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Subject: Science , asked on 12/10/12

# can anyone give me some problems in the lesson electricity and magnetic field of electric current except the problems which is in the book

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Electricity -

Very short answer questions (1 mark)
1. Give the unit of (a) Charge (b) Current
2. Define current
3. Name the unit of (a) electrical resistance (b) resistivity
4. Define One Ohm
5. Define Resistivity
6. What is the resistance of a torch bulb rated at 2.5 V and 500 mA?
7. Two resistances of each 2 ohm are connected in parallel. Find their equalent resistance.
8. On what factors does the resistivity of a material depend?
9. Plot a graph between the Potential difference V and current I through a conductor
10. What happens to the resistance of the circuit if the current through it is doubled?

1. Two wires of same material are having length L and 2L. Compare their resistance and resistivity.
2. Why are coils of electric toaster and electric iron made of an alloy rather then a pure metal?
3. Two wires are of same length and radius but one of them is copper and the other is of iron. Which will have more resistance? (Given the resistivity of copper = 1.62 x 10 -8 ohm meter and resistivity of iron = 10 x 10-8 ohm meter.
4. Define 1KWh. Give the relation between 1kwh and Joule.
5. State which has a higher resistance. A 50W or 25W lamp. Also find the ratio of their resistances.
6. A wire of resistance 5 Ohm is spent in the form of closed circle. What is the resistance between 2 points at the ends of any diameter of the circle?
7. Calculate the amount of charge that would flow in one hour through the element of an electric iron drawing a current of 0.4 amps.
8. A electric toaster of resistance 20 Ohm takes a current of 5A. Calculate the heat developed in 30 s.
9. A bulb is rated at 5V, 100mA. Calculate its (1) Power (2) Resistance
10. Name two special characteristics of a heater coil.

1. Define resistance and resistivity. Give the relation between them. Explain the dependence of resistance on temperature.
2. With the help of neat circuit, derive the expression for the equalent resistance of 3 resistances connected in series.
3. With the help of neat circuit, derive the expression for the equivalent resistance of 3 resistances connected in parallel
4. (a ) Draw the circuit consisting of a battery of five 2V cells, 5ohm resistor, 10 ohm resistor, 15 ohm resistor and a plug key. All connected in series (b) Calculate the current passing through the above circuit when key is closed.
5. Two identical resistors each of resistance 2 Ohm are connected in turn (1) in series (2) in parallel to a battery of 12 V. Calculate the ratio of power consumed in two cases.
6. A piece of wire is redrawn by pulling it until its length is tripled. Compare the new resistance with the original value.
7. An electric kettle is rated 500W, 200V. IT is used to heat 400 gm of water for 30 secs. Assuming the voltage to be 220V calculate the rise in temperature of water. Specific heat capacity of water is 4200 J/Kg ºC.
8. In an experiment the current flowing through a resistor and potential difference across it are measured. The values are given below. Show that these values confirm Ohm’s Law and also find the resistance of the resistor.
I (ampere) I(ampere) 1.0 1.0 2 1.5 2.0 2.0 2.5 2.5 3.0 3.0
V (volt) V(volt) 4.0 4.0 6.0 6.0 8.0 8.0 10.0 10.0 12.0 12.0

9. A heater draws 1100 W at 220V. (a) Find the resistance of the heater (b) Calculate the energy in KWh consumed in a week if the heater is used daily for 4 hours.

1. State Ohms law with a neat circuit. Explain how this law can be verified and also plot the expected v-I graph.
2. (a) Differentiate resistance and resistivity
(b) the ratio of resistivities of two materials a and b is 1:2, ratio of their length is 3:4 and if the ratio of radii is 2:3 find the ratio of resistance of a and b.

Magnetic Effects of Electric Current -

Q.1 A straight wire carrying electric current is moving out of plane of paper and is perpendicular to it. What is the direction and type of induced magnetic field?

Q.2 How can it be shown that magnetic field existaround a wire carrying current?

Q.3 How can a solenoid be used to magnetise a steel bar.

Q.4 Why can’t two magnetic field lines ever intersect?

Q.5 Can a 5 A fuse be used in wire carrying 15 A current? Why?

Q.6 Give the factors that affect strength of magnetic field at a point due to a straight conductor carrying current.

Q.7 Where do we connect a fuse: with live wire or with neutral wire?

Q.8 Give two uses of electromagnets.

Q.9 Name any two devices which use permanent magnets.

Q.10 Draw the magnetic field lines representing uniform magnetic field.

Q.11 A current-carrying straight conductor is placed in the east-west direction. What will be the direction of the force experienced by this conductor due to earth’s magnetic field? How will this force get affected on? (a) reversing the direction of flow of current (b) doubling the magnitude of current.

