Current Electricity - Questions & Answers

EXERCISE-9(A)

(A) MULTIPLE CHOICE TYPE :
(Choose the correct answer from the options given below).

1. In the given figure, the flow of electrons would be from :
(a) B to A
(b) A to B
(c) both A to B and B to A
(d) none of the above
Answer: (b) A to B

2. Current is the rate of flow of charge across a cross-section ............ the flow of current.
(a) in any direction of
(b) in a parallel direction to
(c) in the same direction as
(d) normal to the direction of
Current is the rate of flow of charge across a cross-section (d) normal to the direction of the flow of current.

3. In metals, current flows due to the movement of :
(a) protons
(b) neutrons
(c) electrons
(d) electrons and protons
Answer: (c) electrons

4. The conventional current is in a direction ............ to the direction of motion of electrons.
(a) parallel
(b) normal
(c) opposite
(d) none of the above
The conventional current is in a direction (c) opposite to the direction of motion of electrons.

5. Insulators have ............ electrons and conductors have ............ electrons.
(a) no free, large number of free
(b) large number of free, no free
(c) no free, small number of free
(d) small number of free, no free
Insulators have (a) no free electrons and conductors have large number of free electrons.

6. An insulator of electricity is :
(a) copper
(b) acidulated water
(c) human body
(d) silk
Answer: (d) silk

7. Select the insulator among the following :
(a) Pure water
(b) Human body
(c) Aluminium
(d) Mercury
Answer: (a) Pure water

8. A conductor having an excess of electrons is said to be at ............ potential while a conductor having a deficit of electrons is at ............ potential.
(a) lower, lower
(b) lower, higher
(c) higher, lower
(d) higher, higher
A conductor having an excess of electrons is said to be at (b) lower potential while a conductor having a deficit of electrons is at higher potential.

9. The force between the two charges is ............, when they are at an infinite separation.
(a) finite
(b) infinite
(c) zero
(d) none of the above
The force between the two charges is (c) zero, when they are at an infinite separation.

10. The unit of potential difference is :
(a) ampere
(b) volt
(c) ohm
(d) coulomb.
Answer: (b) volt

11. The S.I. unit of potential is :
(a) Joule/coulomb
(b) Volt
(c) Ampere
(d) Both (a) and (b)
Answer: (d) Both (a) and (b)

12. On increasing the resistance in a circuit, the current in it :
(a) decreases
(b) increases
(c) remains unchanged
(d) nothing can be said.
Answer: (a) decreases

13. A larger wire offers ............ resistance than a shorter wire.
(a) less
(b) equal
(c) more
(d) none of the above
A larger wire offers (c) more resistance than a shorter wire.

14. A thicker wire offers ............ resistance than a thin wire :
(a) less
(b) equal
(c) more
(d) none of the above
A thicker wire offers (a) less resistance than a thin wire.

15. Upon increasing the temperature, the resistance of a wire :
(a) decreases
(b) does not change
(c) increases
(d) first increases then decreases
Answer: (c) increases

16. The resistance of a conductor would be ........ if a current of 2 A flows through it when the potential difference across its ends is 2 volt.
(a) 2 Ω
(b) 0 Ω
(c) 1 Ω
(d) 4 Ω
The resistance of a conductor would be (c) 1 Ω if a current of 2 A flows through it when the potential difference across its ends is 2 volt.

17. While performing an experiment, a student reduces the resistance to one half keeping the potential difference same. New reading of the current will be :
(a) half
(b) double
(c) four times
(d) one fourth
Answer: (b) double

(B) VERY SHORT ANSWER TYPE :

1. What is the net charge on an atom ?
Zero. (An atom is electrically neutral).

2. What are free electrons ?
The weakly bound electrons present in the outermost orbit of an atom which can detach and move freely through a substance are called free electrons.

3. Name three constituents of an atom and state the nature of charge on each of them.
Electrons (negative charge), Protons (positive charge), and Neutrons (no charge).

4. 'The charge on a body is 1.5 e'. Comment on this statement.
This statement is incorrect because according to the quantum nature of charge, charge is quantized and always exists as an integral multiple of the charge on an electron (e). It can never be a fraction like 1.5 e.

5. How much is the charge on an electron ?
-1.6 x 10^-19 Coulomb.

6. Select conductors of electricity from the following : Copper wire, silk thread, pure water, acidulated water, human body, glass, mercury.
Copper wire, acidulated water, human body, mercury.

7. What is the other name for the unit :
(a) Joule per Coulomb
(b) Coulomb per second?
(a) Volt (b) Ampere

8. For what purpose is a battery connected in a circuit ?
A battery is connected in a circuit to act as a source of continuous direct current by maintaining a potential difference across the circuit components.

