Current Electricity

In the given figure, the flow of electrons would be :

1. B to A
2. A to B
3. both A to B and B to A
4. none of the above

Answer

A to B

Reason — Conductor A is negatively charged while conductor B is positively charged. Electrons flow from the negatively charged conductor A to positively charged conductor B.

Current is the rate of flow of charge across a cross-section ............... the flow of current.

1. in any direction of
2. in a parallel direction to
3. in the same direction as
4. normal to the direction of

Answer

normal to the direction of

Reason — Current is the rate of flow of charge across a cross-section normal to the direction of the flow of current.

In metals, current flows due to the movement of :

1. protons
2. neutrons
3. electrons
4. electrons and protons

Answer

electrons

Reason — In metals, current flows due to the movement of electrons.

The conventional current is in a direction ............... to the direction of motion of electrons.

1. parallel
2. normal
3. opposite
4. none of the above

Answer

opposite

Reason — Conventionally, the direction of current is taken positive in the direction of flow of positive charge. Therefore, conventionally, the current will be negative in the direction of flow of electrons.

Hence, current is in a direction opposite to the direction of motion of electrons.

Insulators have ............... electrons and conductors have ............... electrons.

1. no free, large number of free
2. large number of free, no free
3. no free, small number of free
4. small number of free, no free

Answer

no free, large number of free

Reason — Insulators have no free electrons and offer very high resistance and conductors have large number of free electrons and they offer very small resistance to the flow of electrons.

An insulator of electricity is:

1. copper
2. acidulated water
3. human body
4. silk

Answer

silk

Reason — Silk does not conduct electricity, hence is an insulator.

Select the insulator among the following :

1. Pure water
2. Human body
3. Aluminium
4. Mercury

Answer

Pure water

Reason — Pure water is a good insulator as it has a low concentration of ions so it does not conduct electricity very well.

On the other hand, the human body, aluminum, and mercury are not good insulators because

• The human body contains electrolytes and can conduct electricity.
• Aluminum is a metal and is highly conductive.
• Mercury is a liquid metal and also conducts electricity well.

A conductor having an excess of electrons is said to be at ............... potential while a conductor having a deficit of electrons is at ............... potential.

1. lower, lower
2. lower, higher
3. higher, lower
4. higher, higher

Answer

lower, higher

Reason — A conductor having an excess of electrons is said to be at negative (lower) potential while a conductor having a deficit of electrons is at positive (higher) potential.

The force between the two charges is ............... when they are at an infinite separation.

1. finite
2. infinite
3. zero
4. none of the above

Answer

zero

Reason — The force between the two charges is zero when they are at an infinite separation.

The unit of potential difference is:

1. ampere
2. volt
3. ohm
4. coulomb

Answer

volt

Reason — Potential difference is measured in joule per coulomb which is named as volt (V).

The S.I. unit of potential is :

1. Joule/coulomb
2. Volt
3. Ampere
4. Both joule/coulomb and volt

Answer

Both joule/coulomb and volt

Reason — The S.I. unit of potential is joule/coulomb or volt (symbol V).

On increasing the resistance in a circuit, the current in it:

1. decreases
2. increases
3. remains unchanged
4. nothing can be said.

Answer

decreases

Reason — Current (I) is inversely proportional to the resistance (R) hence it decreases.

A larger wire offers ............... resistance than a shorter wire.

1. less
2. equal
3. more
4. none of the above

Answer

more

Reason — The resistance of a wire increases with its length because the moving electrons experience more collisions when traveling a longer distance.

Hence, resistance of a wire is directly proportional to the length of the wire.

A thicker wire offers ............... resistance than a thin wire.

1. less
2. equal
3. more
4. none of the above

Answer

less

Reason — In a thick wire, electrons get a larger area of cross section to flow as compared to a thin wire, therefore, a thick wire offers less resistance (i.e., resistance of wire is inversely proportional to the area of cross section of the wire).

Upon increasing the temperature, the resistance of a wire :

1. decreases
2. does not change
3. increases
4. first increases then decreases

Answer

increases

Reason — If the temperature of the wire increases, ions in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of the wire increases with the increase in it's temperature).

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.

