Define e..m.f. of a cell. How can you compare the emf of two cells using potentiometer ?

Electromotive force or e.m.f. is defined as the electric potential produced by either an electrochemical cell or by changing the magnetic field. In fact, an electromotive force is work done on a unit electric charge, by generator or a battery for the conversion of energy from one form to another, when, one terminal becomes positively charged while the other becomes negatively charged. EMF is defined as the maximum potential difference that is delivered by the battery when there is no flow of current. It is given by ε = V + ir .........................................................................................(1) Where V =is the voltage of the cell i = is the current across the circuit, r is the internal resistance of the cell ε = is the electromotive force. Comparison of emf of two cells. To compare the emf of two cells, the circuit connections are made as shown in the figure. The potentiometer wire AB is connected to a battery E p and a key K in series. This is the primary circuit. The end A of the wire is connected to the terminal X of a DPDT (Double Pole Double Throw) switch via the first cell E 1 and the other terminal Y is connected to a jockey through a galvanometer G . The second cell is similarly connected to the DPDT switch as shown in the figure. The positive terminals of E 1 and E 2 should be connected to the same end. The DPDT switch can select any of the cells in the circuit to be included in the secondary circuit and the balancing length l 1 and l 2 are found by adjusting the jockey for zero deflection, Let the current in the circuit is I , then we have, Since the potential difference is the same for the two cases, we get. E = I l 1 and E 2 = I l 2 Hence we have E 1 E = l 1 l 2 . Therefore just by comparing the position of null defelction with each cell, we can get the ratio of their e.m.f.

E M B I B E

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Punjab Board-2019>Q 6

MEDIUM

12th Punjab Board

IMPORTANT

Earn 100

State Wheatstone bridge principle. Use Kirchhoff's Laws to obtain the relation between the resistance in four arms of the Wheatstone bridge by drawing circuit diagram.

Important Questions on Current Electricity

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12th Punjab Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Punjab Board-2018>Q 1

For alloys the value of temperature coefficient of resistance is very high.

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12th Punjab Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Punjab Board-2019>Q 4

For metal conductors the electrical resistivity deceases with the rise in temperature

MEDIUM

12th Punjab Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Punjab Board-2019>Q 9

Explain the principle, construction and working of a cyclotron with the help of a labelled diagram. State its two limitations.

MEDIUM

12th Punjab Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Punjab Board-2018>Q 2

A wire with an area of cross section $10 {mm}^{2}$ has resistance $5 Ω$ ,when a potential difference across its ends is $25 V$ .calculate the drift velocity of the electron. Given the number density of electrons is $5 \times 10^{20} electron / m^{3}$ .