9. How will the resistance of a wire change if it is stretched ?
Its resistance will increase because stretching the wire increases its length and decreases its cross-sectional area.

10. How does current change with voltage when it is altered across a given resistance ?
Current is directly proportional to the voltage across a given resistance. If voltage increases, the current increases proportionally (Ohm's law).

11. Name the physical quantities whose units are :
(a) Coulomb and ampere
(b) Volt and ohm.
(a) Electric charge and Electric current.
(b) Potential difference and Electrical resistance.

12. How is the direction of flow of current between two charged conductors determined by their potentials ?
Current conventionally flows from the conductor at a higher potential to the conductor at a lower potential.

13. Define the term potential difference.
The potential difference between two conductors is defined as the work done in transferring a unit positive charge from one conductor to the other.

14. State and define the S.I. unit of potential difference.
The S.I. unit of potential difference is volt (V). Potential difference between two points is said to be 1 volt if 1 joule of work is done in transferring 1 coulomb of charge from one point to the other.

15. State and define the S.I. unit of resistance.
The S.I. unit of resistance is ohm (Ω). The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when the potential difference across its ends is 1 volt.

16. State Ohm's law.
Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends provided its temperature remains constant.

17. How are the potential difference (V), current (I) and resistance (R) related ?
V = I x R

18. State whether the resistance of filament of a bulb will decrease, remain unchanged or increase when it glows.
Increase (because the temperature of the filament increases when it glows).

19. n electrons flow through a cross section of a conductor in time t. If charge on an electron is e, write an expression for the current in the conductor.
I = ne / t

(C) SHORT ANSWER TYPE :

1. What do you mean by quantum nature of charge ?
The quantum nature of charge means that electric charge always exists as an integral multiple of the fundamental electronic charge (e), i.e., q = ±ne.

2. What do you understand by the term resistance ?
Resistance is the obstruction offered to the flow of current by a conductor.

3. How is the current flowing in a conductor changed if the resistance of conductor is doubled keeping the potential difference across it the same ?
The current will be halved.

4. Fig. 9.9 shows two conductors A and B. Their charges and potentials are given in the diagram. State the direction of (i) flow of electrons, and (ii) flow of current, when both the conductors are joined by a metal wire.
(i) The flow of electrons will be from conductor A to conductor B.
(ii) The flow of conventional current will be from conductor B to conductor A.

5. How is the resistance of a wire affected if its (a) length is doubled, (b) radius is doubled ?
(a) The resistance becomes doubled (twice).
(b) The resistance becomes one-fourth.

6. 'The potential difference between two conductors is 1 volt'. Explain the meaning of this statement.
It means that 1 joule of work is done in transferring 1 coulomb of positive charge from one conductor to the other conductor.

7. 'The resistance of a wire is 1 ohm'. Explain the meaning of this statement.
It means that a current of 1 ampere will flow through the wire when a potential difference of 1 volt is applied across its ends.

8. What is the S.I. unit of charge ?
Coulomb (C).

9. State the charge on a proton in coulomb.
+1.6 x 10^-19 C.

10. State two differences between a conductor and an insulator of electricity.
1. Conductors contain a large number of free electrons, whereas insulators contain almost negligible free electrons.
2. Electric conduction is easily possible in conductors, whereas electric conduction is not possible in insulators.

(D) LONG ANSWER TYPE :

1. Explain the concept of electric potential difference in terms of work done in transferring the charge.
Like charges repel each other, so work must be done against the repulsive force to move charges. The potential difference between two conductors is defined as the work done (W) in transferring a unit positive charge (q) from one conductor to the other. Thus, V = W / q.

2. Explain why does a metal wire when connected to a cell offer resistance to the flow of current.
In a metal wire, free electrons move in a random manner. When connected to a cell, these electrons drift towards the positive terminal. During this movement, they constantly collide with fixed positive ions and other free electrons, causing their speed to decrease and their direction to change. These collisions offer an obstruction to the flow of electrons, which is known as resistance.

3. State three factors on which the resistance of a wire depends. Explain how does the resistance depend on the factors stated by you.
1. Length of the wire: Resistance is directly proportional to length. A longer wire offers more resistance because electrons suffer more collisions.
2. Area of cross-section: Resistance is inversely proportional to the area. A thicker wire offers less resistance because electrons get a larger area to flow.
3. Material of the wire: Different materials have different concentrations of free electrons. Good conductors have a higher concentration of free electrons and offer less resistance.

