JEE 2020 Study Notes: Electric Instruments

By Avinash Kumar|Updated : June 28th, 2019

Current Electricity is an important topic from JEE Main / JEE Advanced Exam Point of view. Every year there are 1-3 questions asked from this topic. This short notes on Current Electricity will help you in revising the topic before the JEE Main & IIT JEE Advanced Exam. This is the third part of Current Electricity in which we have discussed Electric Instruments.

Wheatstone bridge, Meter Bridge, and Potentiometer

Wheatstone bridge

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The above shown arrangement is a case of Wheatstone bridge.

For balanced condition: byjusexamprep the current in the arm BD is zero, i.e. there will be no deflection in the galvanometer G.

Wheat stone bridges are used in calculation of unknown resistances.

Meter Bridge

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It consists of a wire of length 1m. and of uniform cross sectional area strained tight and fastened between two thick metallic strips fixed at right angles.

The end points are connected to a cell through a key and one end of a galvanometer is connected to the metallic strip midway between the two gaps. The other end of the galvanometer is connected to a ‘jockey’ as in the figure.

It uses the balanced Wheatstone bridge to determine the unknown resistance:

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Potentiometer

An instrument which has several metre long wire connected across a cell. The potentiometer draws no current from the voltage source is measured because it is unaffected by the internal resistance of the source. It is used to compare the Emf’s of a cell and the unknown internal resistance of the cell.

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Principle: When a constant current flows through a uniform cross sectional wire the potential difference between two points is directly proportional to the length of the wire between the two points.

Applications of Potentiometer

Comparing the Emf of the cell: The Emf of a cell is compared by observing the balancing length byjusexamprep

From the principle:

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Comparing the Internal resistance of the cell:

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As it draws no current from voltage source the internal resistance of any cell is measured using the potentiometer by finding the balancing lengths for the cell.

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