Charge
and Current
•The charge carriers are usually electrons; q = e= -1.60 x 10-19 C; but be careful when a current of ions exists.
The value of n for copper is 8.0 x 1028
m3.
- Calculate 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 mms-1
- In a silicon transistor with
area of cross section 3.8 x 10-6 m2 there is a
- The beam of electrons in a
cathode-ray tube contains electrons travelling at 8.4 x 106 ms-1
and the current through the tube is 2.8 μA.
(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?
- 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 l0-6
m2. By what factor does the drift speed of the delocalised electrons increase as they move from the
wide to the narrow section? Suggest how this indicates that a damaged wire
will overheat.
- Explain why a light comes on
almost immediately when switched on, although the drift speed of the
electrons in the wires to the light is so small.