1. 2 Ω
2. 0 Ω
3. 1 Ω
4. 4 Ω

Answer

1 Ω

Reason — Given,

• I = 2A
• V = 2 volt

From Ohms's law,

$\text R = \dfrac{\text{V}}{\text{I}} \\[1em] = \dfrac{2}{2} \\[1em] = 1\ \text Ω$

Hence, R = 1 Ω

While performing an experiment, a student reduces the resistance to one half keeping the potential difference same. New reading of the current will be :

1. half
2. double
3. four times
4. one fourth

Answer

double

Reason — Let initial and final current be I and I_new, initial and final resistance is R and R_new and initial and final potential be V and V_new

From Ohm's law,

Case 1 :

$\text V = \text {IR}$

Case 2 :

$\text V_{\text {new}} = \text I_{\text {new}}\text R_{\text {new}}$

According to question,

$\text V_{\text {new}} = \text V\ \text {and}\ \text { R}_{\text {new}}=\dfrac {1}{2}\text R$

Then

$\Rightarrow \text I_{\text {new}}\text R_{\text {new}} = \text {IR} \\[1 em] \Rightarrow \text I_{\text {new}}\times \dfrac{1}{2}\text R = \text {IR} \\[1 em] \Rightarrow \dfrac{\text I_{\text {new}}}{2} = \text I \\[1 em] \Rightarrow \text I_{\text {new}} = 2\text I$

Hence, the current becomes double.

What is the net charge on an atom ?

Answer

The net charge on an atom is zero i.e. it is electrically neutral as the number of protons is equal to the number of electrons.

What are free electrons ?

Answer

The electrons in the orbits which are far from the nucleus are free to move and can easily leave their atoms. These electrons are called free electrons.

Name three constituents of an atom and state the nature of charge on each of them.

Answer

The three constituents of an atom are electrons, protons and neutrons. The electrons are negatively charged, protons are positively charged and neutrons have no charge (i.e., they are neutral).

'The charge on a body is 1.5 e'. Comment on this statement.

Answer

The statement is incorrect.

Electric charge is quantised, which means the charge on a body can exist only as an integral multiple of the elementary charge (e).

q = ±ne

where (n = 1, 2, 3, ......)

So, a body can have charge like (+e), (-2e), (+3e), etc., but not (1.5e), because 1.5 is not an integer.

Hence, a charge of (1.5e) is not possible on a body.

How much is the charge on an electron?

Answer

The charge on an electron is -1.6 x 10^-19 coulomb.

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

Answer

The conductors of electricity are — copper wire, acidulated water, human body and mercury.

What is the other name for the unit :

(a) Joule per Coulomb

(b) Coulomb per second

Answer

(a) Volt (V)

(b) Ampere (A)

For what purpose is a battery connected in a circuit?

Answer

A battery is connected in a circuit when a strong current is needed.

How will the resistance of a wire change if it is stretched ?

Answer

The resistance of a wire will increase when it is stretched due to increase in length and decrease in area of cross section.

How does current change with voltage when it is altered across a given resistance ?

Answer

On increasing the voltage, the current increases and vice versa because voltage (potential difference) is directly proportional to current.

Name the physical quantities whose units are —

(a) Coulomb and ampere

(b) Volt and ohm.

Answer

The physical quantities are —

(a) Charge and Current

(b) Potential difference and Resistance

How is the direction of flow of current between two charged conductors determined by their potentials?

Answer

The direction of flow of current is from a body at a higher potential to the one at a lower potential i.e., in direction opposite to the direction of flow of electrons.

Define the term potential difference.

Answer

Potential difference between two conductors is equal to the work done in transferring a unit positive charge from one conductor to the other conductor.

If work W is done in transferring a test charge q from one conductor to the other, the potential difference between them is

$\text V_1 - \text V_2 = \dfrac{\text W}{\text q}$

Potential difference is a scalar quantity.

State and define the S.I. unit of potential difference.

Answer

The S.I. unit of potential difference is volt (V) or joule per coulomb.

The potential difference between two points is said to be 1 volt if work done in transferring 1 coulomb of charge from one point to the other point is 1 joule.

State and define the S.I. unit of resistance.

Answer

S.I. unit of resistance is ohm (Ω) or volt per ampere.

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 it's ends is 1 volt.

State Ohm's law.

Answer

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

I ∝ V

or

V ∝ I

or

V = IR

where

• R is the resistance of the conductor.

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

Answer

V = IR

State whether the resistance of filament of a bulb will decrease, remain unchanged or increase when it glows.

