Friday, 5 September 2014

Electric - charge and current Problems and Questions


                  Electric - charge and current

Where needed take the charge on the electron to be –1.6x10-19 C

1. Why does the resistance of a metal increase with temperature?

2. (a) What is meant by free electrons?
(b) how are they moving within a solid?

3. Why does an insulator not conduct electricity?

4. What would you have to do to a sample of semiconductor to decrease its resistance? Explain your answer.

5. What is the approximate drift velocity of a ‘cloud’ of free electron through the bulb in a torch?

6. Express the following in AMPS:
(a) 2mA
(b) 5μA
(c) 6MA
(d) 12pA
(e) 102mA

7. How could the drift velocity of electons be increased?

8. Draw a diagram of a Thermionic diode and explain how the number of electrons crossing the tube varies with the applied voltage.

9. Calculate the charge passing a point in a wire when the following currents flow in a wire for the times given:
(a) 25mA 20s 
(b) 3A 5m 
(c) 10 μA 5 weeks

10. Calculate the current in a wire when the following charges flow past in the times given:
(a) 20 C in 5s
(b) 0.5 C in 25
(c) 1015 electrons in 2s
(d) 12 C in 2 minutes