31kg

u = 1.66 x 10−27kg

1. An electron enters a cloud chamber, passing into a 0.1T magnetic field. The initial curvature (the reciprocal of its radius) of its path is 100m−1. At what speed was it moving when it entered the magnetic field?

This is too close to the speed of light to ignore special relativity, however we just did.

2. The electron spirals inwards in a clockwise direction, as show in the diagram on the right. What would the path of a positron, moving with an identical speed, look like?

3. Using a 2T magnetic field, what electric field strength must be used to get a velocity selector to select only particles which are moving at 100ms−1?

4. Some uranium (atomic number 92) ions (charge +3e) of various isotopes are put through the velocity selector described in question 3. They then enter an 0.00002T uniform magnetic field. What radius of circular motion would uranium-235 have?