Answer

The resistance of the filament increases, when a bulb glows as the temperature of the filament increases.

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.

Answer

Current in conductor $\bold {I = \dfrac{Q}{t} = \dfrac{ne}{t}}$.

What do you mean by quantum nature of charge ?

Answer

The quantum nature of charge means that charge cannot exist in any random amount. It is always found in fixed amounts.

The charge on a body is always an integral multiple of the charge of one electron.

q = ±ne

where (n = 1, 2, 3, 4, ......) and
e = charge on an electron = 1.6 × 10^-19 C.

So, a body can have charge like (+e), (-e), (+2e), (-3e) etc., but it cannot have charge like (1.5e) or (0.9e).

Thus, charge is said to be quantised.

What do you understand by the term resistance?

Answer

The obstruction offered to the flow of current by a conductor is called its electrical resistance.

How is the current flowing in a conductor changed if the resistance of conductor is doubled keeping the potential difference across it the same?

Answer

According to Ohm's law,

$\text{V} = \text{I}\text{R} \\[1em] \Rightarrow \text{I} = \dfrac{\text V}{\text R}$

Let the doubled resistance be 2R. Potential difference remains the same

$\text{I}_{\text{new}} = \dfrac{\text V}{2\text R} \\[1em] \Rightarrow \text{I}_{\text{new}} = \dfrac{\text I}{2}$

∴ If the resistance of conductor is doubled keeping the potential difference across it the same then current flowing in a conductor will be halved.

Figure below 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.

Answer

(a) The direction of flow of electrons is from A to B (i.e., from a low potential to a high potential)

(b) The direction of flow of current when both the conductors are joined by a metal wire is from B to A. (i.e., from a high potential to a low potential)

How is the resistance of a wire affected if its (a) length is doubled, (b) radius is doubled?

Answer

(a) If the length is doubled, the resistance of the wire doubles as resistance of a wire is directly proportional to the length of the wire.

(b) The resistance of a wire becomes one-fourth when the radius of the wire is doubled as resistance of a wire is inversely proportional to the area of cross section of the wire.

'The potential difference between two conductors is 1 volt'. Explain the meaning of this statement.

Answer

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

'The resistance of a wire is 1 ohm'. Explain the meaning of this statement.

Answer

'The resistance of wire is 1 ohm' means that current of 1 ampere flows through it when the potential difference across its ends is 1 volt.

What is the S.I. unit of charge ?

Answer

The S.I. unit of charge is the coulomb (C).

One coulomb is defined as the amount of charge transferred by a current of one ampere in one second.

State the charge on a proton in coulomb.

Answer

The charge on a proton is +1.6 × 10⁻¹⁹ coulomb.

State two differences between a conductor and an insulator of electricity.

Answer

The differences between a conductor and an insulator of electricity are as follows —

Explain the concept of electric potential difference in terms of work done in transferring the charge.

Answer

We know that, like charges repel and unlike charges attract, therefore to create an excess or deficit of electrons at a point, some work is to be done in moving the charges (or electrons) against the forces between them. The force between the two charges is zero when they are at infinite separation.

Hence, quantitatively, potential at a point is measured in terms of work done in bringing a charge q from infinity to that point. If work W^' is done in bringing a charge q from infinity to a point, then potential at that point is:

$\text V = \dfrac{\text W'}{\text q}$

Hence, the potential difference between two conductors is measured in terms of work done in transferring the charge from one conductor to the other, through a metallic wire.

Potential difference is a scalar quantity.

Explain why does a metal wire when connected to a cell offer resistance to the flow of current.

Answer

A metal wire has free electrons which move in a random manner in the absence of any cell connected across it.

When the ends of the wire are connected to a cell, the electrons start moving from the negative terminal of cell to it's positive terminal through the metal wire.

During their movement, they collide with the fixed positive ions and other free electrons of the wire due to which their speed decreases and direction of motion changes.

After each collision, they again accelerate towards the positive terminal and suffer collision with other positive ions and free electrons again. This process continues.

As a result, the electrons do not move in bulk with increasing speeds from one end to the other but they drift towards the positive terminal. This is how a wire offers resistance to the flow of electrons (or current) through it.

State three factors on which the resistance of a wire depends. Explain how does the resistance depend on the factors stated by you.