(E) NUMERICALS :

1. A charge 0.5 C passes through a cross section of a conductor in 5 s. Find the current.
Current (I) = Q / t = 0.5 C / 5 s = 0.1 A.

2. A current of 1.5 A flows through a conductor for 2.0 s. What amount of charge passes through the conductor ?
Charge (Q) = I x t = 1.5 A x 2.0 s = 3.0 C.

3. When starter motor of a car is switched on for 0.8 s, a charge 24 C passes through the coil of the motor. Calculate the current in the coil.
Current (I) = Q / t = 24 C / 0.8 s = 30 A.

4. In transferring 1.5 C charge through a wire, 9 J of work is done. Find the potential difference across the wire.
Potential difference (V) = W / Q = 9 J / 1.5 C = 6 V.

5. A cell of potential difference 12 V is connected to a bulb. The resistance of filament of bulb when it glows, is 24 Ω. Find the current drawn from the cell.
Current (I) = V / R = 12 V / 24 Ω = 0.5 A.

6. A bulb draws current 1.5 A at 6.0 V. Find the resistance of the filament of bulb while glowing.
Resistance (R) = V / I = 6.0 V / 1.5 A = 4.0 Ω.

7. A current 0.2 A flows in a wire of resistance 15 Ω. Find the potential difference across the ends of the wire.
Potential difference (V) = I x R = 0.2 A x 15 Ω = 3.0 V.

EXERCISE-9(B)

(A) MULTIPLE CHOICE TYPE :
(Choose the correct answer from the options given below).

1. A cell is used to :
(a) measure current in a circuit
(b) provide current in a circuit
(c) limit current in a circuit
(d) prevent current in a circuit.
Answer: (b) provide current in a circuit

2. During charging of a secondary cell, the ............ energy changes into ............ energy and gets stored in the cell
(a) mechanical, chemical
(b) chemical, electrical
(c) mechanical, electrical
(d) electrical, chemical
During charging of a secondary cell, the (d) electrical energy changes into chemical energy and gets stored in the cell.

3. Primary cells provide current as a result of ............ chemical reaction and secondary cells provide current as a result of ............ reaction.
(a) reversible, reversible
(b) reversible, irreversible
(c) irreversible, reversible
(d) irreversible, irreversible
Primary cells provide current as a result of (c) irreversible chemical reaction and secondary cells provide current as a result of reversible reaction.

4. Identify the correct statement(s) :
(i) Primary cell is capable of giving low as well as high current whereas secondary cells provide a weak current only.
(ii) The internal resistance of primary cells is low and that of secondary cells is high.
(a) (i)
(b) (ii)
(c) both (i) and (ii)
(d) none of the above
Answer: (d) none of the above

5. A battery is correctly represented by :
(a)
(b)
(c)
(d)
Answer: (d)

6. A ............ is a device by which resistance in a circuit can be varied continuously.
(a) key
(b) switch
(c) rheostat
(d) voltmeter
A (c) rheostat is a device by which resistance in a circuit can be varied continuously.

7. Rheostat is used in a circuit to provide :
(a) fixed current
(b) fixed voltage
(c) fixed resistance
(d) variable resistance
Answer: (d) variable resistance

8. An instrument used to measure the magnitude of current flowing in a circuit is :
(a) voltmeter
(b) galvanometer
(c) ammeter
(d) rheostat
Answer: (c) ammeter

9. A ............ is either used to detect the presence of a weak current or to only know the direction of flow of current in a circuit.
(a) ammeter
(b) galvanometer
(c) Voltmeter
(d) rheostat
A (b) galvanometer is either used to detect the presence of a weak current or to only know the direction of flow of current in a circuit.

10. The presence of ............ in the path makes a circuit incomplete and current does not flow.
(a) conductor
(b) ammeter
(c) voltmeter
(d) insulator
The presence of (d) insulator in the path makes a circuit incomplete and current does not flow.

11. Connecting wires are made of substances that have :
(a) highly conducting material
(b) negligible resistance
(c) free electrons in a large number
(d) all of these
Answer: (d) all of these

12. Out of the following circuits shown, the bulb glows in which one ?
(a)
(b)
(c)
(d)
Answer: (d)

(B) VERY SHORT ANSWER TYPE :

1. Name one d.c. source and one a.c. source.
d.c. source : cell (or battery). a.c. source : mains in our house.

2. What is an electric cell ?
An electric cell is a source of direct current that converts chemical energy into electrical energy when it sends current in a circuit.

3. What transformation of energy takes place when current is drawn from a cell ?
Chemical energy changes into electrical energy.

