Cells, emf, Internal Resistance Important Questions

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Cells, emf and internal resistances are interrelated concepts. A cell has both emf and internal resistance. An ideal cell is supposed to have zero internal resistance. Cells, emf and Internal resistance topic is part of Class 12 Physics Chapter 3 Current Electricity.

Cell: A cell is a device which provides necessary potential difference to an electric circuit to maintain continous flow of electric current in it.

EMF: Emf (Electromotive force) of a cell may be defined as the potential difference between the terminals of the cell when no current is drawn from it.

Internal resistance: Internal resistance of a cell is defined as the opposition offered by electrolytes and electrodes of a cell to the flow of electric current through it.

The formula for emf E is given by,

E = W/q

Where

W = work done
q = charge

The formula of internal resistance is given by,

E - V = Ir

Where

E = emf
V = Terminal potential or Voltage
I = Current
r = internal resistance

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Important Questions on Cells, emf, and Internal Resistance

Ques. Why does emf of a cell depend upon the concentration of electrolyte? (2 Marks)

Ans. Emf of a cell depends on the distance of electrodes, nature of the material of the electrodes and area of electrodes. It is inversely proportional to the concentration of electrolyte. Thus higher the concentration, lower the emf and vice versa.

Ques. Three identical cells, each of emf. 2v and unknown internal resistance are connected in parallel. This combination is connected to a 5ohm resistor. If the terminal voltage across the cell is 1.5volt. What is the internal resistance of each cell .hence define internal resistance of cell? (3 Marks)

Ans. Given

Emf = 2v,
Voltage =1.5v,
Resistance = 5 ohms

Resistance = 5 ohms

r = 50 ohms

Ques. What is internal resistance? What are the factors that affect internal resistance? (2 Marks)

Ans. Internal resistance is the resistance offered by the cell due to the nature of the materials of the cell. The internal resistance is based on factors such as:

Distance between electrodes
Nature of electrode and electrolyte
Area of electrode

Ques. A 5A current passes through an electric circuit for 6 minutes and 3600 J of work is done. Find the emf. (2 Marks)

Ans. Given

Current = 5A
Time = 6 minutes = 360 sec
Work done = 3600J

We have,

Power = Work done/time

Power = 3600/360 = 10W

Power = emf x current

Therefore,

Emf = Power/ current

Emf = 10/5

Emf = 2V

Ques. A current passes through an electric circuit for 10 minutes and 1000 J of work is done. The emf is 20V. Find the current. (3 Marks)

Ans. Given

Time = 10 minutes = 600 sec
Work done = 1000J

We have

Power = Work done/time

Power =1000/600 = 5/3W

Power = emf x current

Therefore

Emf = Power/ current

20 = 5/3x I

I = 5/3 X20

I = 1/12

I = 0.8A

Ques. What is emf? (2 Marks)

Ans. Emf or Electromotive force is a force that is developed from the source which is used to drive the charges from one point to another. It is denoted by the symbol ε.

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Ques. The emf of a battery is 10v, current of 5A and internal resistance is 2 ohm. If the cell is connected to a resistor, find the resistance. (2 Marks)

Ans: Given

emf = 10v
Current = 5A
Internal resistance = 2 ohm

We know that,

I = e/R + r

5 = 10/ R + 2

3/10 = 1/R

R = 3.3 ohms

Ques. The emf of a battery is 5v, current of 5A and internal resistance is 5 ohm. If the cell is connected to a resistor, find the resistance. (2 Marks)

Ans. Given

emf = 10v
Current = 5A
Internal resistance = 2 ohm

We know that,

I = e/R + r

5 = 10/ R + 2

3/10 = 1/R

R = 3.3 ohms

Ques. A cell of emf E and internal resistance r is connected across an external resistance R. Plot a graph showing the variation of P.D. across R, versus R. (2 Marks)

Ans. The graph to show the variation of internal resistance r and external resistance R is given below.

Ans. The graph to show the variation of internal resistance r and external resistance R is given below.

Ques. The emf of a cell is always greater than its terminal voltage. Why? (2 Marks)

Ans. All cells have a definite amount of internal resistance due to the materials which it is made up of. Due to this, when current passes through a cell, there is a drop in potential across it due to its internal resistance. This is called the lost voltage. Thus the emf of a cell is greater than its terminal voltage.

