Physics

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 R₁, R₂, R₃, etc. are combined in series, then the equivalent resistance (R) is given by,
R = R₁ + R₂ + R₃ + .....
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 R₁, R₂, R₃, 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.