Charge, current and voltage

Current is a flow of charge

$$I = \frac{q}{t}$$

• $I$ — current (amperes, A)

• $q$ — charge that flows past a point (coulombs, C)

• $t$ — time taken (seconds, s)

• One ampere is one coulomb per second.

• Current is measured with an ammeter, which is always connected in series — the charge must flow through it to be counted.

• In a metal, the moving charges are free electrons.

Conventional current

• Conventional current is drawn flowing from the positive terminal of the supply, round the circuit, to the negative terminal.
• The electrons actually move the other way, from negative to positive, because they are negatively charged.
• This is a historical convention, fixed before the electron was discovered. Use conventional current for every circuit diagram and every hand rule — but be ready to explain the difference if asked.

Voltage is energy per coulomb

$$V = \frac{\Delta E}{q}$$

• $V$ — voltage, or potential difference (volts, V)

• $\Delta E$ — energy transferred (joules, J)

• $q$ — charge that moved (C)

• One volt is one joule per coulomb.

• Voltage measures how much energy each coulomb of charge carries or delivers.

• Voltage is measured with a voltmeter, always connected in parallel across the component — it compares the energy of the charge on either side.

• Two words that describe the same quantity in different places:

  • Supply voltage (emf) — the energy given to each coulomb by the battery or cell.
  • Potential difference — the energy delivered by each coulomb to a component.

Where the energy comes from and goes

• The battery does not supply the charge — the charges are already in the wires.
• The battery supplies energy, pushing the charges around the circuit.
• Each coulomb picks up energy in the battery and delivers it to the components.
• Charge is conserved: the same current leaves a component as enters it. A bulb does not "use up" current.
  • This is one of the most persistent misconceptions in the whole topic. Energy is used up; charge is not.

Circuit symbols and diagrams

You are expected to draw and read standard circuit diagrams:

• cell — a long thin line (positive) and a short thick line (negative)

• battery — two or more cells in a row

• resistor — a plain rectangle

• variable resistor — a rectangle with an arrow through it

• lamp — a circle with a cross in it

• switch — a break in the line with a hinged lever

• ammeter — a circle with A, connected in series

• voltmeter — a circle with V, connected in parallel

• Draw the wires as straight lines with right-angled corners, and keep components spaced out. A neat diagram earns the mark; a sketchy one often does not.