Answer

The factors on which the resistance of a wire depend are —

1. The length of the wire — The number of collisions suffered by the moving electrons will be more if they have to travel a longer distance in a wire, therefore a long wire offers more resistance than a short wire (i.e., resistance of a wire is directly proportional to the length of the wire).
2. The area of cross section of wire — In a thick wire, electrons get a larger area of cross section to flow as compared to a thin wire, therefore, a thick wire offers less resistance (i.e., resistance of wire is inversely proportional to the area of cross section of the wire).
3. The temperature of the wire — If the temperature of the wire increases, ions in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of the wire increases with the increase in it's temperature.

A charge 0.5 C passes through a cross section of a conductor in 5 s. Find the current.

Answer

Given,

• Charge (Q) = 0.5 C
• Time (t) = 5 s
• Current (I) = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$\text I = \dfrac{0.5}{5} \\[1em] = 0.1 \text { A}$

Hence, current = 0.1 A

A current of 1.5 A flows through a conductor for 2.0 s. What amount of charge passes through the conductor?

Answer

Given,

• Current (I) = 1.5 A
• Time (t) = 2 s
• Charge (Q) = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$1.5 = \dfrac{\text Q}{2} \\[1em] \Rightarrow \text Q = 1.5 \times 2 \\[1em] = 3 \text { C}$

Hence, the amount of charge that passes through the conductor is 3 coulombs.

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.

Answer

Given,

• Time = 0.8 s
• Charge = 24 C
• Current = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$\text I = \dfrac{24}{0.8} \\[1em] = 30 \text { A}$

Hence, the current in the coil is 30 A.

In transferring 1.5 C charge through a wire, 9 J of work is done. Find the potential difference across the wire.

Answer

Given,

• Charge (q) = 1.5 C
• Work = 9 J

Potential difference across the wire is given by,

$\text V = \dfrac{\text W}{\text q}$

Substituting the values in the formula, we get,

$\text V = \dfrac{9}{1.5} \\[1em] = 6\ \text V$

Hence, the potential difference across the wire is 6 volts.

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.

Answer

Given,

• V = 12 V
• R = 24Ω
• I = ?

From Ohm's Law,

$\text V = \text {IR}$

Substituting the values in the formula, we get,

$12 = \text I \times 24 \\[1em] \Rightarrow \text I = \dfrac{12}{24} \\[1em] = 0.5\ \text A$

Hence, the current drawn from the cell is 0.5 amperes.

A bulb draws current 1.5 A at 6.0 V. Find the resistance of the filament of bulb while glowing.

Answer

Given,

• I = 1.5 A
• V = 6 V
• R = ?

From Ohm's Law,

$\text V = \text {IR}$

Substituting the values in the formula, we get,

$6 = 1.5 \times \text R \\[1em] \Rightarrow \text R = \dfrac{6}{1.5} \\[1em] = 4\ \text Ω$

Hence, the resistance of filament of bulb while glowing is 4 Ω.

A current 0.2 A flows in a wire of resistance 15 Ω. Find the potential difference across the ends of the wire.

Answer

Given,

• I = 0.2 A
• R = 15 Ω
• V = ?

From Ohm's Law,

$\text V = \text {IR}$

Substituting the values in the formula, we get,

V = 0.2 x 15 = 3 V

Hence, the potential difference across the ends of the wire is 3 V.

A cell is used to :

1. measure current in a circuit
2. provide current in a circuit
3. limit current in a circuit
4. prevent current in a circuit

Answer

provide current in a circuit

Reason — The cell or a battery is a source of direct current (d.c.) which provides current as a result of irreversible reaction.

During charging of a secondary cell, the ............... energy changes into ............... energy and gets stored in the cell.

1. mechanical, chemical
2. chemical, electrical
3. mechanical, electrical
4. electrical, chemical

Answer

electrical, chemical

Reason — During charging of a secondary cell, the electrical energy changes into chemical energy and gets stored in the cell.

Primary cells provide current as a result of ............... chemical reaction and secondary cells provide current as a result of ............... reaction.

1. reversible, reversible
2. reversible, irreversible
3. irreversible, reversible
4. irreversible, irreversible

Answer

irreversible, reversible

Reason — Primary cells provide current as a result of irreversible chemical reaction and secondary cells provide current as a result of reversible reaction.

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.

