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Electric Current, Potential Difference and Electric Circuits

Electric current is the rate of flow of charge, I = Q/t, measured in amperes with an ammeter joined in series. Potential difference is the work done to move a unit charge between two points, V = W/Q, measured in volts with a voltmeter joined in parallel. Charge flows only in a closed circuit.

🎬 Step-by-step story

  1. Here is a simple circuit: a cell, a switch, a bulb and an ammeter joined by copper wire. The switch is open, so the path is broken and nothing flows.
  2. Close the switch. Now the path is complete, the tiny charges start to move and the bulb glows. The amount of charge passing each second is the current: I = Q ÷ t.
  3. Look at the two arrows. Electrons really move from the − end to the + end. But by old agreement we draw the current from + to −. That is conventional current.
  4. A green voltmeter is joined across the bulb. Now we add a second cell. The push, called potential difference, rises from 1.5 V to 3 V, and the current rises too.
  5. Every part has a short name and a symbol we use in circuit diagrams: cell, switch (key), bulb, ammeter in series, voltmeter in parallel.
  6. Your turn: switch it on and off, change the number of cells, and watch the ammeter, voltmeter and the charge counter below.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does the bulb not glow when the switch is open?

An open switch breaks the path. Charge can only keep flowing round a closed loop, so the current is zero everywhere in the circuit.

Does current get used up in the bulb?

No. The ammeter reading is the same before and after the bulb because the same charges come out that went in. The bulb takes energy from the charges, not the charges themselves.

If electrons go from − to +, why do we draw current from + to −?

The direction was fixed before electrons were known. We keep the old rule (conventional current) so that all books and diagrams agree.

Why is the voltmeter connected across the bulb and not in the loop?

It measures the difference between two points, so its two leads must touch both ends of the bulb. Its very high resistance means almost no current goes through it.

Why does adding a cell make the current bigger?

Two cells give a bigger push (3 V instead of 1.5 V). The same bulb then lets more charge through every second.

Do charges move very fast in a wire?

Electrons drift slowly, less than a millimetre per second, but the whole wire is already full of them, so they all start moving almost at once when you close the switch.

What is electric charge and electric current?

All matter has tiny charged particles. In a metal wire, some electrons are free to move. When they drift in one direction, charge flows. The electric current is how much charge passes a point every second:

I = Q / t

Charge Q is measured in coulombs (C), time t in seconds and current I in amperes (A). So 1 ampere means 1 coulomb of charge passing every second. One coulomb is the charge of about 6.25 × 1018 electrons, and one electron carries 1.6 × 10−19 C. Small currents are written in milliamperes (1 mA = 10−3 A) or microamperes (1 µA = 10−6 A).

Direction of electric current

Scientists agreed on the direction of current before electrons were discovered. They chose the direction in which positive charge would move: from the positive terminal to the negative terminal outside the cell. This is conventional current. Electrons are negative, so they actually drift the other way, from − to +. In all circuit diagrams we show conventional current.

Electric circuit and the ammeter

An electric circuit is a closed, unbroken path for charge. If the switch (plug key) is open or a wire is broken, the circuit is open and no current flows anywhere in it.

Current is measured with an ammeter. It is always joined in series, so that the same current passes through it. An ammeter has very low resistance, so it hardly changes the current it measures. Its + terminal goes towards the + side of the cell.

What is potential difference?

Charge does not flow by itself. It needs a push, just as water needs a height difference to flow down a pipe. A cell does chemical work to keep one terminal at a higher electric potential than the other. The potential difference (p.d.) between two points is the work done to move one unit of charge from one point to the other:

V = W / Q

Its SI unit is the volt (V): 1 volt = 1 joule per coulomb. So if 1 J of work moves 1 C of charge between two points, the p.d. is 1 V. Potential difference is measured with a voltmeter, joined in parallel (across) the part. A voltmeter has very high resistance, so it takes almost no current.

Circuit diagram symbols

Drawing real batteries and bulbs takes time, so we use standard symbols:

In the exam you are often asked to draw a circuit with a cell, key, resistor, ammeter in series and voltmeter across the resistor. Practise drawing it neatly with straight lines.

Key formulas and definitions

Worked examples

1. A charge of 30 C passes through a bulb in 1 minute. Find the current.

t = 60 s. I = Q / t = 30 / 60 = 0.5 A.

2. A current of 2 A flows for 5 minutes. How much charge flows?

t = 5 × 60 = 300 s. Q = I × t = 2 × 300 = 600 C.

3. How much work is done in moving 4 C of charge across a p.d. of 12 V?

W = V × Q = 12 × 4 = 48 J.

4. How many electrons make up a charge of 1 C?

n = Q / e = 1 / (1.6 × 10⁻¹⁹) = 6.25 × 10¹⁸ electrons.

5. A current of 0.4 A flows in a wire. How many electrons pass any point in 2 seconds?

Q = I × t = 0.4 × 2 = 0.8 C. n = Q / e = 0.8 / (1.6 × 10⁻¹⁹) = 5 × 10¹⁸ electrons.

6. A battery does 360 J of work to push charge for 2 minutes through a circuit with a 3 V p.d. Find the charge and the current.

Q = W / V = 360 / 3 = 120 C. t = 120 s, so I = Q / t = 120 / 120 = 1 A.

Common mistakes

Practice quiz

1. The SI unit of electric current is:
2. An ammeter is always joined:
3. 1 volt is equal to:
4. 10 C of charge flows in 5 s. The current is:
5. The direction of conventional current is:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is electric current in simple words?

It is the amount of electric charge passing a point in one second. I = Q / t, measured in amperes.

What is the difference between current and potential difference?

Current is the flow of charge; potential difference is the push (work per unit charge) that makes it flow. Current is measured in amperes with an ammeter in series; p.d. in volts with a voltmeter in parallel.

Is this in the CBSE Class 10 syllabus?

Yes. Electric current, potential difference and circuit diagrams open the Electricity chapter of CBSE Class 10 Science (unit Effects of Current).

Where this is taught

Canada (Ontario)Grade 9D. Physics
NetherlandsVWO 3 (onderbouw)Electricity and energy
NetherlandsVWO 5Charge and field (part 1)
PolandSzkoła podstawowa, klasa VIIIElectricity
RomaniaClasa a VIII-aElectric and magnetic phenomena
RomaniaClasa a IX-aElectrical
RomaniaClasa a IX-aElectronics and automation
Spain2º ESOEnergy
Spain2º ESOProblem-solving process
Spain3º ESOEnergy
Spain3º ESOProblem-solving process
Spain1º BachilleratoElectrical and electronic systems
CBSE (India)Class 10Effects of Current
England (GCSE, A level)Year 9Physics: Electricity and electromagnetism
USA (Common Core, NGSS, AP)Grade 9Energy
Japan中学2年Field 1 (3): Electric current and its uses
Japan高校（専門学科）1〜3年Electrical Theory
Germany (Bavaria)Jahrgangsstufe 8Electric current
FranceQuatrièmeEnergy
FrancePremièrePhysics-chemistry: Energy
FranceTerminalePhysics-chemistry: Energy
FranceTerminalePhysics-chemistry: Energy conversions and transfers
Russia8 классElectric and magnetic phenomena
Russia8 классElectric and magnetic phenomena
China九年级（初三）Ch.15 Current and circuits
China九年级（初三）Ch.16 Voltage and resistance

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