Basic Concept

Energy is carried by electric current

Electric current is the rate flow of electric charge past a point.

I=\frac{Q}{t}

The unit of charge is Coulomb (C), and the unit of current is Ampere(S.I. Unit)

Charge is Quantised: charge values are not continuous; they are discrete. All charges are multiples of charges of

1e=-1.6\times10^{-19}

Potential difference: If two points have a potential difference of 1V if the work required to move 1C of charge between them is 1 joule

W=VQ\\
P=VI; P=I^2R; P=\frac{V^2}{R}

Cells in the circuit give energy to charges, which transfer to useful energy and heat.

The e.m.f. is the total work a battery does per unit charge when the unit charge goes around a complete circuit.

Conductors

Electrons move in a specific direction when p.d. is applied across a conductor, causing current.

I=nAvq

Where I = Current, A = Arear, v = velocity of the electron, n = number density, q = the charge carried.

Resistance and Resistivity

Resistance is the opposition to the passage of current within a component. The unit of resistance is Ohm

R=\frac{V}{I}

R=\rho \frac{L}{A}

1 ohm is the resistance when 1-volt drives 1 ampere through.

I-V Characteristic

D.C. Circuit

Kirchhoff's Law

First Law

The sum of current into a junction = Sum of current out of that junction

It's about the conservation of charges

Second Law

Total e.m.f. in any loop = Total p.d. in that loop

It's about conservation of energy

Resistance in Series and Parallel

Series

R=R_1+R_2

Parallel

\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}

Potential Divider

The potential divider divides the voltage into smaller parts

Potentiometer

A potentiometer is a continuously variable potential divider used to compare potential differences.

If the galvanometer needle didn't deflect, the current passed through it = 0; thus, it is the same p.d.

\frac{E_1}{E_2}=\frac{L_{XY}}{L_{XT}}