GCSE Physics: The flow of current

Conventional Current flows from positive to negative. On the diagrams below this is from left to right. The current is either carried by the negative particles moving to the left, or the positive particles moving to the right. It is easiest to say that the current flows in the direction that the positive charge moves in. So we say the current flows from the positive terminal to the negative terminal.

Positive particles in a semi-conductor:

Electrons in metal:

Positive and negative ions in a solution:

Measuring Charge and Current

We measure the current in a circuit using an ammeter. The unit of current is the ampere (A), though we call it an "Amp" for convenience. When the curren is big (10A), the charge moved round the circuit quickly. When the current is small (0.01A) the charge moves round the circuit slowly. Current is the rate at which charge flows around a circuit.

Current (I) = Charge flowing (Q) / Time (t)

We could measure the charge by counting the number of electrons flowing. However, electrons only have a very small charge. Our unit of charge is the coulomb. On coulomb is equivalent to the charge on six million million (6 * 10^18) electrons.