4. Name the constituents of a cell.
A vessel containing two conducting rods called electrodes, immersed in a solution called the electrolyte.

5. State the two kinds of cell. Give one example of each.
Primary cells (e.g., simple voltaic cell or dry cell) and Secondary cells (e.g., lead accumulator).

6. Name the instrument used to control current in an electric circuit.
Rheostat.

7. What is the function of a key (or switch) in an electric circuit ?
A key is used to put the current on and off in a circuit. It allows you to make the circuit open or closed.

(C) SHORT ANSWER TYPE :

1. What is a primary cell ? Name two such cells.
A primary cell is a 'use and throw' type of cell that provides current as a result of an irreversible chemical reaction. Examples: dry cell, simple voltaic cell.

2. What is a secondary cell ? Name one such cell.
A secondary cell is a cell that provides current as a result of a reversible chemical reaction, meaning it can be recharged after use. Example: lead accumulator.

3. What are conductors and insulators of electricity ? Give two examples of each.
Conductors are materials that allow electric current to pass through them due to the presence of free electrons (e.g., silver, copper). Insulators are materials that do not allow electric current to pass through them as they contain almost negligible free electrons (e.g., wood, glass).

4. Write the condition required for a circuit to be a closed circuit.
For an electric circuit to be a closed circuit, each component of it must pass current through it, meaning it should be conducting and there should be no insulator in the path (the key must be closed).

5. How will you obtain a source of large direct current ? Show it with the help of a diagram.
A source of large direct current can be obtained by joining a number of cells together in series (the cathode of one cell connected to the anode of another) to form a battery.

6. Which material is used to make wires for a resistance box ? Why is this particular material used ?
An alloy like manganin or constantan is used. This material is used because it has a fixed resistance depending on its length and thickness, which acts as a reliable standard resistance that does not alter much with temperature changes.

7. A galvanometer has an 'O' mark at the middle, while a voltmeter and ammeter has the mark at the left hand corner of the dial. Explain.
A galvanometer has a zero mark at the centre because its main function is to detect both the presence and the direction of weak current, deflecting left or right accordingly. Ammeters and voltmeters only measure magnitude in one direction, so they start from zero at the left.

8. What is meant by load in an electrical circuit ? Give some examples.
An appliance which is connected in a circuit to consume electrical energy is called a load. Examples: bulb, heater, resistance.

(D) LONG ANSWER TYPE :

1. Distinguish between d.c. and a.c.
Direct current (d.c.) is a current of constant magnitude that flows in only one direction. Alternating current (a.c.) is a current for which both the magnitude and direction change with time, repeating its value after a fixed period.

2. Distinguish between a closed circuit and an open circuit, with the use of suitable labelled diagrams.
A closed circuit provides a continuous, unbroken conducting path allowing current to flow (switch is turned on). An open circuit has a break in the path (e.g., an open switch or broken wire), preventing the flow of current.

3. State three differences between primary and secondary cells.
1. Chemical reaction is irreversible in primary cells, whereas it is reversible in secondary cells.
2. Primary cells cannot be recharged, whereas secondary cells can be recharged.
3. Primary cells provide a weak current and have high internal resistance, while secondary cells can provide a high current and have low internal resistance.

4. Write symbols and state the functions of each of the following components in an electric circuit : (i) key, (ii) cell, (iii) rheostat, (iv) ammeter, and (v) voltmeter.
(i) Key: Used to turn the current on and off in a circuit.
(ii) Cell: Used as a source of direct current to provide potential difference.
(iii) Rheostat: Used to adjust the magnitude of current in a circuit by continuously varying the resistance.
(iv) Ammeter: Connected in series, it is used to measure the magnitude of current flowing in the circuit.
(v) Voltmeter: Connected in parallel, it is used to measure the potential difference across two points of a circuit.

5. In the electric circuit shown in Fig. 9.25, label the parts A, B, C, D, E, and F. State the function of each part. Show in the diagram the direction of flow of current.
A = Cell (Provides potential difference/energy to the circuit).
B = Switch / Key (Used to make or break the circuit).
C = Ammeter (Measures the magnitude of current in series).
D = Rheostat (Varies resistance to adjust the current).
E = Voltmeter (Measures potential difference across the load in parallel).
F = Load / Resistance wire (Provides standard resistance or consumes energy).
Direction of current flow: From the positive terminal of the cell (A) through the external circuit back to the negative terminal.

