What an electric current is
- Electric current is the rate of flow of electric charge around a circuit
- Current is measured in amperes, symbol A (often shortened to "amps")
- One ampere is the current that flows when one coulomb of charge passes a point in one second, so 1 A = 1 C/s
- For current to flow, there must be a complete (unbroken) conducting loop with a source of voltage in it, such as a cell or battery
Charge carriers in different materials
- In a metallic conductor, the moving charges are negatively charged electrons; the heavy positive metal ions are fixed in a lattice and only the outermost electrons drift through it
- In a liquid electrolyte or ionised gas, the moving charges include both positive and negative ions, which drift in opposite directions
Conventional current vs electron flow
- Long before electrons were discovered, scientists agreed that current flows from the positive terminal of a cell, around the external circuit, and back to the negative terminal. This historical choice is called the conventional current direction, and it is the direction shown by every arrow on every IGCSE circuit diagram
- In a metal wire, the electrons actually drift the other way, from the negative terminal, through the wire, back to the positive terminal
- Both descriptions refer to exactly the same physical situation; they just measure the flow in opposite senses. In exam answers, always use the conventional direction unless asked for electron flow explicitly
Measuring current
- An ammeter is the instrument that measures current
- An ammeter must be wired in series with the component whose current is being measured, because the same current that flows through the component must also flow through the ammeter
- An ideal ammeter has zero resistance, so inserting it does not affect the circuit it is measuring
The charge equation
Q = I × t
- where:
- Q = charge that has passed (coulombs, C)
- I = current (amps, A)
- t = time the current has been flowing (s)
- Rearrangements:
- I = Q / t (current is charge per second)
- t = Q / I
- Convert any milliamp values into amps before substituting: 1 mA = 1 × 10⁻³ A. A current of 250 mA, for example, is 0.250 A
Example — a current of 5.0 A passes through a hairdryer element for 6.0 s. Calculate the charge that has flowed.
- Q = I × t = 5.0 × 6.0 = 30 C