Friday, 5 September 2014

CURRENT ELECTRICITY -RESISTIVITY, DRIFT VELOCITY& INTERNAL RESISTANCE PROBLEMS


                                  Current electricity

Charge on the electron (e) = -1.6x10-19 C

1. What is meant by the free electron concentration in a metal?

2. Why might a good electrical conductor also be a good thermal conductor?

3. Calculate the current flowing in a copper wire of cross-sectional area 2x10-7m2 if the drift velocity is 3.5x10-4 ms-1

4. Calculate the drift velocity in a silver wire of diameter 0.26 mm if a current of 20 mA flows through it. 

5. Calculate how much electrical energy is supplied by a 1.5V battery when: 
(a) a charge of 1000C passes through it 
(b) a current of 2.5A flows from it for 2s 

6. How much energy is drawn from a 12V car battery if it is used to supply 200A for 1.5s to the starter motor.

7. You have five 1.5V cells.
Explain how you would connect them to give: 
(a) the highest output potential 
(b) a given current for the longest possible time 
(c) an output potential of 4.5V (you must use all the cells)

8. 10 identical torch bulbs are connected in series across a 12 V d.c supply. (a) what is the p.d across each bulb (b) what is the potential at the join of the second and third bulbs from the negative terminal? 

9. Why does the resistance of a metal increase as its temperature is raised?

10. Calculate the current through the following resistors:
(a) 100Ω connected to 200V 
(b) 4700Ω connected to 12V 
(c) 150kΩ connected to 3V 
(d) 2.0MΩ connected to 10V 

11. What is the resistance of the following? 
(a) a torch bulb that draws 0.3 A from a 12V supply
(b) an immersion heater that draws 10 A from a 240V supply

12. What is meant by:
(a) the E.M.F. of a cell
(b) the internal resistance of a cell

13. How does the internal resistance of a cell affect the current drawn from it?

14. A cell of e.m.f 3.0 V is connected to a resistor of 2700 Ω and when a voltmeter of very high resistance is connected across its terminals the voltmeter reads 2.8 V
(a) Explain the difference between these two voltages
(b) Calculate the internal resistance of the cell
(c) Calculate the new reading of the voltmeter if the voltmeter has a resistance of 1500 Ω

15. Define resistivity.

16. Calculate the length of a constantan wire of cross sectional area 0.8x10-7 m2 that would have a resistance of 2 Ω resistivity of constantan 47x10-8 Ωm.)

17. Calculate the resistivity of a material if a 1.5 m length of wire of that material with a diameter of 0.28 mm has a resistance of 2 Ω

18. A 240 V mains lamp draws a current of 2 A from the supply when operating normally.
Calculate:
(a) the resistance of the lamp when hot
(b) the power of the lamp when operating normally
(c) the number of electrons passing through the lamp filament each second
(d) the energy transferred to each coulomb by the supply
(e) the energy transferred to each electron by the supply

19. What is the power of an electrical heater operating from 110 V if the resistance of the heater is 15 Ω?

20. What is the power loss down a copper connecting lead 75cm long with a resistance of 0.13 Ω per metre when a current of 4.5 A flows through it?