Q.12 An electron enters a magnetic field at right angles to it as shown in fig. The direction of the force acting on the electron will be: (a) to the right (b) to the left (c) out of the page (d) into the page

Q.13 Why is the earth pin thicker and longer than the live and the neutral pins?

Q.14 A coil of insulated copper wire is connected to a galvanometer. What would happen if a bar magnet is

(i) Pushed into the coil?

(ii) Withdrawn from inside the coil?

(iii) Held stationary inside the coil?

#### ANSWERS OF THE ABOVE QUESTIONS

A.1 Induced magnetic field will be in the form of concentric circles in the plane of paper.

A.2 By using magnetic compass which, shows deflection.

A.3 By inserting the steel bar inside thesolenoid and switching on electric current.

A.4 If so then at the point of intersection there will be two different directions of magnetic field which is not possible.

A.5 No, because both of them would then be ineffective in controlling the amount of current flowing.

A.6 Magnitude of electric current, perpendicular distance between that point and conductor.

A.7 It is always connected with live wire.

A.8 (i) It is used in cranes for lifting heavy loads.

(ii) used in electric bells.

A.9 Loudspeakers, Galvanometer, voltmeter.

A.10 **

A.11 The direction of earth’s magnetic field is from G-south to G-north. Let current is from west to east. Therefore force is vertically upwards.

(a) By reversing the direction of current, the direction of will be reversed i.e. vertically downwards.

(b) The magnitude of the force is doubled.

A.12 When a conductor carrying current is placed perpendicular to the direction of magnetic field, the acting on it is given by Fleming’s left hand rule. Since the direction of current is the same as that of the motion of a positive charge, the direction of force acting on it when moving perpendicular to the direction of magnetic field is the same as that acting on a current-carrying conductor placed perpendicular to the direction of magnetic field. Obviously, the force acting on an electron is opposite to that. Therefore in this case it is into the page.

A.13 It is thicker so that it does not enter into the live or neutral sockets. It is made longer so that it gets connected to the earth terminal earlier than the live and  neutral pins. This ensures the safety of the user.

A.14 (i) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards right).

(ii) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards left opposite to that in case one).

( ii i) As it is stationary no change in magnetic flux linked with coil, so galvanometer shows no deflection.

Hope this helps!! Good luck for your exams!!

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Here are some numericals on Electricity & don't forget to pracitce the NCERT NUMERICALS FIRST ;-)

1. The current passing through a room heater has been halved. What will happen to the heat produced by it?

2. An electric iron of resistance 20 ohm draws a current of 5 amperes. Calculate the heat produced in 30 seconds.

3. An electric heater of resistance 8 ohm takes a current of 15 A from the mains supply line. Calculate the rate at which heat is developed in the heater.

4. A resistance of 40 ohms and one of 60 ohms are arranged in series across 220 volt supply. Find the heat in joules produced by this combination in half a minute.

5. A resistance of 25 ohm is connected to a 12 V battery. Calculate the heat energy in joules generated per minute.

6. 100 joules of heat is produced per second in a 4 ohm resistor. What is the potential difference across the resistor?

7. An electric iron is connected to the mains power supply of 220 V. When the electric iron is adjusted at minimum heating’ it consumes a power of 360 W but at ‘maximum heating’ it takes a power of 840 W. Calculate the current and resistance in each case.

8. Ten bulbs are connected in a series circuit to a power supply line. Ten identical bulbs are connected in a parallel circuit to an identical power supply line.
1. Which circuit would have the highest voltage across each bulb
2. In which circuit would the bulbs be brighter?
3. In which circuit, if one bulb blows out, all others will stop glowing?
4. Which circuit would have less current in it?

9. Calculate the cost of operating a heater of 500 W for 20 hours at the rate of Rs. 3.90 per unit.

10. Which has a greater resistance, a 100 watt bulb or a 60 watt bulb?

11. How much energy is consumed when a current of 5 amperes flows through the filament (or element) of a heater having resistance of 100 ohms for two hours? Express it in joules.

12. An electric bulb is rated at 220 V, 100 W. What is its resistance?

13. An electric bulb is connected to a 220 V power supply line. If the bulb draws a current of 0.5 A, calculate the power of the bulb.

14. In which of the following cases more electrical energy is consumed per hour?
1. A current of 1 ampere passed through a resistance of 300 ohms
2. A current of 2 amperes passed through a resistance of 100 ohms.

15. Electric kettle rated at 220 V, 2.2 kW, works for 3 hours. Find he energy consumed and the current drawn. W

16. In a house two 60 W electric bulbs are lighted for 4 hours, and three 100 W bulbs for 5 hours everyday. Calculate the electric energy consumed in 30 days.

