Electrical quantities

Electricity is all about the movement of electric charge. In metal wires, charge is carried by tiny negatively charged particles called electrons. Electric charge (symbol Q) is a fundamental property of matter and is measured in coulombs (C). When these charges flow through a conductor, we call it an electric current. The size of the current (symbol I) is the rate at which charge flows past a point in the circuit. Think of it like water in a pipe: the charge is the water itself, and the current is how much water flows per second. Current is measured in amperes (A) using an ammeter connected in series. A current of 1 ampere means 1 coulomb of charge is flowing every second. By convention, we talk about 'conventional current' flowing from the positive terminal to the negative terminal, even though in metals it's the electrons that flow the opposite way.

For a current to flow, there needs to be an 'electrical push'. This push is provided by a power source like a battery or power supply. We have two key terms for this 'push': Electromotive Force (e.m.f.) and Potential Difference (p.d.). E.m.f. is the energy supplied per unit charge by the source (e.g., a battery). It's the total energy given to each coulomb of charge. Potential difference (also called voltage) is the energy transferred per unit charge when it passes through a component (e.g., a lamp or resistor). It's the energy 'used' by each coulomb to do work, like creating light and heat. Both e.m.f. and p.d. are measured in volts (V) using a voltmeter connected in parallel across the component or source. One volt is equal to one joule of energy per coulomb of charge (1 V = 1 J/C).

Resistance (symbol R) is a measure of how much a component opposes the flow of electric current. A component with high resistance will allow less current to flow for a given potential difference compared to a component with low resistance. It's like a narrow pipe resisting the flow of water. Resistance is measured in ohms (Ω). The relationship between resistance, potential difference (V), and current (I) is given by the equation R = V / I. This is the definition of resistance. For some components, like a metal wire at a constant temperature, the resistance is constant. This special case is called Ohm's Law, which states that the current through an ohmic conductor is directly proportional to the potential difference across it, provided the temperature and other physical conditions remain unchanged.

The resistance of a wire is not just a random property; it depends on several physical factors. For a wire of a uniform material, the resistance is determined by: 1. Length (L): The longer the wire, the more resistance it has. This is because the electrons have to travel further and collide with more ions in the metal lattice. Resistance is directly proportional to length (R ∝ L). 2. Cross-sectional Area (A): The thicker the wire (larger cross-sectional area), the lower its resistance. A wider path provides more space for electrons to flow, reducing congestion. Resistance is inversely proportional to the cross-sectional area (R ∝ 1/A). 3. Material: Different materials inherently resist current by different amounts. This property is called resistivity (symbol ρ, rho). Materials like copper and silver have very low resistivity, making them excellent conductors. Nichrome has a higher resistivity and is used for heating elements. This is summarised in the equation R = ρL/A.

Electrical power (P) is the rate at which an appliance transfers energy. A 100 W lamp converts 100 joules of electrical energy into light and heat every second. Power is measured in watts (W). It can be calculated by multiplying the potential difference (V) across an appliance by the current (I) flowing through it. The formula is P = V × I. To find the total electrical energy (E) transferred, we multiply the power by the time(t) the appliance is on for. This gives the formula E = P × t. By substituting P = VI into this, we get a very useful equation: E = V × I × t. Energy is measured in joules (J) when power is in watts and time is in seconds. For billing purposes, electricity companies use a larger unit of energy called the kilowatt-hour (kWh). 1 kWh is the energy used by a 1 kW appliance running for 1 hour.