10th Class Physics Chapter 7 Notes PDF 2026 – Electricity

Definition

Electric current is the rate of flow of charge through a conductor — the amount of charge passing a point per second.

Conventional current is defined as flowing from the positive terminal to the negative terminal through the external circuit — this is the opposite direction to the actual flow of electrons (negative to positive), a convention fixed before electrons were discovered and kept ever since.

Direct current (DC) flows in one direction only (e.g. from a battery). Alternating current (AC) periodically reverses direction (e.g. mains household supply).

$I = \frac{Q}{t}$ (current $=$ charge $\div$ time)

Example 1 — current from charge and time

A charge of $25$ C passes through a circuit in $10$ s. Find the current.

$I = \frac{Q}{t} = \frac{25}{10}$ .

Answer: $I = 2.5$ A

Remember

An ammeter measures current by being connected in series (so all the circuit's current passes through it) and must have very low resistance, so it doesn't itself reduce the current it's trying to measure. Connecting it incorrectly (in parallel) would let most current bypass the ammeter through the rest of the circuit, giving a wrong (usually much too low) reading, and could also draw a very large current through the ammeter's low resistance, damaging it.

Definition

Electromotive force (e.m.f.) is the energy a source (like a battery) gives to each unit of charge it pushes around the whole circuit — it's what drives current, even accounting for energy lost inside the source itself.

Potential difference (voltage) is the energy transferred per unit charge between two specific points in a circuit — a battery creates this by maintaining an excess of charge at one terminal relative to the other, using chemical energy to keep pushing charge against the field building up inside it.

Rule

Ohm's law: at constant temperature, the current through a conductor is directly proportional to the potential difference across it — so voltage divided by current gives a constant, the resistance.

$V = I R$

$R = \frac{ρ L}{A}$ ( $ρ$ = resistivity, $L$ = length, $A$ = cross-sectional area)

Definition

Resistivity is a property of the material itself (not its shape), describing how strongly it opposes current flow. It depends on the material's atomic structure — metals have low resistivity (good conductors), insulators have very high resistivity.

Remember

A metal conductor's resistance increases with temperature, because hotter atoms vibrate more and get in the way of flowing electrons more often — this is why resistivity rises with temperature for metals. Semiconductors behave oppositely: heating them frees more charge carriers, so their resistivity typically falls with rising temperature.

Replacing copper wire with aluminium of the same dimensions increases resistance, since aluminium has higher resistivity than copper — for the same voltage, current (and so power, $P = V I$ ) would decrease.

Series: $R_{total} = R_{1} + R_{2} + R_{3} + \dots$ (same current through each resistor)

Parallel: $\frac{1}{R _{total}} = \frac{1}{R _{1}} + \frac{1}{R _{2}} + \frac{1}{R _{3}} + \dots$ (same voltage across each resistor)

Example 2 — resistors in series

Three resistors $4 Ω$ , $6 Ω$ , $8 Ω$ are connected in series to a $24$ V battery. Find the total resistance and the current.

$R_{total} = 4 + 6 + 8$ .
$R_{total} = 18 Ω$ .
$I = \frac{V}{R} = \frac{24}{18}$ .

Answer: $R_{total} = 18 Ω$ , $I \approx 1.33$ A

Example 3 — resistors in parallel

Resistors $10 Ω$ and $30 Ω$ are connected in parallel across a $20$ V supply. Find the total resistance and total current drawn.

$\frac{1}{R _{total}} = \frac{1}{10} + \frac{1}{30} = \frac{3}{30} + \frac{1}{30} = \frac{4}{30}$ .
$R_{total} = \frac{30}{4} = 7.5 Ω$ .
$I = \frac{V}{R} = \frac{20}{7.5}$ .

Answer: $R_{total} = 7.5 Ω$ , $I \approx 2.67$ A

Example 4 — batteries in series

Four $1.5$ V batteries are connected in series in a flashlight. Find the total e.m.f.

e.m.f.s add directly in series: $1.5 \times 4$ .

Answer: $6.0$ V

Remember

A voltmeter measures potential difference by being connected in parallel across a component, and needs very high resistance so it draws almost no current itself and doesn't disturb the circuit being measured. Connected incorrectly (in series), its huge resistance would choke off nearly all current in the circuit.

Household bulbs are wired in parallel (not series) so each gets the full mains voltage and works independently — if one bulb in a series circuit fails or is removed, the circuit breaks and every bulb on it goes out.

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

Energy $= P \times t$

Example 5 — power and energy dissipated

A $5$ A current passes through a $7 Ω$ resistor. Find the power dissipated and the energy consumed in $10$ minutes.

$P = I^{2} R = (5)^{2} (7) = 25 \times 7$ .
$P = 175$ W.
$t = 10 \times 60 = 600$ s.
Energy $= P \times t = 175 \times 600$ .

Answer: $P = 175$ W, Energy $= 105, 000$ J

Definition

A fuse is a thin wire that melts and breaks the circuit if current exceeds a safe rated value, protecting the device and wiring from overheating.

Example 6 — choosing a fuse rating

Design a fuse for a device rated $230$ V, $5$ A. What rating should the fuse have, and what goes wrong if it's too low or too high?

Normal operating current is $5$ A, so the fuse should be rated slightly above this (commonly the next standard rating up, e.g. $6$ – $7$ A) to allow for normal small surges without blowing unnecessarily.
Too low a rating (e.g. $3$ A): the fuse blows during normal operation, cutting power unnecessarily.
Too high a rating (e.g. $13$ A): a genuine fault current well above the device's safe limit could flow without the fuse blowing, risking overheating, fire, or damage to the device before protection kicks in.

Answer: A rating just above $5$ A (e.g. $\sim 6$ – $7$ A) — too low trips needlessly, too high fails to protect

Remember

AC is more suitable than DC for power transmission across cities because AC voltage can be stepped up (via transformers) to very high values for transmission, cutting current and so power loss ( $P = I^{2} R$ ) in the wires, then stepped back down for safe household use — transformers only work with the changing current of AC, not steady DC. DC remains preferred in batteries and portable electronics because it provides a steady, unidirectional voltage that matches how electronic circuits and chemical cells naturally operate.

Quick Recap

● $I = Q / t$ ; conventional current flows $+ \to -$ , opposite to actual electron flow
●Ammeter: series, very low resistance. Voltmeter: parallel, very high resistance
●Ohm's law: $V = I R$ ; $R = ρ L / A$ ; metal resistivity rises with temperature
●Series: $R_{total} = \sum R$ . Parallel: $\frac{1}{R _{total}} = \sum \frac{1}{R}$
● $P = V I = I^{2} R = V^{2} / R$ ; Energy $= P \times t$ ; fuse rated just above normal operating current
●AC suits power transmission (transformers step voltage up/down); DC suits batteries/electronics

10th Class Physics Chapter 7 Notes

Electric Current

EMF and Potential Difference

Ohm's Law and Resistivity

Series and Parallel Combinations

Electric Power, Energy, and Fuses

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