#### Physics (NTSE/Olympiad)

# 1. Electricity

__Points to Be Remember (Electricity)__

**Points to Be Remember** **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 =

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.

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