Electric Current and Charge

Static electricity is charge that stays put; current electricity is charge on the move. The moment you close a switch and complete a circuit, charge begins to flow round it and energy is delivered to lamps, motors and heaters. But what exactly is "current", how is it linked to the charge you met in the last lesson, and what has to be true for a current to flow at all? This lesson, part of Topic P3 of OCR Gateway Combined Science A, defines electric current as the rate of flow of charge, introduces the key equation $Q = It$Q=It, sorts out the difference between conventional current and the flow of electrons, and works through calculations with the correct units.

By the end of this lesson you should be able to define electric current as the rate of flow of charge, recall and use $Q = It$Q=It, state the conditions needed for a current to flow, explain the difference between conventional current and the flow of electrons, and carry out worked calculations.

This lesson combines AO1 understanding of current and conventional current direction with substantial AO2 application when you rearrange and use $Q = It$Q=It in worked calculations, converting units correctly.

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What Is an Electric Current?

An electric current is a flow of electric charge. In a metal wire the charge is carried by the electrons that are free to move through the metal; when a cell or battery is connected, it pushes these free electrons so that they drift around the circuit, and this flow of charge is the current.

More precisely, the current is the rate of flow of charge — that is, how much charge passes a point each second. A large current means a lot of charge flows past every second; a small current means only a little. Current is given the symbol $I$I and is measured in amperes (often shortened to amps), symbol $\text{A}$A. One ampere means that one coulomb of charge passes each second. Current is measured with an ammeter, which is connected in series so the same charge that flows through the component also flows through the meter.

Exam Tip: Define current precisely as the rate of flow of charge — the charge passing a point per second. "A flow of electrons" earns partial credit, but the full definition examiners want is in terms of charge per second.

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The Conditions for a Current to Flow

Two things must be true before any current can flow.

First, there must be a complete circuit — an unbroken conducting loop from one terminal of the cell, round the components, and back to the other terminal. If the circuit is broken anywhere (for example by an open switch or a snapped wire), no charge can flow and the current stops everywhere at once, not just at the break.

Second, there must be a source of potential difference — something, such as a cell or battery, that gives the charges energy and pushes them round the loop. Without this "push" the free electrons in a wire simply jiggle about randomly and there is no net flow in any direction.

It is also worth being clear that current is not "used up" as it flows. In a single loop the same current passes through every component, because the charge that enters one end of a component must leave the other end — charge cannot pile up or vanish. The size of the current depends on the potential difference of the supply (a bigger push drives a bigger current) and the resistance of the circuit (more opposition allows less current); this is the relationship $V = IR$V=IR, which you will meet in full in the next lesson.

Exam Tip: For a current to flow you need both a complete circuit and a source of potential difference (a cell or battery). A common misconception is that the current is "used up" going round — in a single loop the current is the same everywhere.

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The Charge Equation: $Q = It$Q=It

The link between current and charge is one of the most important equations in the whole topic:

$Q = It$Q=It

where $Q$Q is the charge in coulombs ($\text{C}$C), $I$I is the current in amperes ($\text{A}$A), and $t$t is the time in seconds ($\text{s}$s). In words: the charge that flows equals the current multiplied by the time for which it flows. This makes sense from the definition — if $I$I coulombs pass each second, then in $t$t seconds a total of $I \times t$I×t coulombs pass.

The equation rearranges to make any quantity the subject:

$Q = It \qquad I = \dfrac{Q}{t} \qquad t = \dfrac{Q}{I}$Q=ItI=tQ​t=IQ​

Worked Example 1 — Finding charge from current and time

A current of $3 \text{ A}$3 A flows through a lamp for $20 \text{ s}$20 s. How much charge passes through the lamp?

Step 1 — write the equation: $Q = It$Q=It.

Step 2 — substitute: $Q = 3 \times 20$Q=3×20.

Step 3 — calculate: $Q = 60 \text{ C}$Q=60 C.

Answer: $60 \text{ C}$60 C of charge passes through the lamp.

Worked Example 2 — Rearranging for current

A charge of $90 \text{ C}$90 C flows through a resistor in $30 \text{ s}$30 s. Calculate the current.

Step 1 — rearrange for current: $I = \dfrac{Q}{t}$I=tQ​.

Step 2 — substitute: $I = \dfrac{90}{30}$I=3090​.

Step 3 — calculate: $I = 3 \text{ A}$I=3 A.

Answer: the current is $3 \text{ A}$3 A.

Worked Example 3 — Rearranging for time, with a unit conversion

A torch draws a current of $0.5 \text{ A}$0.5 A. How long does it take for $300 \text{ C}$300 C of charge to flow through it? Give your answer in minutes.

Step 1 — rearrange for time: $t = \dfrac{Q}{I}$t=IQ​.

Step 2 — substitute: $t = \dfrac{300}{0.5}$t=0.5300​.

Step 3 — calculate: $t = 600 \text{ s}$t=600 s.

Step 4 — convert to minutes: $600 \div 60 = 10 \text{ minutes}$600÷60=10 minutes.

Answer: it takes $600 \text{ s}$600 s, which is $10 \text{ minutes}$10 minutes.

Worked Example 4 — A current given in milliamps

A current of $250 \text{ mA}$250 mA flows through an LED for $40 \text{ s}$40 s. How much charge passes through it?

Step 1 — convert the current to amperes: $250 \text{ mA} = 0.25 \text{ A}$250 mA=0.25 A.

Step 2 — write the equation: $Q = It$Q=It.

Step 3 — substitute: $Q = 0.25 \times 40$Q=0.25×40.

Step 4 — calculate: $Q = 10 \text{ C}$Q=10 C.

Answer: $10 \text{ C}$10 C of charge passes through the LED. (Left as $250$250, the answer would have come out $1000$1000 times too big.)

Exam Tip: In $Q = It$Q=It the time must be in seconds and the current in amperes. If a question gives a time in minutes ($\times 60$×60) or a current in milliamps ($\div 1000$÷1000), convert it first. Forgetting to convert is the commonest slip in these calculations.

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What Is Really Moving? A Closer Look at the Electrons

It helps to picture what is happening inside the wire when a current flows, because a common exam mistake is to imagine electrons whizzing round the circuit at enormous speed the instant the switch closes. The reality is more subtle, and understanding it makes several later ideas easier.