Physics · 4 Electricity and magnetism

4.2.2 Electric current

Before you start

Charge is measured in coulombs, and conductors contain free electrons (4.2.1).
Rearranging an equation of the form $a = b / c$ .
Reading a scale on a measuring instrument.

Syllabus objectives covered (6)

Know that electric current is related to the flow of charge
Describe the use of ammeters (analogue and digital) with different ranges
Describe electrical conduction in metals in terms of the movement of free electrons
Know the difference between direct current (d.c.) and alternating current (a.c.)
Define electric current as the charge passing a point per unit time; recall and use the equation I = Q / t
State that conventional current is from positive to negative and that the flow of free electrons is from negative to positive

Part 1 of 5

What a current is

An electric current is a flow of charge. In a metal wire, the things that flow are the free electrons you met in 4.2.1.
Without a cell, those free electrons move randomly in all directions, going nowhere on average. Connect a cell and they drift steadily in one direction along the wire, on top of that random motion.
That steady drift is the current. The electrons themselves move surprisingly slowly — a fraction of a millimetre per second — even though the lamp lights instantly, because they are already everywhere in the wire and all start moving together.
So a full-mark answer to 'describe conduction in a metal' is: metals contain free electrons which, when a p.d. is applied, move (drift) through the metal in one direction, and this flow of charge is the current.
Current is measured in amperes (amps), symbol A.

Part 2 of 5

Defining current properly

The formal definition: electric current is the charge passing a point per unit time.
As an equation, with $I$ the current in amps, $Q$ the charge in coulombs and $t$ the time in seconds:
$I = \frac{Q}{t}$
Rearranged, $Q = I t$ . That form is the one you usually need, because questions tend to give a current and a time and ask for the charge.
This equation also tells you what an amp is: one amp is one coulomb of charge passing a point every second.
Time must be in seconds. A question quoting minutes is testing whether you convert — five minutes is 300 s, and forgetting that is the single most common error here.
Remember there is no formula sheet in Paper 4. You must be able to write $I = Q / t$ from memory.

Part 3 of 5

Two directions, and why both exist

Electricity was studied and named long before anyone knew electrons existed. Scientists guessed that current flowed from positive to negative, and built the convention on that.
When electrons were discovered, they turned out to be negative and to flow the other way — but by then every circuit diagram, rule and instrument used the original convention, so it stayed.
Conventional current flows from positive to negative, outside the cell.
Free electrons flow from negative to positive — the opposite direction.
Both statements are correct and both are examined. Read the question carefully: it will say either 'the conventional current' or 'the electrons', and the two answers are opposites.
Every rule you meet later in this unit — and in electromagnetism — uses conventional current unless it says otherwise.

Part 4 of 5

Direct and alternating current

Direct current (d.c.) flows in one direction only, and its size is usually steady. This is what a cell or battery produces.
Alternating current (a.c.) repeatedly reverses direction. Its size rises and falls, going one way then the other, many times a second. This is what the mains supply and a generator produce.
The single most reliable way to state the difference in an exam: d.c. flows in one direction only; a.c. repeatedly changes direction.
On a graph of current against time, d.c. is a horizontal line above the axis and a.c. is a wave that crosses the axis, spending equal time positive and negative.
Crossing the axis is what reversing direction looks like. If a graph never crosses the axis, the current never reverses, so it is d.c. however much it wobbles.

Part 5 of 5

Using an ammeter

An ammeter measures current, and it must be connected in series with the component whose current you want. The current has to pass through it to be counted.
It should have a very low resistance, so that inserting it changes the circuit as little as possible.
Digital ammeters show a number directly. They are quick to read and free of the reading errors below, though the last digit may fluctuate.
Analogue ammeters have a needle and a scale. Read them with your eye directly in front of the needle, to avoid a parallax error from viewing at an angle.
Choosing the range is what this objective is really about. Pick a range whose maximum is just above the current you expect.
Too large a range and the needle barely moves, so a small deflection has to be read off a coarse scale and the reading is imprecise. Too small a range and the needle goes off the end — no reading at all, and possibly a damaged meter.
So a current of about 0.3 A is best read on a 0–1 A range: roughly a third of full scale, easy to read precisely, with no risk of over-range.

Worked examples

Example 1. A current of $0.25$ A flows through a lamp for $4.0$ minutes. Calculate the charge that passes through the lamp.

Show the working

Note what is given and what is wanted: current and time in, charge out. So use $I = Q / t$ rearranged.
Convert the time to seconds before doing anything else: $4.0$ minutes $= 4.0 \times 60 = 240$ s.
Rearrange the equation to make $Q$ the subject.
$Q = I t$
Substitute the values.
$Q = 0.25 \times 240 = 60 C$
Give the unit. Charge is measured in coulombs, and the final answer mark requires it.

Answer: $Q = 60$ C.

Example 2. A charge of $180$ C passes a point in a circuit in $1.5$ minutes. Calculate the current.

Show the working

This time charge and time are given and the current is wanted, so use the equation as it stands.
Convert: $1.5$ minutes $= 90$ s.
Substitute into $I = Q / t$ .
$I = \frac{180}{90} = 2.0 A$
Check the answer is sensible — 2 A is a typical current for a household appliance, so nothing has gone badly wrong.

Answer: $I = 2.0$ A.

Example 3. A student expects a current of about $0.4$ A. Three analogue ammeters are available, with ranges 0–0.1 A, 0–1 A and 0–10 A. Determine which the student should use, and explain the problem with each of the others.

Show the working

Start with the rule: choose the range whose maximum is just above the expected current.
The 0–0.1 A meter has a maximum below $0.4$ A. The needle would go off the scale, giving no reading and risking damage to the meter.
The 0–10 A meter would work, but $0.4$ A is only 4% of full scale. The needle would barely move and the reading would be imprecise.
The 0–1 A meter puts the reading at about 40% of full scale, where the divisions are usable and the reading is most precise.
Add the reading technique: look at the needle straight on, to avoid a parallax error.

Answer: The 0–1 A meter. The 0–0.1 A range is too small — the needle would go off scale. The 0–10 A range is too large — the deflection would be tiny and the reading imprecise.

Where people go wrong

Mistake: Saying current flows from negative to positive.

Instead: Conventional current flows positive to negative; the electrons flow negative to positive.

Both are true statements about different things. The question always specifies which, and the answers are opposites.

Mistake: Connecting an ammeter in parallel with a component.

Instead: An ammeter goes in series, so the current passes through it.

You are counting the charge going past a point, so the meter has to be in the path.

Mistake: Using minutes directly in $I = Q / t$ .

Instead: Convert to seconds first.

The amp is defined as one coulomb per second. Using minutes makes the answer 60 times too small or too large.

Mistake: Saying a.c. 'changes size' and leaving it there.

Instead: The defining feature is that it repeatedly changes direction.

A d.c. supply can vary in size without ever reversing. Direction is what separates the two.

Mistake: Choosing the largest available ammeter range to be safe.

Instead: Choose the range whose maximum is just above the expected current.

An over-large range gives a tiny deflection and an imprecise reading, which is a measurement error, not a safety gain.

Check yourself

Describe electrical conduction in a metal.
Answer
A metal contains free electrons. When a p.d. is applied they drift through the metal in one direction, and this flow of charge is the electric current.
Define electric current and give the equation.
Answer
The charge passing a point per unit time: $I = Q / t$ .
A current of $3.0$ A flows for $20$ s. How much charge passes?
Answer
$Q = I t = 3.0 \times 20 = 60$ C.
In which direction does conventional current flow, and in which direction do the free electrons move?
Answer
Conventional current flows from positive to negative; free electrons flow from negative to positive.
State the difference between d.c. and a.c., and name a source of each.
Answer
D.c. flows in one direction only, from a cell or battery. A.c. repeatedly changes direction, from the mains supply or a generator.
How should an ammeter be connected, and why must its resistance be low?
Answer
In series with the component. A low resistance means it has as little effect as possible on the current it is measuring.
Why is it a mistake to measure a $0.2$ A current on a 0–10 A ammeter?
Answer
The needle would barely deflect, so the reading would be imprecise. A 0–1 A range would give a much larger, more readable deflection.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A torch bulb is connected to a cell. A current of $0.30$ A flows for $5.0$ minutes.

(a) Define electric current. 1 mark No calculator

Mark scheme

B1 the charge passing a point per unit time (accept the equation with symbols defined)

(b) Calculate the charge that flows through the bulb. 3 marks

Mark scheme

C1 time converted to 300 s
C1 Q = It used
A1 90 C; unit required

(c) State how an ammeter should be connected to measure this current. 1 mark No calculator

Mark scheme

B1 in series with the bulb

Question 2. A student compares the output of a cell with the output of a laboratory a.c. supply using an oscilloscope.

(a) State the difference between direct current and alternating current. 2 marks No calculator

Mark scheme

B1 d.c. flows in one direction only
B1 a.c. repeatedly changes / reverses direction

(b) Describe how the two traces would differ on the oscilloscope screen. 2 marks No calculator

Mark scheme

B1 d.c. gives a horizontal line, not crossing the zero axis
B1 a.c. gives a wave that crosses the axis, going above and below zero

(c) State the direction in which the free electrons move in a wire carrying a direct current. 1 mark No calculator

Mark scheme

B1 from negative to positive (the opposite of conventional current)

Question 3. A student is measuring a current expected to be about $0.05$ A, and is given ammeters with ranges 0–0.1 A, 0–1 A and 0–5 A.

(a) Determine which ammeter the student should use, and justify the choice. 2 marks No calculator

Mark scheme

B1 the 0–0.1 A meter
B1 the reading is about half of full scale, so the deflection is large and the reading precise; the other ranges give too small a deflection

(b) Suggest one advantage of a digital ammeter over an analogue one for this measurement. 1 mark No calculator

Mark scheme

B1 no parallax error when reading (accept: gives a direct numerical reading / easier to read small values precisely)

(c) Describe, in terms of electrons, what is happening in the connecting wires while this current flows. 2 marks No calculator

Mark scheme

B1 free electrons in the metal drift in one direction through the wire
B1 this flow of charge constitutes the current

Words to know

Electric current: The charge passing a point per unit time, measured in amperes.
Ampere (A): The unit of current. One ampere is one coulomb per second.
Conventional current: The direction from positive to negative, used in all circuit rules.
Direct current (d.c.): Current that flows in one direction only.
Alternating current (a.c.): Current that repeatedly reverses direction.
Parallax error: A reading error caused by viewing an analogue scale from an angle rather than straight on.

All Physics lessons Next: 4.2.3 Electromotive force and potential difference