Lesson 01 · Electricity basics

Charge, current and voltage

Every circuit runs on three ideas. Charge is what electrons carry, counted in coulombs. Voltage is the push, the energy each coulomb gets, in volts. Current is how much charge flows past a point each second, in amperes. Wire a 1.5 V battery, a bulb and a switch on a board, picture it as water in pipes for a moment, then close the loop and watch current flow from + to − while the electrons drift the other way.

10 min10 steps5 questionsBeginner

Fig 1 · Interactive

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01Electric charge is measured in coulombs (C). What is one coulomb?

One electron carries a tiny charge, 1.602176634 × 10⁻¹⁹ C. So it takes about 6.24 × 10¹⁸ (6.24 billion billion) electrons to make 1 coulomb.

02A 1.5 V battery: how much energy does each coulomb of charge get from it?

Voltage is energy per charge: 1 V = 1 J/C. So 1.5 V gives each coulomb 1.5 joules (J) of energy. Coulombs (C) count charge and amperes (A) measure current, not energy.

03A meter counts 2 coulombs passing in 4 seconds. What is the current?

Current is charge per second: 1 A = 1 C/s. So current = 2 C / 4 s = 0.5 A.

04The switch is open but the battery is still connected. Why does the bulb stay dark?

The battery still has its 1.5 V push, but current needs a closed loop. With a gap anywhere in the loop, no current flows anywhere.

05Which way do the electrons in the wire drift while the bulb is lit?

Conventional current is drawn from + to −, the way positive charge would move. Electrons are negative, so they drift the other way, from − to +, and slowly: well under a millimetre per second here.

Key terms

Glossary

Electric charge: A property of particles such as electrons (negative) and protons (positive). It is measured in coulombs (C).
Electron: A tiny particle with a negative charge of 1.602176634 × 10⁻¹⁹ C. In a metal wire, some electrons are free to move.
Coulomb (C): The unit of charge: the charge of about 6.24 billion billion (6.24 × 10¹⁸) electrons.
Current: How much charge flows past a point each second. Measured in amperes: 1 A = 1 C/s.
Voltage: The push that drives current: the energy, in joules, that each coulomb of charge gets, measured between two points. 1 V = 1 J/C.
Circuit: A closed loop that charge can flow all the way round. A gap anywhere stops the current everywhere.
Conventional current: The direction current is drawn in: from the + end to the − end, the way positive charge would move. Electrons drift the opposite way.
Joule (J) and watt (W): The joule is the unit of energy. The watt is the unit of power, energy used each second: 1 W = 1 J/s. In a simple circuit, power = voltage × current.

Fine print

Notes and sources

The board circuit is a teaching model: the 0.5 A bulb current and the 0.5 mm² wire are round numbers chosen to show the idea. The 1.5 V is the nominal voltage of one AA alkaline cell.
Units: the coulomb (C) is the unit of charge, the joule (J) of energy, the ampere (A) of current (1 A = 1 C/s), the volt (V) of energy per charge (1 V = 1 J/C) and the watt (W) of power (1 W = 1 J/s), as defined in the SI.
The elementary charge e = 1.602176634 × 10⁻¹⁹ C is exact since the 2019 redefinition of the SI, so one coulomb is about 6.24 × 10¹⁸ electrons.
Current is drawn in the conventional direction (from + to −); the electrons in the wires drift the opposite way.
Drift speed: v = I / (n e A) with n = 8.34 × 10²⁸ free electrons per m³ for copper (OpenStax). For 0.5 A in 0.5 mm² of copper that is about 0.075 mm/s.
The water picture (pump = battery, pressure = voltage, flow rate = current) is only an analogy; it is set aside after step 3.

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