Electricity

Electric current is the rate of flow of electric charge. Imagine a river; the water flowing is like the charge, and the speed of the river's flow is the current. In metal wires, the charge carriers are tiny, negatively charged particles called electrons. Current is measured in amperes (A), often called 'amps'. For a current to flow, there must be a complete circuit and a source of potential difference, like a battery. The total charge (Q) that flows is the current (I) multiplied by the time(t) it flows for. Charge is measured in coulombs (C). One coulomb is the charge that passes a point in one second when the current is one amp. A key principle is that charge is 'quantised', meaning it comes in discrete packets. The smallest packet of charge is the 'elementary charge' (e), which is the magnitude of the charge on a single electron or proton, approximately 1.60 x 10⁻¹⁹ C.

While we imagine current flowing smoothly, on a microscopic level it's about the collective movement of charge carriers, usually electrons in a metal. These electrons don't zoom straight through the wire. They constantly collide with the metal ions in the lattice structure, causing them to move in a chaotic, random way. When a potential difference is applied, they are given a slow, net 'nudge' in one direction. This slow net speed is called the mean drift velocity (v). It's surprisingly slow, often less than 1 mm per second. So why does a light turn on instantly? Because the electric field that pushes the electrons propagates at nearly the speed of light, so all electrons in the wire start drifting almost simultaneously. The current (I) is related to this drift velocity by the equation I = nAve, where 'n' is the number density of charge carriers (how many free electrons per cubic metre), 'A' is the cross-sectional area of the wire, 'v' is the drift velocity, and 'e' is the elementary charge.

What makes charge flow? An 'electrical push' is needed. This push is provided by a source like a battery or power supply. We have two key terms for this: electromotive force (e.m.f.) and potential difference (p.d.). Both are measured in volts (V). Electromotive Force (e.m.f.): This is the energy supplied *by the source* to each coulomb of charge. Think of it as the total energy given to each charge to make a full lap of the circuit. It is the work done per unit charge in converting chemical or other energy into electrical energy. Potential Difference (p.d.): This is the energy converted *by a component* from electrical energy into other forms (like light, heat, sound) for each coulomb of charge that passes through it. It is the work done per unit charge as it passes through the component. The definition of the volt links them: 1 Volt is 1 Joule per Coulomb (1 V = 1 J/C). So, a 6V battery gives 6 joules of energy to every coulomb of charge. A lamp with a 2V p.d. across it turns 2 joules of electrical energy into light and heat for every coulomb that passes through.

Resistance is a measure of how much a component opposes the flow of electric current. Think of it as electrical friction. A component with high resistance will allow less current to flow for a given potential difference compared to a component with low resistance. Resistance (R) is measured in ohms (Ω). The relationship between potential difference (V), current (I), and resistance (R) for many components is described by Ohm's Law. It states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature and other physical conditions remain constant. This relationship is summarised in the famous equation V = IR. A component that obeys Ohm's Law is called an 'ohmic' conductor (e.g., a resistor at constant temperature), and its V-I graph is a straight line through the origin. Components like filament lamps or diodes are 'non-ohmic' as their resistance changes, and their V-I graphs are curved.

Electrical power is the rate at which electrical energy is transferred or converted. Since power is energy per unit time (P = E/t) and potential difference is energy per unit charge (V = E/Q), we can combine these ideas. We know current is charge per unit time (I = Q/t). By substituting, we get the fundamental equation for electrical power: P = VI. This tells you the power dissipated in a component is the product of the potential difference across it and the current flowing through it. Power is measured in watts (W), where 1 watt is 1 joule per second. Using Ohm's Law (V=IR), we can derive two other useful power equations: P = I²R and P = V²/R. These are great for when you don't know V or I respectively. Electrical energy (E) is simply power multiplied by time (E = Pt). Combining this with our power equations gives us E = VIt, E = I²Rt, and E = (V²/R)t.