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?

**Short answer questions (2 marks)**

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.

**Short answer questions (3 marks)**

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.

**Long Answer questions (5 marks)**

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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