1. (i)
2. (ii)
3. both (i) and (ii)
4. none of the above

Answer

none of the above

Reason — As :

• Primary cell is capable of giving a weak current only and secondary cells provides a low as well as high current.
• The internal resistance of primary cells is high and that of secondary cells is low.

So, none of the above statement is correct.

A battery is correctly represented by :

Answer

Reason — A battery is formed by joining a number of cells together in series (i.e., cathode of one cell connected to the anode of another cell).

A ............... is a device by which resistance in a circuit can be varied continuously.

1. key
2. switch
3. rheostat
4. voltmeter

Answer

rheostat

Reason — A rheostat is a device by which resistance in a circuit can be varied continuously.

It is used to adjust the magnitude of current in a circuit by changing the length of the resistance wire included in the circuit.

Rheostat is used in a circuit to provide :

1. fixed current
2. fixed voltage
3. fixed resistance
4. variable resistance

Answer

variable resistance

Reason — A rheostat is a device by which resistance in a circuit can be varied continuously. It is used to adjust the magnitude of current in a circuit by changing the length of the resistance wire included in the circuit.

An instrument used to measure the magnitude of current flowing in a circuit is :

1. voltmeter
2. galvanometer
3. ammeter
4. rheostat

Answer

ammeter

Reason — An ammeter is an instrument used to measure the magnitude of current flowing in a circuit.

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.

1. ammeter
2. galvanometer
3. voltmeter
4. rheostat

Answer

galvanometer

Reason — A galvanometer is used to either detect the presence of a weak current or to only know the direction of flow of current in a circuit. It does not measure the magnitude of current in a circuit.

The presence of ............... in the path makes a circuit incomplete and current does not flow.

1. conductor
2. ammeter
3. voltmeter
4. insulator

Answer

insulator

Reason — The presence of insulator in the path makes a circuit incomplete and current does not flow.

Connecting wires are made of substances that have :

1. highly conducting material
2. negligible resistance
3. free electrons in a large number
4. all of these

Answer

all of these

Reason — Connecting wires are typically made of substances, such as copper or aluminum, that possess highly conducting materials, negligible resistance, and a large number of free electrons. These characteristics allow for efficient flow of electric current through the wires.

Out of the following circuits shown, the bulb glows in which one?

Answer

option 4

Reason — In circuit 4, the key is closed, completing the circuit. In the other circuits the key (K) is left open, so the circuits are incomplete. For an electric circuit to function, every component must conduct current. If the circuit is broken at any point, the circuit remains open and no current flows.

Name one d.c. source and one a.c. source.

Answer

The cell or a battery is a source of direct current (d.c.).

The mains in our house or an a.c. generator are the source of alternating current (a.c.).

What is an electric cell?

Answer

An electric cell is a device that converts chemical energy into electrical energy when it sends current in a circuit. It consists of a vessel containing two conducting rods, called the electrodes, at some separation, placed or immersed in a solution (or jelly), called the electrolyte.

What transformation of energy takes place when current is drawn from a cell?

Answer

When current is drawn from a cell, chemical energy changes to electrical energy.

Name the constituents of a cell.

Answer

A cell consists of two electrodes and an electrolyte placed in a vessel.

State the two kinds of cell. Give one example of each.

Answer

The two kinds of cells with examples are —

1. Primary cell : Example — Daniel cell
2. Secondary cells or accumulators : Example — Ni-Fe (or alkali) accumulator

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

Answer

Rheostat.

What is the function of a key (or switch) in an electric circuit?

Answer

A key is used to put the current on or off in a circuit.

What is a primary cell? Name two such cells.

Answer

Primary cell — These cells provide current as a result of irreversible chemical reaction. The cells are discarded after use when the entire chemical energy in them has been converted into electrical energy. Thus, these are 'use and throw' types of cells and cannot be recharged.

Depending upon the material of electrodes and electrolyte, we have different types of primary cells e.g., simple voltaic cell, dry cell, etc.

What is a secondary cell? Name one such cell.

Answer

Secondary cells or accumulators also provide current as a result of a chemical reaction. In these cells, the chemical reaction is reversible and so they can be recharged after use. e.g., Lead accumulator.

What are conductors and insulators of electricity? Give two examples of each.

Answer

Conductors — The substances which allow the current to flow through them easily are called conductors. They have large number of free electrons and they offer a very small resistance to the flow of current.

