A simple circuit consists of a lamp connected to a battery.
In a circuit:
Electric current is a flow of electric charges. The lamp lights up because charged particles are moving through it. The charged particles move through conductors.
A copper wire consists of millions of copper atoms. Most of the electrons are held tightly to their atoms, but each copper atom has 1 or 2 electrons which are loosely held. Since these electrons are negatively charged, an atom that loses an electron is left with a positive charge and is called an ion.
The diagram shows that the copper wire is made up of a lattice of positive ions surrounded by free electrons. The ions can only vibrate about their fixed positions, but the electrons are free to move from one ion to another through the the lattice.
All metals have a structure like this (lattice).
When a battery is attached to the copper wire, the free electrons are repelled by the negative terminal and attracted to the positive one. They still have a random movement but they now also all move slowly in the same direction through the the wire with a steady drift velocity. ****Electrons in a circuit flow from negative to positive.
Conductors and insulators
Some materials are conductors, while others are insulators. This property is determined by the number of free electrons per m$^3$.
free electrons per m$^3$ $\propto$ conductivity
| type of material | number of free electrons per mm^3 | n, number of free electrons per m^3 |
|---|---|---|
| conductor | ~ 1 * 10^20 | ~ 1 * 10^29 |
| semiconductor | ~ 1 * 10^10 | ~ 1 * 10^19 |
| insulator | ~ 1 | ~1 * 10^9 |
Current is measured in Amperes (A) using an ammeter. The ammeter is placed in the circuit so that the electrons pass through it. The more electrons that pass through it in one second, the higher the current in amps. The position of an ammeter in a circuit does not matter; current is constant throughout a circuit.
1 Ampere is a flow of about 1 $\times$ 10$^{18}$ electrons per second . The single electron is too small to be used as the basic unit of of charge, so instead a bigger unit is used, the coulomb (C). The charge on one electron is 1.6 $\times$ 10$^{-19}$ C (1A = 1 C s$^{-1}$). You can picture a coulomb as a group of a large number of electrons. The more coulombs of charge that pass through an ammeter per second, the bigger the current.
$\text{current, }I = \frac{\text{charge, }Q}{\text{time, }t}$ or $I=\frac{Q}{t}$ so $Q = I\times t$
1 coulomb is the amount of charge that passes a point when a current of 1 ampere flows for 1 second, and current is the rate of of flow of charge.