# Homework Help: Help! pls

1. Sep 23, 2007

### alyx

help! pls :)

1. What is the current in a copper wire with area of cross section 2.0 x 10^-5 m² when electrons drift through it with a mean speed of 0.80 mm s^-¹? The number density of free electrons for copper is 8.0 x 10^28 m^-³.

2. In a silicon transistor with area of cross section 3.8 x 10^-6 m² there is a d.c current of 200 mA. The current is a flow of electrons and the number density of free electrons for silicon is 8.3 x 10^23 m^-³. Determine the mean drift speed of the electrons.

3. The beam of electrons in a cathode-ray tube contains electrons travelling at 8.4 x 10^6 m s-¹ and then current through the tube is 2.8 uA.
a) How many electrons are emitted per second from the cathode of the tube?
b) What is the number of electrons per unit length of the beam?

4. A direct current of 3.0 A through a copper wire reaches a place where the area of cross section of the wire changes from 2.0 x 10^-6 m² to 0.090 x 10^-6 m². By what factor does the drift speed of the electrons increase as they move from the wide to the narrow section? Suggest how this indicates that a damaged wire will overheat.

2. Sep 23, 2007

### mgb_phys

You have to attempt a solution yourself and we will help - see the sticky thread.