## Important Points of Electricity

**Current :** The rate of flow of charge (Q) through a conductor is called current .

Current (I) is given by,

Current = or I =

The SI unit of current is ampere (A) : 1A = 1 C/s

The current flowing through a circuit is measured by a device called ammeter. **Ammeter **is connected in series with the conductor. The direction of the current is taken as the direction of the flow of positive charge.

**Ohm’s law :** At any constant temperature, the current (I) flowing through a conductor is directly proportional to the potential (V) applied across it.

Mathematically,

I = V/R or V = IR

**Resistance :** Resistance is the property of a conductor by virtue of which it opposes the flow of electricity through it. Resistance is measured in ohms. Resistance is a scalar quantity.

**Resistivity : **The resistance offered by a cube of a substance having side of 1 meter, when current flows perpendicular to the opposite faces, is called its resistivity (ρ). The SI unit of resistivity is ohm.m.

**Equivalent resistance :**A single resistance which can replace a combination of resistances so that current through the circuit remains the same is called*equivalent resistance.***Law of combination of resistances in series :**When a number of resistance are connected in series, their equivalent resistance is equal to the sum of the individual resistances.

If R_{1}, R_{2}, R_{3}, etc. are combined in series, then the equivalent resistance (R) is given by,

R = R_{1} + R_{2} + R_{3} + …..

The equivalent resistance of a number of resistances connected in series is higher than each individual resistance.

**Law of combination of resistances in parallel :**When a number of resistances are connected in parallel, the reciprocal of the equivalent resistance is equal to the sum of the reciprocals of the individual resistances.

If R_{1}, R_{2}, R_{3}, etc. are combined in parallel, then the equivalent resistance (R) is given by.

The equivalent resistance of a number of resistances connected in parallel is less than each of all the individual resistances.