Definitions [9]
Define the term current.
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire.
The electrical energy consumed in a circuit is defined as the total work done in maintaining the current in the electric circuit for a given time.
Electrical Energy = \[VIt=I^2Rt=\frac{V^2t}{R}\]
S.I. unit of electric energy is joule (1 kWh = \[3.6\times10^6\mathrm{~J}\])
The rate of doing work is called power.
Define power.
Power is defined as the rate of doing work or work done per second.
i.e., Power = `("Work done in joule")/("Times in second")`
or, p = `("W (in joule)")/("t (in second)")`
Specific resistance of a material is the resistance of a wire of that material of unit length and unit area of cross section.
The reciprocal of specific resistance is known as conductivity.
When no current is drawn from a cell i.e., when the cell is in open circuit, the potential difference produced by the chemical reaction between the terminals of the cell is called its electro-motive force (or e.m.f.).
Define the following:
Electromotive force (e.m.f.)
The e.m.f of an electrical energy source is one volt if one joule of work is done by the source to drive one coulomb of charge completely around the circuit.
The e.m.f. of a cell is defined as the energy spent (or the work done) per unit charge in taking a positive test charge around the complete circuit of the cell (i.e., in the circuit outside the cell as well as in the electrolyte inside the cell).
Formulae [1]
Power P = \[\frac{\text{Work done }W}{\text{Time taken }t}\]
or
P = \[\frac {W}{t}\]
Key Points
- S.I. unit: If 1 joule of work is done in 1 second, the power spent is said to be 1 watt.
- C.G.S. unit: The C.G.S. unit of power is erg per second (erg s-1).
- Relationship between S.I. and C.G.S. units:
1 W = 1 J s-1 = 107 erg s-1 - 1 horse power (H.P.) = 746 W = 0.746 kW
- Specific resistance is a characteristic property of a substance and differs among metals, semiconductors, and insulators.
- Specific resistance depends on temperature: it increases with temperature for metals and decreases with temperature for semiconductors, while it remains nearly constant for some alloys.
- Specific resistance does not depend on the shape and size of the conductor and remains unchanged when a wire is stretched or doubled.
