Current Electricity — Part 1: Ohm’s law and resistance: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

What keeps the charge moving?

Part 1 — a cell pushes charge round a circuit. How much flows, how hard it is pushed, and what holds it back are measured by current, potential difference and resistance.

In this lesson you will

Relate charge, current and potential difference, and use Q = It and V = W/Q
State Ohm’s law, describe its verification and find resistance from a V–I graph
Distinguish ohmic and non-ohmic conductors
Use R = ρl/A and explain how temperature and material affect resistance; describe superconductors

Table of charge, current, potential difference and resistance with their symbols, units and measuring instruments
Quantity	Symbol	SI unit	Measured with
Charge	Q	coulomb (C)	—
Current	I	ampere (A)	ammeter (in series)
Potential difference	V	volt (V)	voltmeter (in parallel)
Resistance	R	ohm (Ω)	V ÷ I

Concept

Charge and electric current

Charge is carried in metals by free electrons (charge of one electron $e = 1.6 \times 10^{-19}$ C). Electric current is the rate of flow of charge: $I = \dfrac{Q}{t}$ . 1 ampere = 1 coulomb passing a point each second. Conventional current flows from the positive terminal of the cell, through the outside circuit, to the negative terminal; electrons drift the opposite way.

Table of charge, time and the resulting current
Charge Q (C)	Time t (s)	Current I = Q ÷ t (A)
60	30	2
15	60	0.25
1	1	1

Concept

Potential difference and resistance

The potential difference between two points is the work done per unit charge in moving charge between them: $V = \dfrac{W}{Q}$ ; 1 volt = 1 joule per coulomb. Resistance is the opposition a conductor offers to the flow of current: $R = \dfrac{V}{I}$ ; 1 ohm is the resistance of a conductor that carries 1 A when the p.d. across it is 1 V.

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