# Electromotive Force and Internal Resistance

Card 1: What is electromotive force?

## Electromotive Force

In a circuit, electromotive force is the energy per unit charge converted from the other forms of energy into electrical energy to move the charge across the whole circuit.

Unit:

The unit of e.m.f. is JC-1 or V (Volt)

Card 2: Formula of Electromotive Force

## Formula of Electromotive Force Card 3: Comparing E.M.F. and otential Difference

## Comparing E.M.F. and otential Difference

 Electromotive Force Potential Difference Similarities: Have same unit (Volt) Can be measured by Voltmeter Definition The electromotive force (e.m.f.) is defined as the energy per unit charge that is converted from chemical, mechanical, or other forms of energy into electrical energy in a battery or dynamo. Definition The potential difference (p.d.) between two points is defined as the energy converted from electrical to other forms when one coulomb of positive charge passes between the two points. Symbol: Denote by the symbol, E. Symbol: Denote by the symbol, V

Card 4: What is internal resistance?

## Internal resistance

The internal resistance of a source (cell or generator) is the resistance against the moving charge in the source.

The load resistance in a circuit is the effective resistance against the moving charge outside the source of electric.

Card 6: Terminal Potential Difference

## Terminal Potential Difference

• Terminal potential difference or terminal voltage is the potential difference across the two terminal (the positive terminal and the negative terminal) of an electric source (cell or generator).
• If the internal resistance of the cell is ignored, the terminal potential difference is equal to the e.m.f.
Card 7: Formulae of Electromotive Force and Internal Resistance

## Formulae of Electromotive Force and Internal Resistance Card 8: Finding E.M.F and Internal Resistance - The Open Circuit Method

## Finding E.M.F and Internal Resistance - The Open Circuit Method In open circuit ( when the switch is off), the voltmeter shows the reading of the e.m.f. In close circuit ( when the switch is on), the voltmeter shows the reading of the potential difference across the cell.

With the presence of internal resistance, the potential difference across the cell is always less than the e.m.f.

Card 9: Finding E.M.F and Internal Resistance - Linear Graph Method

## Finding E.M.F and Internal Resistance - Linear Graph Method Gradient od the grapf, m = - internal resistance
Y intercept of the graph, c = e.m.f.

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