Ques. Derive a relation between the internal resistance, emf and terminal potential difference of a cell from which current l is drawn. Draw V vs l graph for a cell and explain its significance. (5 Marks)

Ans. Consider the circuit shown.

circuit

By Kirchhoff’s rules we have

E – lR – rl = 0

E – V – lr = 0

E = V + lr

The V-l graph is as shown.

The V-l graph

Significance of Graph: To find emf and internal resistance of the cell.

Ques. A voltmeter of resistance 998 ohms is connected across a cell of emf 2 V and internal resistance 2 Ohms. Find the potential difference across the voltmeter and also across the terminals of the cell. Estimate the percentage error in the reading of the voltmeter. (3 Marks)

Ans. Consider the following circuit diagram.

Consider the following circuit diagram

V= E – lr

998 × l = 2 – 2l

1000 l = 2

l = 0.002 A

Therefore,

V= 0.002 × 998= 1.996 V

Percentage error = (2-1.996)/2 x 100

= 0.004/2 = 0.2%

Ques. Gives the difference between Emf and the potential difference of a cell. (3 Marks)

Ans. The Difference between Emf and the potential difference of a cell is given below:

Emf	Potential Difference
The difference of potential between two terminals of a cell when no current is drawn from it is called the Emf of a cell.	The difference of potential between two terminals of a cell when current is drawn from it is called the potential difference of a cell.
The emf of a cell is independent of the resistance of the external resistance.	The potential difference is directly proportional to the external resistance.
The emf of a cell is greater than the potential difference.	The potential difference is greater than the emf.
It is denoted by a symbol ε.	It is denoted by V.

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Cells, emf, Internal Resistance Important Questions

Important Questions on Cells, emf, and Internal Resistance

CBSE CLASS XII Related Questions

2.
The magnetic field in a plane electromagnetic wave travelling in glass (\( n = 1.5 \)) is given by \[ B_y = (2 \times 10^{-7} \text{ T}) \sin(\alpha x + 1.5 \times 10^{11} t) \] where \( x \) is in metres and \( t \) is in seconds. The value of \( \alpha \) is:

3.
Assertion : Induced emf produced in a coil will be more when the magnetic flux linked with the coil is more. Reason (R): Induced emf produced is directly proportional to the magnetic flux.

4.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.
The figure represents the variation of the electric potential \( V \) at a point in a region of space as a function of its position along the x-axis. A charged particle will experience the maximum force at:

Similar Physics Concepts

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Cells, emf, Internal Resistance Important Questions

Important Questions on Cells, emf, and Internal Resistance

CBSE CLASS XII Related Questions

2.The magnetic field in a plane electromagnetic wave travelling in glass (\( n = 1.5 \)) is given by \[ B_y = (2 \times 10^{-7} \text{ T}) \sin(\alpha x + 1.5 \times 10^{11} t) \] where \( x \) is in metres and \( t \) is in seconds. The value of \( \alpha \) is:

3.Assertion : Induced emf produced in a coil will be more when the magnetic flux linked with the coil is more. Reason (R): Induced emf produced is directly proportional to the magnetic flux.

4.Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.The figure represents the variation of the electric potential \( V \) at a point in a region of space as a function of its position along the x-axis. A charged particle will experience the maximum force at:

Similar Physics Concepts

Comments

SUBSCRIBE TO OUR NEWS LETTER

2.
The magnetic field in a plane electromagnetic wave travelling in glass (\( n = 1.5 \)) is given by \[ B_y = (2 \times 10^{-7} \text{ T}) \sin(\alpha x + 1.5 \times 10^{11} t) \] where \( x \) is in metres and \( t \) is in seconds. The value of \( \alpha \) is:

3.
Assertion : Induced emf produced in a coil will be more when the magnetic flux linked with the coil is more. Reason (R): Induced emf produced is directly proportional to the magnetic flux.

4.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.
The figure represents the variation of the electric potential \( V \) at a point in a region of space as a function of its position along the x-axis. A charged particle will experience the maximum force at: