Electrostatics studied charges sitting still. Now we let them move. Electric current is nothing more mysterious than charge flowing past a point, count the coulombs per second and you have the current in amps. There is one famous historical twist: the current we draw on diagrams flows the opposite way to the electrons. This lesson untangles it.
Two wires carry charge. In one, $4\ \text{C}$ passes a point every second; in the other, $4\ \text{C}$ passes every two seconds.
Before reading: which wire carries the larger current, and what do you think "current" is measuring? Write your prediction.
Count the coulombs passing each second
Current just measures how fast charge flows, the more coulombs that pass a point each second, the bigger the current.
Electric current is the rate at which charge flows past a point in a circuit. If a charge $q$ passes a point in a time $t$, the current is the NESA formula-sheet relationship:
$$I = \frac{q}{t}$$
From this definition:
Electric current is the rate of flow of charge: $I = \dfrac{q}{t}$, measured in amperes (A), where $1\ \text{A} = 1\ \text{C s}^{-1}$. The charge that flows in time $t$ is $q = It$.
Pause, copy the highlighted definition into your book before moving on.
Did you get this? True or false: a current of one ampere means one coulomb of charge passes a point every second.
Charge and time into $I = \dfrac{q}{t}$
We just saw what current means. That raises a question: how do we find it from a given charge and time? This card answers it → divide the charge by the time using $I = \dfrac{q}{t}$.
Current calculations are direct division, the only care needed is consistent SI units of coulombs and seconds.
Use charge in coulombs and time in seconds; the current comes out in amperes.
A charge of $36\ \text{C}$ flows past a point in a wire over $1.5\ \text{minutes}$. Find the current in the wire.
To find current, convert the time to seconds and divide the charge by it: $I = \dfrac{q}{t}$ (answer in amperes). Rearrange to $q = It$ to find charge, or $t = \dfrac{q}{I}$ to find time.
Add the highlighted worked method to your notes before the check below.
Quick check: A current of $2.0\ \text{A}$ flows for $10\ \text{s}$. How much charge passes a point in that time ($q = It$)?
Pattern A, Calculate then reason
A torch globe draws a current of $0.30\ \text{A}$. In your book: (1) calculate the charge that flows through it in $2.0\ \text{minutes}$ using $q = It$; (2) work out how many electrons this represents (each electron carries $1.6 \times 10^{-19}\ \text{C}$); (3) state in one sentence why the current would be larger if more charge flowed each second.
Fill the blanks: Complete the current calculation.
Current is the rate of flow of charge, $I = q/t$, measured in . A charge of $12\ \text{C}$ passing a point in $4.0\ \text{s}$ is a current of A.
Dot point: Calculate electric current using I = q/t
Why diagrams show current flowing the "wrong" way
We just learned to measure how much charge flows. That raises a question: which way does it flow, and why do diagrams disagree with the electrons? This card answers it → a historical convention defines current as positive-charge flow.
Long before electrons were discovered, scientists guessed that current was positive charge moving, and we kept that convention even after finding it is really electrons.
The direction of conventional current is defined as the direction in which positive charge would flow: from the positive terminal of a source, around the external circuit, to the negative terminal. In a metal conductor, however, the charge carriers are electrons, which are negative, so they actually drift the other way, from the negative terminal to the positive terminal.
Conventional current is defined as the direction positive charge would flow: from + to − in the external circuit. In a metal the carriers are electrons (negative), so they flow the opposite way, from − to +. All circuit equations use the conventional-current direction.
Pause, copy the highlighted convention into your book before the check below.
Last check: Decide if the statement is true or false.
In a metal wire, the electrons drift in the opposite direction to the conventional current.
Two truths, one lie: three of these statements about current DIRECTION are correct. Which one is the lie?
Dot point: Relate the flow of charge to the direction of conventional current
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
ApplyBand 5(4 marks) 1. A current of $1.5\ \text{A}$ flows through a wire for $40\ \text{s}$. Calculate the total charge that passes a point in the wire, and explain the difference between the direction of conventional current and the direction of electron flow in the wire.
1 mark: uses $q = It$ · 1 mark: $q = 60\ \text{C}$ · 1 mark: conventional current flows + to − (direction of positive-charge flow) · 1 mark: electrons (carriers in the metal) flow the opposite way, − to +
ApplyBand 4(3 marks) 2. A charge of $18\ \text{C}$ passes a point in a wire in $45\ \text{s}$. Calculate the current, and determine how long it would take for $60\ \text{C}$ to pass the same point at that current.
1 mark: $I = q/t = 0.40\ \text{A}$ · 1 mark: rearranges to $t = q/I$ · 1 mark: $t = 150\ \text{s}$
Dot point: Calculate electric current using I = q/t
ExplainBand 5(3 marks) 3. Explain the difference between conventional current and electron flow in a metal conductor, and explain why the conventional direction is still used even though it is not the direction the charge carriers move.
1 mark: conventional current is the direction positive charge would flow, from + to − externally · 1 mark: in a metal the carriers are electrons, which drift the opposite way · 1 mark: the convention predates the discovery of the electron and all circuit rules are written consistently with it, so it gives correct results
Dot point: Relate the flow of charge to the direction of conventional current
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): The total charge is $q = It = (1.5)(40) = 60\ \text{C}$. Conventional current is defined as the direction in which positive charge would flow, from the positive terminal, around the external circuit, to the negative terminal. In a metal wire, however, the actual charge carriers are electrons, which are negatively charged, so they drift in the opposite direction, from the negative terminal to the positive terminal. The two directions are therefore opposite, but all circuit calculations are based on the conventional-current direction.
Q2 (3 marks): The current is $I = q/t = 18/45 = 0.40\ \text{A}$. To find the time for a different charge at the same current, rearrange the same relationship: $t = q/I = 60/0.40 = 150\ \text{s}$, which is two and a half minutes. The relationship is linear, so tripling the charge from $18\ \text{C}$ to just over three times that value triples the time in the same proportion.
Q3 (3 marks): Conventional current is defined as the direction in which positive charge would flow, which in the external circuit is from the positive terminal of the supply to the negative terminal. In a metal conductor the only mobile charge carriers are electrons, and because they are negative they drift in exactly the opposite direction, from the negative terminal towards the positive one. The convention was fixed before the electron was discovered, so it was a reasonable guess that turned out to be backwards. It is retained because a flow of negative charge one way is electrically equivalent to a flow of positive charge the other way, and because every circuit rule and equation has been written to be consistent with it. Using the convention therefore gives correct answers, provided you remember which direction the electrons themselves move when asked.
You can now calculate current with $I = \dfrac{q}{t}$ and explain conventional current versus electron flow. Next lesson looks at how charge moves through conductors and compares direct current with alternating current.
↩ Back to focus-area hubAt the start you compared two wires and guessed what current measures. Current is the rate of flow of charge, $I = \dfrac{q}{t}$, so the wire carrying $4\ \text{C}$ every second has twice the current of the one carrying $4\ \text{C}$ every two seconds. The twist: conventional current points opposite to the electrons that actually move in a metal.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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