Electricity is the presence and movement of electric charge. In a solid metal conductor the charges that move are free electrons; in an electrolyte, in a plasma or inside a semiconductor other charge carriers can move as well. Circuit analysis describes that movement as conventional current, a sign convention that flows from the positive terminal through the external circuit to the negative terminal.
Charge and current
Charge is measured in coulombs (C). The practical meaning of current is the rate at which charge passes a chosen point: twice the charge in the same time is twice the current.
Charge through a lamp
Problem. A charge of 12 C passes through a lamp in 4 s. What is the current?
Solution. I = Q ÷ t = 12 C ÷ 4 s = 3 A.
Potential difference and energy
Voltage, or potential difference, is the energy transferred per unit charge between two points. A source raises the energy of the charge; a load lets that energy leave the circuit as heat, light or mechanical work.
Voltage from energy and charge
Problem. A source transfers 24 J of energy to 2 C of charge. What is the potential difference?
Solution. V = W ÷ Q = 24 J ÷ 2 C = 12 V.
- Current is a flow and is measured in series with the circuit.
- Voltage is a difference between two points and is measured in parallel.
- Charge is conserved: it is not created or destroyed, only moved.
- A complete path is needed before a steady current can flow.
- Current is charge per unit time: 1 A = 1 C/s.
- Voltage is energy per unit charge: 1 V = 1 J/C.
- Conventional current flows from positive to negative outside the source.
- A circuit needs a source, a load and a closed path.
Worked example
A 12 V battery supplies 3 A for 60 s. The charge moved is Q = I × t = 180 C and the energy delivered is W = V × Q = 12 × 180 = 2160 J.