Electricity — RBSE Class 10 (Science)

Flip a switch in Jaipur and a fan 3 metres away spins instantly. Nothing visibly travelled — yet energy moved from the power station to your room in a blink. This chapter is the rulebook for that invisible flow: how much charge moves (current), what pushes it (voltage), what resists it (resistance), and how they lock together in one tidy equation, V = IR.

1. Electric current and circuit

Electric current is the rate of flow of electric charge:

$I=\frac{Q}{t}$

where Q is charge in coulombs (C) and t is time in seconds. The unit of current is the ampere (A): 1 A = 1 C/s.

One coulomb is the charge of about $6.25\times 10^{18}$ electrons. By convention, current flows in the direction opposite to electron flow (from + to − outside the cell). A device that measures current is an ammeter, connected in series.

2. Potential difference — the push

To make charge flow, you need a "push" — a potential difference (voltage) provided by a cell or battery.

$V=\frac{W}{Q}$

Potential difference is the work done to move a unit charge between two points. Its unit is the volt (V): 1 V = 1 J/C. A voltmeter measures it, connected in parallel across the component.

3. Ohm's law — the central equation

At constant temperature, the current through a conductor is directly proportional to the potential difference across it.

$V=IR$

A graph of V (y-axis) against I (x-axis) is a straight line through the origin, and its slope is the resistance R.

Resistance (R) opposes the flow of current; its unit is the ohm (Ω): 1 Ω = 1 V/A. A good conductor has low resistance; an insulator very high. A rheostat is a variable resistor used to change the current in a circuit.

4. What decides resistance? — Resistivity

The resistance of a wire depends on four things:

$R=\rho \frac{l}{A}$

• Length (l): R ∝ l — a longer wire has more resistance.
• Area of cross-section (A): R ∝ 1/A — a thicker wire has less resistance.
• Material: captured by resistivity ρ (rho), a property of the material. Unit: Ω·m.
• Temperature: resistance generally rises with temperature.

Metals like copper and aluminium have very low resistivity → used for wires. Alloys like nichrome have high resistivity and don't oxidise easily at high temperature → used in heating elements (irons, toasters, geysers). Filaments are made of tungsten (very high melting point).

5. Combinations of resistors

Series — same current, voltages add

Resistors joined end to end carry the same current; the total (equivalent) resistance is the sum:

$R_s=R_1+R_2+R_3+\dots$

R_s is larger than any individual resistor. A drawback: if one component fails (a fused bulb), the whole circuit breaks.

Parallel — same voltage, currents add

Resistors joined across the same two points have the same voltage; the reciprocal of the total resistance is the sum of reciprocals:

$\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}+\dots$

R_p is smaller than the smallest individual resistor. Advantages — each device gets the full voltage, each can be switched independently, and one failing doesn't stop the rest. That's why home wiring uses parallel connections.

6. Heating effect of electric current

When current flows through a resistor, electrical energy turns into heat — Joule's law of heating:

$H=I^{2}Rt$

This is useful (heaters, geysers, electric irons, the filament of an incandescent bulb, and the fuse — a thin wire that melts and breaks the circuit when current gets dangerously high) and sometimes wasteful (heat lost in transmission lines).

7. Electric power and the commercial unit

Electric power is the rate at which electrical energy is consumed:

$P=VI=I^{2}R=\frac{V^2}{R}$

The unit is the watt (W): 1 W = 1 J/s. A 60 W bulb uses 60 joules every second.

Energy = power × time. The SI unit is the joule, but electricity bills use the kilowatt-hour (kWh), the commercial unit of energy, also called 1 unit:

$1\text{kWh}=1000\text{W}\times 3600\text{s}=3.6\times 10^6\text{J}$

So a 1000 W (1 kW) geyser running for 1 hour uses exactly 1 unit of electricity.

8. Closing thought

Almost everything in this chapter flows from one relationship — V = IR — and one definition — power = energy per second. Combine them and you can:

• predict the current any appliance draws,
• choose between series and parallel wiring,
• size a fuse safely, and
• read your own electricity bill in kWh.

It is the most numerical chapter of Class 10 Science and one of the most rewarding for the RBSE board — if you can rearrange V = IR and P = VI confidently and watch your units, the marks are almost guaranteed. Write the formula, substitute with units, then solve.