CBSE · Class 10 · Physics · Chapter 11

Electricity — Formula Sheet

Board Formulas

10 formulas

1.Electric Current
$I = \frac{Q}{t}$
$I$ : Current (ampere, A) · $Q$ : Charge (coulomb, C) · $t$ : Time (second, s)
Rate of flow of charge. 1 A = 1 C/s. Ammeter connected in series.
2.Potential Difference
$V = \frac{W}{Q}$
$V$ : Potential difference (volt, V) · $W$ : Work done (joule, J) · $Q$ : Charge moved (C)
Work done per unit charge to move it from one point to another. Voltmeter in parallel.
3.Ohm's Law★
$V = I R$
Current through a metallic conductor is directly proportional to potential difference (constant T).
4.Resistance and Resistivity★
$R = ρ \frac{l}{A}$
$R$ : Resistance (ohm, Ω) · $ρ$ : Resistivity of the material (Ω·m) · $l$ : Length of the conductor (m) · $A$ : Area of cross-section (m²)
Resistance is proportional to length and inversely proportional to area of cross-section.
5.Resistors in Series
$R_{s} = R_{1} + R_{2} + R_{3} + \dots$
Same current through each resistor; voltages add. R_s is always LARGER than the largest resistor.
6.Resistors in Parallel★
$\frac{1}{R _{p}} = \frac{1}{R _{1}} + \frac{1}{R _{2}} + \frac{1}{R _{3}} + \dots$
Same voltage across each; currents add. R_p is always SMALLER than the smallest resistor.
7.Electric Power
$P = V I = I^{2} R = \frac{V ^{2}}{R}$
Three forms — pick whichever avoids the unknown quantity. Unit: watt (W).
8.Electrical Energy
$E = P \times t = V I t$
Commercial unit: 1 kWh = 3.6 × 10⁶ J (also called 1 unit of electricity).
9.Joule's Law of Heating★
$H = I^{2} R t$
Heat produced in a resistor when current flows through it. Basis of electric heater, iron, bulb filament.
10.Commercial Unit of Energy
$1 kWh = 3.6 \times 1 0^{6} J$
One 'unit' on the electricity bill. Multiply kW by hours to get kWh.