Examples — impure water (or acidulated water) and mercury are conductors of electricity.

Insulators — The substances which do not allow current to flow through them are called insulators. They have almost no electrons and offer a very high resistance to the flow of current.

Example — cotton, rubber.

Write the condition required for a circuit to be a closed circuit.

Answer

For an electric circuit to be closed, each component of it must pass current through it, i.e., it should be conducting

How will you obtain a source of large direct current ? Show it with the help of a diagram.

Answer

When a strong direct current is needed, either we join a number of cells together in series (cathode of one cell connected to the anode of another cell), to form a battery or we use a storage cell.

Which material is used to make wires for a resistance box ? Why is this particular material used ?

Answer

Manganin is an alloy which is used for preparing wires in a resistance box. It is used because of its property that its resistance does not vary much with temperature.

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.

Answer

A galvanometer is used to know the direction of flow of current in a circuit. Current can enter or leave through any of its terminals, therefore it has just an 'O' mark at the middle and no +/- signs at its terminals.

On the other hand, Ammeter and Voltmeter measure the current and voltage in the circuit, respectively. Therefore, the + sign marked at the left hand corner indicates that current must enter through this terminal.

What is meant by load in an electrical circuit ? Give some examples.

Answer

An appliance which is connected in a circuit is called a load. It may be just a resistance or a combinations of different electrical components. Examples : bulb, refrigerator, microwave etc.

Distinguish between d.c. and a.c.

Answer

The differences between d.c. and a.c are as follows —

Distinguish between a closed circuit and an open circuit, with the use of suitable labelled diagrams.

Answer

The differences are —

The diagram below shows closed and open circuits:

State three differences between primary and secondary cells.

Answer

The differences between a primary and a secondary cell are —

Write symbols and state functions of each of the following components in an electric circuit —

(i) key,

(ii) cell,

(iii) rheostat,

(iv) ammeter, and

(v) voltmeter.

Answer

The symbol and function of the components are as follows —

(i) Key

Symbol:

Function:
A key is used to put the current on and off in a circuit.

(ii) Cell

Symbol:

Function:
A cell acts as a source of direct current for the circuit.

(iii) Rheostat

Symbol:

Function:
It controls the flow of current in a circuit.

(iv) Ammeter

Symbol:

Function:
An ammeter is an instrument used to measures the magnitude of current flowing in a circuit.

(v) Voltmeter

Symbol:

Function:
A voltmeter is used to measure the potential difference between two points of a circuit.

In the electric circuit shown in below figure, 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.

Answer

The diagram below shows the direction of flow of current marked by red arrows and the labelled parts — A, B, C, D, E, and F

The name and function of each part is as follows —

A → Ammeter.

An ammeter is an instrument used to measure the magnitude of current flowing in a circuit.

B → Cell.

It acts as a source of direct current for the circuit.

C → key.

It is used to put the current on and off in the circuit.

D → Load.

An appliance which is connected in a circuit. It may be a resistance (e.g., bulb, heater etc.) or a combination of different electrical components.

E → Voltmeter.

A voltmeter is used to measure the potential difference between two points of a circuit.

F → Rheostat.

A rheostat is a device by which resistance in a circuit can be varied continuously.

(a) Complete the circuit given in below figure 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.

Answer

(a) The circuit with ammeter between the terminals A and C with polarity marked is shown below:

(b) The direction of flow of current in the completed circuit with ammeter (polarity marked) is shown below:

(c) Rh is the rheostat. It controls the flow of current in a circuit.

Assertion (A) : Primary cells are use and throw types.

Reason (R) : Primary cells provide current as a result of irreversible chemical reaction.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because primary cells (like dry cells, alkaline batteries) are non-rechargeable. Once their chemical reaction is completed, they cannot be reused, so they are discarded after use.

Reason (R) is true because in primary cells, the chemical reaction is irreversible, which means the chemicals cannot be restored to their original form by passing a current therefore, the cell cannot be recharged. Hence, reason correctly explains assertion.

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because electronic current refers to the actual flow of electrons, which move from the negative terminal to the positive terminal of a power source and the rate at which electrons flow defines the magnitude of current.

Reason (R) is true because conventional current is defined as the flow of positive charge, which is taken to be from positive to negative terminal of a cell opposite to the direction of electron flow.