17. If the potential difference between the ends of a wire of fixed resistance is doubled, by how much does the electric power increase?

18. A bulb is rated as 250 V; 0.4 A. Find its : (i) power, and (ii) resistance.

19. For a heater rated at 4 kW and 220 V, calculate (a) the current, b) the resistance of the heater, ) the energy consumed in 2 hours, and d) the cost if 1 kWh is priced at Rs. 4.60.

20. An electric motor takes 5 amperes current from a 220 volt supply line. Calculate the power of the motor arid electrical energy consumed by it in 2 hours.

21. Which uses more energy: a 250 W TV set in 1 hour or a 1200 W Toaster in 10 minutes?

22. An electric bulb is rated as 10 W, 220 V. How many of these bulbs can be connected in parallel across the two wires of 220 V supply line if the maximum current which can be drawn is 5 A.

23. How much work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts?

24. What possible values of resultant resistance one can get by combining two resistances, one of value 2 ohm and the other 6 ohm?

25. If 3 resistances of 3 ohm each are connected in parallel, what will be their total resistance?

26. If five resistances, each of value 0.2 ohm, are connected in series, what will be the resultant resistance?

27. Four resistances of 16 ohms each are connected in parallel. Four such combinations are connected in series. What is the total resistance?

28. An electric bulb of resistance 20 and a resistance wire of 4 are connected in series with a 6 V battery. Draw the circuit diagram and calculate (i)the total resistance of the circuit.(ii) Current through the circuit. (iii)Potential difference across the electric bulb.(iv)Potential difference across the resistance wire.

29. How will you connect three resistors of 2 ohm, 3 ohm and 5 ohm respectively so as to obtain a resultant resistance of 2.5 ohm? Draw the diagram to show the arrangement.

30. How will you connect three resistors of resistances 2ohm, 3 ohm and 6 ohm to obtain a total resistance of: (a) 4 ohm, and (b) 1 ohm?

31. A wire of resistance R is cut into five equal pieces. These five pieces of wire are then connected in parallel. If the resultant resistance of this combination be R then the ratio of resultant to the original will be?

32. A copper wire has a diameter of 0.5 mm and resistivity of 1.6 x 10 m.
1. What will be the length of this wire to make its resistance 10 2ohm?
2. How much does the resistance change if the diameter is doubled?

33. An electric heater which is connected to a 220 V supply line has two resistance coils A and B of 24 resistances each. These coils can be used separately (one at a time), in series or in parallel. Calculate the current drawn when
1. Only one coil A is used.
2. Coils A and B are used in series.
3. Coils A and B are used in parallel.

34. If the length of a wire is doubled by taking more of wire, what happens to its resistance?

35. How does the resistance of a wire change when
1. Its length is tripled?
2. Its diameter is tripled?
3. Its material is changed to one whose resistivity is three times?

36. How much energy is given to each coulomb of charge passing through a 6 V battery?

37. The potential difference between the terminals of an electric iron is 240 V and the current is 5.0 A. What is the resistance of the electric iron?

38. A potential difference of 20 volts is applied across the ends of a resistance of 5 ohms. What current will flow in the resistance?

39. A resistance of 20 ohms has a current of 2 amperes flowing in it. What potential difference is there between its ends?

40. A current of 5 amperes flows through a wire whose ends are at a potential difference of 3 volts. Calculate the resistance of the wire.

41. The resistance of an electric lamp filament is 230 ohms. The lamp is switched on when the line voltage is 115 volts. What is the current in the lamp circuit?

42. What is the potential difference between the ends of a conductor of 16 ohm resistance, when a current of 1.5 A flows through it?

43. Calculate the work done in moving a charge of 4 coulombs from a point at 220 volts to another point at 230 volts.

44. What is the potential difference between the terminals of a battery if 250 joules of work is required to transfer 20 coulombs of charge from one terminal of the battery to the other?

45. How much work is done in moving a charge of 2 C across two points having a potential difference of 12 V?

46. An electric bulb draws a current of 0.25 A for 20 minutes. Calculate the amount of electric charge that flows through the circuit.

47. A radio set draws a current of 0.36 A for 15 minutes. Calculate the amount of electric charge that flows through the circuit.

48. Potential difference between two points of a wire carrying 2 ampere current is 0.1 volt. Calculate the resistance between these points.

49. A simple electric circuit has a 24 V battery and a resistor of 60 ohms. What will be the current in the circuit? The resistance of the connecting wires is negligible.

50. A wire of resistance R is cut into five equal pieces. These five pieces of wire are then connected in parallel. If the resultant resistance of this combination be R then the ratio of resultant to the original will be?