6. (a) Complete the circuit given in Fig. 9.26 by inserting between the terminals A and C, an ammeter. (b) In the diagram mark the polarity at the terminals of ammeter and indicate clearly the direction of flow of current in the circuit, when the circuit is complete. (c) Name and state the purpose of Rh in the circuit.
(a) An ammeter should be drawn in series between the terminals A and C.
(b) The positive (+) terminal of the ammeter is marked towards the positive terminal of the cell, and the negative (-) towards the negative terminal of the cell. The direction of current is marked with an arrow originating from the positive terminal of the cell flowing through the circuit to the negative terminal.
(c) Rh represents a Rheostat. Its purpose is to act as a variable resistance to adjust the magnitude of the current flowing through the circuit.

Competency Focussed Questions

(E) ASSERTION - REASON TYPE QUESTIONS :
(Choose the answer based on the codes given below.)
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are True and R is not the correct explanation of A
(c) Assertion is false but reason is true
(d) Assertion is true but reason is false

(i) Assertion (A) : Primary cells are use and throw types.
Reason (R) : Primary cells provide current as a result of irreversible chemical reaction.
Answer: (a)

(ii) Assertion (A) : The rate of flow of electrons in a direction is called electronic current in that direction.
Reason (R) : The conventional current is in a direction opposite to direction of motion of electrons.
Answer: (a)

(iii) Assertion (A) : An ammeter is used to measure potential difference between two points of circuit.
Reason (R) : An ammeter must have a very low resistance.
Answer: (c)

(iv) Assertion (A) : Cotton and rubber are examples of insulators.
Reason (R) : These materials have a large number of free electrons and they offer less resistance to flow of current.
Answer: (d)

(v) Assertion (A) : A thick wire offers less resistance in comparison to a thin wire.
Reason (R) : Resistance of wire is directly proportional to its area of cross-section.
Answer: (d)

(vi) Assertion (A) : Current flows from body at higher potential to a body at lower potential.
Reason (R) : The direction of flow of electrons is from low potential body to a body at a higher potential.
Answer: (a)

(vii) Assertion (A) : Longer wires have greater resistance and the smaller wires have lesser resistance.
Reason (R) : Resistance of a wire increases with increase in temperature.
Answer: (b)

(F) CASE STUDY BASED QUESTIONS :

1. Two copper wires of equal length but different thickness are connected one by one to the same cell and bulb.
(a) In which wire will the rate of flow of the charge be greater? Give reason.
The rate of flow of charge (current) will be greater in the thicker wire. Reason: A thicker wire offers less resistance to the flow of electrons because resistance is inversely proportional to the cross-sectional area.

(b) Does a higher current mean that electrons move faster from the battery to the bulb?
No, a higher current does not mean electrons move faster. It simply means that a greater number of electrons are passing through a given cross-section of the wire per unit time.

(c) If current flows for a longer time, what happens to the charge passing per second? What happens to the total charge transferred?
The charge passing per second (current) remains the same. However, the total charge transferred increases as time increases.

2. A student connects a bulb to a cell using two wires, but the bulb does not glow. On checking, the bulb is found to be in proper condition. On further inspection, a wire is found to be loose.
(a) Why did the bulb not glow even though it was not faulty?
The bulb did not glow because the loose wire created an open (incomplete) circuit, breaking the path and preventing electric current from flowing.

(b) What is meant by a closed circuit?
A closed circuit is a continuous, unbroken conducting path that allows electric current to flow smoothly from the positive terminal to the negative terminal of a source.

(c) Why is electric current considered as a flow of charges?
Electric current is considered a flow of charges because it is essentially the physical movement of free electrons (which carry a negative charge) through a conductor over time.

(d) What is the conventional direction of electric current?
The conventional direction of electric current is considered to be from the higher potential (positive terminal) to the lower potential (negative terminal), which is opposite to the actual flow of electrons.

3. Two identical bulbs are connected to different circuits. One bulb glows brighter than the other as it is connected to a cell of higher voltage.
(a) Why does the bulb connected to the higher voltage source glow brighter?
The bulb connected to the higher voltage source glows brighter because a higher potential difference drives a larger amount of current through the bulb, dissipating more electrical energy into heat and light.

(b) What does potential difference represent?
Potential difference represents the amount of work done in transferring a unit positive charge from one point to another in an electrical circuit.

(c) How does resistance affect electric current?
Resistance acts as an obstruction to electric current. For a given potential difference, a higher resistance results in a lower current flowing through the circuit.

(d) Between a thin wire and a thick wire, which has greater resistance? Give reason.
A thin wire has greater resistance. Reason: Resistance of a wire is inversely proportional to its area of cross-section. The thin wire offers a smaller area for the electrons to flow through, resulting in more collisions and higher resistance.