Cells, emf, Internal Resistance Important Questions

Content Curator

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

Check Out:

Related Articles
Kirchhoff’s Laws MCQ	Wheatstone Bridge MCQ	Cells in series and parallel MCQ
Kirchhoff’s Laws Important Questions	Wheatstone Bridge Important Questions	Cells in series and parallel Important questions

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.

Check Out:

Related Articles
Circuit Diagram	Resistors in series and parallel	Internal resistance formula
Resistor Colour Codes	Cells in series and parallel	Uses of Resistor
Electromotive Force	Resistance Formula	Electrical Current

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.

Read More:

Guide Articles
Important Physics Formulae	Comparison Articles in Physics	NCERT Solutions for Class 12 Chemistry
Class 12 Maths Book	Class 12 Physics Book Pdf	CBSE Class 12 Physics Practicals
NCERT Solutions for Class 12 Maths	NCERT Solutions for Class 12 Physics	Important Physics Constants
SI Units in Physics	Important Physics MCQs	NCERT Class 12 PCMB Textbook
Class 12 PCMB Textbooks	Class 12 PCMB Notes	Class 12 Biology Book Pdf
NCERT Solutions for Class 12 Biology	Important Physics Derivations	Physics Study Material
Current Electricity Important Questions	Class 12 Physics Notes	Class 12 English Book Pdf

CBSE CLASS XII Related Questions

1.
Find the ratio $\dfrac{\lambda_\alpha}{\lambda_p}$ of the de Broglie wavelength $\lambda_\alpha$ associated with an alpha particle to the de Broglie wavelength $\lambda_p$ associated with a proton if both are moving with the same velocity.
View Solution
2.
A square loop of side 0.50 m is placed in a uniform magnetic field of 0.4 T perpendicular to the plane of the loop. The loop is rotated through an angle of $60^\circ$ in 0.2 s. The value of emf induced in the loop will be:
- 5 V
- 3.5 V
- 2.5 V
- Zero V
View Solution
3.
A magnet held vertically, with its north pole down, is dropped along the axis of a closed solenoid placed vertically on a table. If the observer looks down from the top,
- the induced current will flow in the anticlockwise direction.
- the induced current will flow in the clockwise direction.
- no induced current will flow in the solenoid.
- the magnet will fall with a constant velocity.
View Solution
4.
(I) The real image of an object placed between $f$ and $2f$ from a convex lens can be seen on a screen placed at the image location. If the screen is removed, is the image still there? Explain.
(II) Plane and convex mirrors produce virtual images of objects. Can they produce real images under some circumstances? Explain.
View Solution
5.
A parallel beam of monochromatic light falls normally on a single slit of width $a$ and a diffraction pattern is observed on a screen placed at distance $D$ from the slit. Explain:
(I) the formation of maxima and minima in the diffraction pattern, and
(II) why the maxima go on becoming weaker and weaker with its increasing number $n$.
View Solution
6.
The resistance of a metal wire at $20^\circ\text{C}$ is $1.05\,\Omega$ and at $100^\circ\text{C}$ is $1.38\,\Omega$. Determine the temperature coefficient of resistivity of this metal.
View Solution