Electric Current, Charge and Energy

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 two lessons, and how does a circuit transfer energy? This lesson, part of Topic P3 (Electricity) of OCR Gateway 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 electron flow, and links charge to the energy it carries through $E = QV$E=QV.

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, explain the difference between conventional current and the flow of electrons, use $E = QV$E=QV to find the energy transferred, and carry out worked calculations with the correct units.

---

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.

For a current to flow 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), no charge can flow and the current stops everywhere at once.

The size of the current depends on two things: 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, met fully in the next lesson — for now, simply note that a higher voltage or a lower resistance gives a larger current. 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 disappear.

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 that examiners want is in terms of charge per second.

---

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.

Exam Tip: In $Q = It$Q=It the time must be in seconds. If a question gives a time in minutes or hours, convert it first ($1 \text{ min} = 60 \text{ s}$1 min=60 s, $1 \text{ hour} = 3600 \text{ s}$1 hour=3600 s). Forgetting to convert is the commonest slip in these calculations.

---

Conventional Current and Electron Flow

Here is a subtlety that catches many students out. Long before anyone knew that electrons existed, scientists agreed to define the direction of current as flowing from the positive terminal of the cell, round the circuit, to the negative terminal. This is called conventional current, and it is the direction marked on circuit diagrams and used in all the rules and equations.

When electrons were later discovered, it turned out that in a metal the electrons actually flow the other way — from the negative terminal, round to the positive terminal — because the negatively charged electrons are repelled by the negative terminal and attracted to the positive one. So:

• Conventional current: positive ($+$+) terminal → negative ($-$−) terminal (the agreed direction we always use).
• Electron flow: negative ($-$−) terminal → positive ($+$+) terminal (the actual direction the electrons drift in a metal).

The two are simply opposite in direction. We keep using conventional current because all the established rules were built around it, and changing now would cause confusion; the important thing is to know that the electrons themselves move the opposite way.

graph LR
  Cell["Cell + terminal"] -->|conventional current| Comp["Component"]
  Comp -->|conventional current| CellNeg["Cell − terminal"]
  CellNeg -->|electron flow| Comp2["Component"]
  Comp2 -->|electron flow| Cell

Exam Tip: Conventional current flows from + to − outside the cell; electrons flow the opposite way, from − to +. If a question asks for the current direction, give conventional current ($+$+ to $-$−) unless it specifically asks about the electrons.

---

Charge and Energy: $E = QV$E=QV