What is electric current?
Electric current is the rate of flow of electric charge through a conductor. When free electrons drift steadily along a wire, charge is transported. Current is measured in amperes (A) with an ammeter connected in series. One ampere is one coulomb of charge passing a point each second.
The relationship is I = Q / t, where I is current (A), Q is charge (C) and t is time (s). Conventional current flows from the positive terminal to the negative terminal, opposite to the direction of electron flow.
Key formula
I = Q / t so Q = I t
Current I in A, charge Q in C, time t in s.
What is potential difference?
Potential difference (p.d.), or voltage, across two points is the electrical energy converted to other forms per unit charge passing between the points. It is measured in volts (V) using a voltmeter connected in parallel across the component. One volt means one joule of energy is transferred per coulomb of charge.
The defining equation is V = W / Q, where W is energy (J) and Q is charge (C). A battery raises the potential energy of charge; a resistor converts that energy into heat.
Worked example
A charge of 60 C flows through a lamp in 30 s, transferring 720 J of energy. Find the current and the p.d. across the lamp.
Current: I = Q / t = 60 / 30 = 2 A.
P.d.: V = W / Q = 720 / 60 = 12 V.
Remember
- Ammeter in series; voltmeter in parallel.
- Conventional current is opposite to electron flow.
- 1 A = 1 C s⁻¹ and 1 V = 1 J C⁻¹.