Physics: Electricity and magnetism

Electricity is all about the movement of tiny particles called electrons. These electrons carry a property called electric charge, measured in coulombs (C). When these charges flow through a conductor, like a copper wire, we have an electric current. Think of it like water flowing through a pipe: the water molecules are like the charge, and the flow of water is the current. We define electric current (I) as the rate of flow of electric charge (Q). A current of 1 ampere (A) means that 1 coulomb of charge is flowing past a point every second. Current is measured using an ammeter, which must be connected in series in the circuit.

For a current to flow, it needs a 'push'. This electrical push is called voltage, or potential difference. It's the energy given to each unit of charge by the battery or power supply. Voltage (V) is measured in volts (V) using a voltmeter, connected in parallel across a component. As charges flow through a component, they encounter opposition. This opposition to the flow of current is called resistance (R), measured in ohms (Ω). The relationship between voltage, current, and resistance for many components is described by Ohm's Law, which states that the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant. This gives us the most important equation in basic electricity: V = IR.

Components in a circuit can be connected in two main ways: series or parallel. In a series circuit, components are connected end-to-end, forming a single path for the current. The current is the same through all components (I_total = I1 = I2). The total voltage is shared between the components (V_total = V1 + V2). The total resistance is the sum of individual resistances (R_total = R1 + R2). If one bulb breaks in a series circuit, the entire circuit stops working. In a parallel circuit, components are connected on separate branches. The current splits to go down each branch and then recombines (I_total = I1 + I2). The voltage across each branch is the same (V_total = V1 = V2). The total resistance is found using the formula 1/R_total = 1/R1 + 1/R2. If one bulb breaks in a parallel circuit, the others can remain lit.

Electrical power is the rate at which electrical energy is converted into other forms, such as light, heat, or sound. A more powerful appliance converts energy more quickly. Power (P) is measured in watts (W), where 1 watt is 1 joule per second. The power dissipated by a component is calculated by multiplying the voltage across it by the current through it (P = VI). Using Ohm's Law (V = IR), we can also derive two other useful forms of the power equation: P = I²R and P = V²/R. The total electrical energy (E) converted by an appliance is its power multiplied by the time it is on for (E = Pt). Energy is measured in joules (J).

Magnetism is a force exerted by magnets. Magnets have two poles, north and south. Like poles repel, and opposite poles attract. A magnet creates a magnetic field around it, which is a region where other magnetic materials or magnets experience a force. We can represent these fields using magnetic field lines, which point from the north pole to the south pole. A surprising link exists between electricity and magnetism: an electric current produces a magnetic field. This is called electromagnetism. For a long, straight wire, the field lines are concentric circles around the wire. For a coil of wire, called a solenoid, the field inside is strong and uniform, similar to that of a bar magnet. You can find the direction of the field using the Right-Hand Grip Rule: point your thumb in the direction of the conventional current, and your fingers curl in the direction of the magnetic field.

When a wire carrying an electric current is placed in a magnetic field, the two fields interact, and the wire experiences a force. This is known as the motor effect. The force is greatest when the wire is at 90 degrees to the magnetic field lines. The direction of the force can be found using Fleming's Left-Hand Rule. Hold your left hand with your thumb, first finger, and second finger all at right angles to each other. Your First finger points in the direction of the magnetic Field (North to South). Your seCond finger points in the direction of the Current (conventional, + to -). Your ThuMb then points in the direction of the Motion (the force). This effect is the principle behind electric motors, where a coil of wire in a magnetic field is made to rotate continuously.

Just as electricity can create magnetism, magnetism can be used to create electricity. This process is called electromagnetic induction. If you move a wire through a magnetic field, or move a magnet near a wire, a voltage (also called an electromotive force or e.m.f.) is induced across the ends of the wire. If the wire is part of a complete circuit, a current will flow. This happens because the wire is 'cutting' through the magnetic field lines. To get a bigger induced voltage, you can move the wire faster, use a stronger magnet, or use a coil with more turns of wire. The direction of the induced current can be found using Fleming's Right-Hand Rule, which is similar to the left-hand rule but used for generators. This principle is fundamental to how we generate electricity in power stations.

A transformer is a device that changes the size of an alternating voltage. It consists of two coils of wire, a primary coil and a secondary coil, wrapped around a soft iron core. When an alternating voltage is applied to the primary coil, it creates a continuously changing magnetic field in the iron core. This changing magnetic field passes through the secondary coil and induces an alternating voltage across it. A step-up transformer has more turns on its secondary coil than its primary (Ns > Np) and it increases the voltage. A step-down transformer has fewer turns on its secondary coil (Ns < Np) and it decreases the voltage. The ratio of the voltages is equal to the ratio of the number of turns. For a 100% efficient transformer, the power input equals the power output (VpIp = VsIs). Transformers are crucial for the National Grid, stepping up voltage for efficient transmission over long distances and then stepping it down for safe use in homes.