CURRENT ELECTRICITY

Prepared by

Sheena Punnoose

PGT Physics, JNV Alleppey

CURRENT ELECTRICITY -2

• Internal resistance of a cell
• Cells in series and parallel
• Kirchhoff’s Rules
• Wheatstone’s bridge
• Metre bridge
• Potentiometer
• Comparison of emf using potentiometer
• Internal resistance of a cell using potentiometer

Internal Resistance of a cell

Internal resistance of a cell in terms of E,V & R

Cells in Series

Extending to n number of cells in series combination :� (i) The equivalent emf of a series combination of n cells is just the sum of their individual emfs.�(ii) The equivalent internal resistance of a series combination of n cells is just the sum of their internal resistances.

KIRCHHOFF’S RULES

(II)Loop rule:

• The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop is zero

Or

The algebraic sum of all the potential drops and emfs along any closed path in an electrical network is always zero.

Sign Conventions

1. The emf is taken negative when we traverse from positive to negative terminal of the cell through the electrolyte.
2. The emf is taken positive when we traverse from negative to positive terminal of the cell through the electrolyte.

Metre bridge

Potentiometer

• It is a long piece of uniform wire of a few metres in length, fixed on a wooden board, across which a standard cell is connected
• Principle: If a constant current flows through a wire of uniform cross section, then the potential difference across any length of the wire is directly proportional to its length.

Comparison of emf’s using potentiometer�

Note

• The balance point will not be obtained on the potentiometer wire if the fall of potential along the potentiometer wire is less than the emf of the cell to be measured.
• The working of the potentiometer is based on null deflection method. So the resistance of the wire becomes infinite. Thus potentiometer can be regarded as an ideal voltmeter.

Internal Resistance of Cell Using Potentiometer