Physics 9702 · Paper 1

Current & Charge

Turn on a switch and the light comes on at once. Nothing has to travel from the switch to the lamp first. The electrons were already inside the wire, everywhere along it, and they all start to move together.
Current is a count: how much charge passes a point each second. That one sentence is the whole definition.

What a current is

A metal is a grid of fixed positive ions with free electrons moving between them. Connect a cell and every free electron is pushed and drifts the same way.
Any charged particle that moves makes a current. Moving electrons, protons or ions are all . In a metal the carriers are electrons. In a salt solution they are positive and negative ions, moving opposite ways at the same time.
The arrow we draw on circuits is : from + to −. Electrons are negative, so they actually drift the other way. The convention was fixed before anyone knew about electrons, and we still use it today.

Inside a metal: the positive ions are fixed, and the free electrons drift between them. The electrons drift one way; the conventional current points the other way.

Learn this line for the exam: electric current is the rate of flow of charge.

Charge and the coulomb

Charge is measured in (C). A current of 1 A means 1 C of charge passes each second.

Q = It

Symbols

$Q$ = charge that passes a point (C)
$I$ = current (A)
$t$ = time (s)

The definition to quote: one coulomb is the charge that passes a point in 1 s when the current is 1 A.
The ampere is an SI base unit. The coulomb is derived from it.

Worked example

Charge from a phone charger

A charger supplies a steady current of 0.50 A for 2.0 hours. Calculate the charge that flows.

Time must be in seconds: $t = 2.0 \times 3600 = 7200$ s.
Then $Q = It = 0.50 \times 7200 = 3600$ C.

Answer

Q = 3600 \text{ C}

Your turn— tap to reveal the worked answer (9702-style)

A charge of 180 C passes a point in 2.0 minutes. Find the current.

Answer: $I = \dfrac{Q}{t} = \dfrac{180}{120} = 1.5$ A. The usual error is leaving the time in minutes. (9702-style)

Charge comes in fixed steps

Every electron carries the same tiny charge: $e = 1.6 \times 10^{-19}$ C. This number is called the .
Charge is carried by whole particles, so any charge must be a whole-number multiple of e. We say charge is .
So $3.2 \times 10^{-19}$ C is possible (that is 2e). But $2.5 \times 10^{-19}$ C is impossible: it would need about one and a half electrons, and half an electron does not exist.

Your turn— tap to reveal the worked answer (9702-style)

Which of these charges is possible? $6.0 \times 10^{-19}$ C, $8.0 \times 10^{-19}$ C, $10.0 \times 10^{-19}$ C

Answer: divide each by $1.6 \times 10^{-19}$ . You get 3.75, then 5, then 6.25. Only $8.0 \times 10^{-19}$ C is a whole number of electron charges, so only that one is possible. (9702-style)

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Current & Charge

Turn on a switch and the light comes on at once. Nothing has to travel from the switch to the lamp first. The electrons were already inside the wire, everywhere along it, and they all start to move together.
Current is a count: how much charge passes a point each second. That one sentence is the whole definition.

What a current is

A metal is a grid of fixed positive ions with free electrons moving between them. Connect a cell and every free electron is pushed and drifts the same way.
Any charged particle that moves makes a current. Moving electrons, protons or ions are all . In a metal the carriers are electrons. In a salt solution they are positive and negative ions, moving opposite ways at the same time.
The arrow we draw on circuits is : from + to −. Electrons are negative, so they actually drift the other way. The convention was fixed before anyone knew about electrons, and we still use it today.

Inside a metal: the positive ions are fixed, and the free electrons drift between them. The electrons drift one way; the conventional current points the other way.

Learn this line for the exam: electric current is the rate of flow of charge.

Charge and the coulomb

Charge is measured in (C). A current of 1 A means 1 C of charge passes each second.

Q = It

Symbols

$Q$ = charge that passes a point (C)
$I$ = current (A)
$t$ = time (s)

The definition to quote: one coulomb is the charge that passes a point in 1 s when the current is 1 A.
The ampere is an SI base unit. The coulomb is derived from it.

Worked example

Charge from a phone charger

A charger supplies a steady current of 0.50 A for 2.0 hours. Calculate the charge that flows.

Time must be in seconds: $t = 2.0 \times 3600 = 7200$ s.
Then $Q = It = 0.50 \times 7200 = 3600$ C.

Answer

Q = 3600 \text{ C}

Your turn— tap to reveal the worked answer (9702-style)

A charge of 180 C passes a point in 2.0 minutes. Find the current.

Answer: $I = \dfrac{Q}{t} = \dfrac{180}{120} = 1.5$ A. The usual error is leaving the time in minutes. (9702-style)

Charge comes in fixed steps

Every electron carries the same tiny charge: $e = 1.6 \times 10^{-19}$ C. This number is called the .
Charge is carried by whole particles, so any charge must be a whole-number multiple of e. We say charge is .
So $3.2 \times 10^{-19}$ C is possible (that is 2e). But $2.5 \times 10^{-19}$ C is impossible: it would need about one and a half electrons, and half an electron does not exist.

Your turn— tap to reveal the worked answer (9702-style)

Which of these charges is possible? $6.0 \times 10^{-19}$ C, $8.0 \times 10^{-19}$ C, $10.0 \times 10^{-19}$ C

Answer: divide each by $1.6 \times 10^{-19}$ . You get 3.75, then 5, then 6.25. Only $8.0 \times 10^{-19}$ C is a whole number of electron charges, so only that one is possible. (9702-style)