Friday, 5 September 2014

CURRENT ELECTRICITY QUESTIONS AND PROBLEMS


                           Current electricity

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

2. Comment on a variety of sources of electricity as a means of powering a city.

3. Calculate the current flowing in a copper wire of cross-sectional area 3x10-7m2 if the drift velocity is 0.25 mms-1

4. Calculate the drift velocity in a silver wire of cross- sectional area 1.5x10-7m2 if a current of 30 mA flows through it.

5. Calculate how much electrical energy is supplied by a 12V battery when: 
(a) a charge of 2000C passes through it 
(b) a current of 3.5A flows from it for 25s 

6. How much energy is drawn from a 12V car battery if it is used to supply 220A for 2s 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

8. 20 identical light bulbs are connected in series across a 240V 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. If two cells, one of 4.5V and the other of 3V are connected in parallel what is the p.d across the junctions between them? 

10. Calculate the current through the following resistors: 
(a) 100Ω connected to 240V 
(b) 4700Ω connected to 24V 
(c) 10kΩ connected to 3V 
(d) 2.5MΩ connected to 30V 

11. What is the resistance of the following?
(a) a torch bulb that draws 0.25 A from a 12V supply
(b) an immersion heater that draws 10 A from a 240V supply 

12. What is meant by: (a) the EMF 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 2400Ω and when a voltmeter of very high resistance is connected across its terminals the voltmeter reads 2.5 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 1000Ω

15. A cell of e.m.f 12.0 V is connected to a resistor of 5000 Ω and when a voltmeter of very high resistance is connected across its terminals the voltmeter reads 10.5 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 2000Ω

16. The two graphs below show a specimen of metal wire at two different temperatures.

(a) calculate the resistance of the wire at each temperature 
(b) which graph shows the higher temperature? 
(c) does the material disobey Ohms Law? Explain your answer. 

17. Calculate the length of a copper wire of cross sectional area 0.8x10-7 m2 with a resistance of 2 Ω Resistivity of copper = 1.7x10-8Ωm

18. Calculate the resistivity of a material if a 2.5 m length of wire of that material with a diameter of 0.3 mm has a resistance of 3Ω

19. Calculate the resistance between the large faces of a slab of germanium of thickness 1 mm and area 1.5 cm2. Resistivity of germanium = 0.65 Ωm.

20. Calculate the length of a constantan wire with a diameter of 0.8 mm that has a resistance of 8 Ω Resistivity of constantan = 49x10-8Ωm.

21. Calculate the power loss in an electrical transmission cable, 15km long, carrying a current of 100A at a potential of 200 kV. The resistance per km of the cable is 0.2 Ω.

22. What power is supplied to the heater of an electric bar fire with a resistance of 50 Ω connected to the mains 240V supply? 

23. What is the power loss down a copper connecting lead 50cm long with a resistance of 0.005 Ω per metre when it carries a current of 1.5A? 

24. Calculate the resistance of the following combinations: 
(a) 100 Ω and 50 Ω connected in series
(b) 100 Ω and 50 Ω connected in parallel 
(c) 100 Ω and 50 Ω connected in series and then connected in parallel with 200 Ω

25. Calculate the current flowing through the following when a p.d of 12V is applied across the ends: (a) a resistance of 500 Ω in series 
(c) 500 Ω and 1000 Ω in parallel

26. How does the theory of parallel circuits restrict the use of adapters and multiple output extension leads? 

27. Two equal lengths of the same wire but of different thicknesses are connected to a cell in parallel. Prove that the velocity of the free electrons is the same in each wire. 

28. You are given one 200 Ω resistor and two 100 Ω resistors. How would you connect any combination of them to give a combined resistance of:
(a) 400 Ω 
(b) 250 Ω
(c) 167 Ω

Data for questions on temperature coefficient of resistance:
Material Ωx10-4 K-1 Material Ωx10-4 K-1 Copper 43 Aluminium 38 Lead 43 Nichrome 1.7 Eureka 0.2 Manganin 0.2 Iron 62 Platinum 38 Carbon - 0.5 Tungsten 60 

Materialβx10-4K-1 Materialβx10-4K-1
Copper43 Aluminium38
Lead43 Nichrome1.7
Eureka0.2 Manganin0.2
Iron62 Platinum38
Carbon-0.5 Tungsten60

Where necessary use the values for the temperature coefficient of resistance quoted in the preceding table for the following problems.

29. Calculate the change in the resistance of a platinum wire if the temperature rises from 293 K to 315 K if its resistance at 293 K was 2.5 Ω

30. A tungsten filament has a resistance of 20 Ω at 20oC. What is its resistance at 1500oC

31. A tungsten filament lamp has a filament whose resistance increases from 30 Ω at 20oC to 350 Ω at its operating temperature. Calculate the operating temperature of the lamp.

32. A coil of wire has a resistance of 3.5 Ω at room temperature (18oC) which rises to 65 Ω when placed in boiling water. Calculate the temperature coefficient of resistance of the metal of the coil.

33. A semiconductor used as a thermometer has a resistance of 200Ω at 15oC and this falls to 25 Ω at 100oC. Calculate the mean temperature coefficient of resistance for the material over that range.