The reason correctly explains the difference between electronic current (actual flow of electrons) and conventional current (assumed flow of positive charge).

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Assertion is false but reason is true

Explanation

Assertion (A) is false because an ammeter is used to measure current, not potential difference and to measure potential difference, voltmeter is used.

Reason (R) is true because an ammeter is connected in series in a circuit and to not affect the current it is measuring, its resistance should be as low as possible.

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Assertion is true but reason is false

Explanation

Assertion (A) is true because both cotton and rubber do not conduct electricity under normal conditions since they have almost no free electrons and offer a very high resistance to the flow of current so they are commonly used as insulating materials in electrical applications.

Reason (R) is false because they have almost no free electrons and offer a very high resistance to the flow of current.

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Assertion is true but reason is false

Explanation

Assertion (A) is true because in a thick wire, electrons get a larger area of cross-section to flow as compared to a thin wire, therefore, a thick wire offers less resistance.

Reason (R) is false because resistance is inversely proportional to the area of cross-section, not directly proportional since flow of electrons (i.e., current) depends upon the area of cross-section through which they flow so more area implies high current as number of collisions between electrons decreases and vice versa.

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because conventional current is defined to flow from higher potential to lower potential, even though actual charge carriers (electrons) move in the opposite direction.

Reason (R) is true because electrons are negatively charged, so they move from lower potential to higher potential opposite to the direction of conventional current.

The reason correctly explains why current flows from high to low potential because electrons, which actually move, go the opposite way.

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.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is false but reason is true
4. Assertion is true but reason is false

Answer

Both A and R are true and R is not the correct explanation of A

Explanation

Assertion (A) is true because the number of collisions suffered by the moving electrons will be more if they have to travel a longer distance in a wire, therefore, a long wire offers more resistance than a short wire (i.e., resistance of a wire ∝ length of wire).

Reason (R) is true because if the temperature of wire increases, electrons in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of a wire increases with the increase in its temperature).

The assertion talks about wire length, but the reason explains the effect of temperature. So, although both statements are true, the reason does not explain the assertion.

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.

(b) Does a higher current mean that electrons move faster from the battery to the bulb?

(c) If current flows for a longer time, what happens to the charge passing per second? What happens to the total charge transferred?

Answer

(a) The thicker wire will have a greater rate of flow of charge (higher current) because a thicker wire has a larger cross-sectional area, offering less resistance, so more charge can pass per second.

(b) No, not necessarily.
Current is the amount of charge flowing per second:

$I = \dfrac{Q}{t}$

So, a higher current means that more charge passes through the wire per second. It does not simply mean that electrons travel faster from the battery to the bulb.

(c) Charge flowing through a wire is given by,

$\text {Charge} = \text {Current flowing} \times \text {Time taken}$

So if current flows for a longer time, the charge passing per second (current) remains the same (assuming constant conditions), however, the total charge transferred increases with time.

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?

(b) What is meant by a closed circuit?

(c) Why is electric current considered as a flow of charges?

(d) What is the conventional direction of electric current?

Answer

(a) The bulb did not glow because the circuit was incomplete (open circuit) due to the loose wire, so even though the bulb was fine, current could not flow without a continuous conducting path.

(b) A closed circuit is a complete and continuous path through which electric current can flow from the cell, through the components and back to the cell.

(c) Electric current is considered as a flow of charges because it is defined as the rate of flow of electric charge. In conductors, this charge is carried by electrons.

(d) The conventional direction of electric current is from the positive terminal to the negative terminal of a cell, opposite to the actual flow of electrons.

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?

(b) What does potential difference represent?

(c) How does resistance affect electric current?

(d) Between a thin wire and a thick wire, which has greater resistance? Give reason.

Answer

(a) The bulb connected to the higher voltage source glows brighter because a higher potential difference drives more current through the bulb. As a result, more electrical energy is converted into light and heat per second, so the bulb glows more brightly.

(b) Potential difference represents the work done per unit charge in moving a charge between two points. It indicates how much energy is supplied to the charges to make them flow in a circuit.

(c) Resistance opposes the flow of electric current. According to Ohm’s law, for a given voltage, higher resistance results in lower current, while lower resistance allows more current to flow.

(d) A thin wire has greater resistance because it has a smaller cross-sectional area, which offers more opposition to the flow of electric current than a thick wire.