Electricity and Circuits

Electric charge is a property of particles that causes electrical forces. Protons have a positive charge, electrons have a negative charge and neutrons have no charge. Like charges repel and opposite charges attract. Charge is measured in coulombs (C).

Current is the rate of flow of electric charge, measured in amperes (A). One ampere means one coulomb of charge passes a point every second. Formula: I = Q/t. Conventional current flows from positive to negative, while electrons move in the opposite direction in a metal wire.

Voltage is the energy transferred per unit charge between two points, measured in volts (V). Formula: V = E/Q. A higher potential difference can transfer more energy to each coulomb of charge. In an X-ray tube, voltage accelerates electrons towards the target.

Resistance describes how strongly a component opposes electric current. It is measured in ohms (Ω). For a fixed voltage, greater resistance produces less current. Resistance depends on factors including material, wire length, cross-sectional area and temperature.

Ohm's law states that voltage equals current multiplied by resistance: V = IR. Rearranging gives I = V/R and R = V/I. Example: a 12 V supply across a 4 Ω resistor produces a current of 3 A. This relationship applies to ohmic components when physical conditions remain constant.

Electrical power is the rate of energy transfer, measured in watts (W). Formula: P = VI. For a resistor, P = I²R or P = V²/R. Example: a device using 2 A at 12 V has a power of 24 W.

Electrical energy is the energy transferred by an electrical device over time. Formula: E = Pt, where energy is in joules (J), power in watts and time in seconds. Example: a 100 W device operating for 10 seconds transfers 1,000 J.

In a series circuit, components are connected along one path. The same current flows through every component, while the supply voltage is shared. Total resistance equals the sum of individual resistances: Rtotal = R1 + R2 + ... . If one component breaks the path, current stops throughout the circuit.

In a parallel circuit, components are connected across separate branches. Each branch has the same potential difference as the supply, while total current divides between branches. Formula: Itotal = I1 + I2 + ... . If one branch is disconnected, other branches can continue operating.

Series circuits have one current path, the same current through every component and divided voltage. Parallel circuits have multiple current paths, the same voltage across branches and divided current. Adding resistance in series increases total resistance; adding a parallel branch decreases equivalent resistance.

A switch opens or closes an electrical path. Two switches connected in parallel can create an OR circuit: closing either switch completes a path and allows a lamp to operate. Two switches in series form an AND arrangement because both must be closed for current to flow.

Conductors allow electric charge to move relatively easily because they contain mobile charge carriers. Metals such as copper are good conductors. Insulators such as rubber and plastic strongly resist charge movement and are used to protect people from electric shock.

Direct current (DC) flows in one direction, as from a battery. Alternating current (AC) periodically reverses direction and is used in mains electricity. UK mains electricity has a frequency of 50 Hz. Transformers require changing magnetic flux and are commonly used with AC.

Magnets have north and south poles. Like poles repel and unlike poles attract. Magnetic poles occur together in ordinary magnets, so cutting a bar magnet produces smaller magnets with both north and south poles.

A magnetic field is a region where magnetic forces can act on magnets, moving charges and electric currents. Magnetic field strength is described by magnetic flux density, measured in tesla (T). Stronger fields are represented by more closely spaced field lines.

Magnetic field lines show the direction and pattern of a magnetic field. Outside a bar magnet, they are conventionally drawn from north to south. Iron filings can reveal the field pattern, while a compass indicates its direction.

Electromagnetism describes the relationship between electricity and magnetism. An electric current creates a magnetic field around a conductor. Changing magnetic fields can also induce an electrical voltage. These principles are used in motors, transformers and medical equipment.

When current flows through a straight wire, it produces circular magnetic field lines around the wire. The right-hand grip rule gives their direction: point the right thumb in the conventional current direction and the curled fingers show the magnetic field direction.

A solenoid is a coil of wire that creates a magnetic field when current flows through it. The field resembles that of a bar magnet. Increasing the current, adding turns or using a suitable iron core can strengthen the electromagnet.

Electromagnetic induction occurs when a changing magnetic flux through a conductor or coil produces an electromotive force (induced voltage). This can happen by moving a magnet near a coil or changing the magnetic field. Induction is the operating principle of generators and transformers.

Faraday's law states that the magnitude of induced voltage depends on how quickly magnetic flux linkage changes. Lenz's law states that the induced current acts in a direction that opposes the change producing it. These laws explain electromagnetic induction.

A transformer transfers electrical energy between coils through changing magnetic flux. It normally consists of primary and secondary coils around a magnetic core. An ideal transformer follows Vs/Vp = Ns/Np, where V is voltage and N is number of turns.

A step-up transformer has more turns in the secondary coil than the primary and increases voltage. A step-down transformer has fewer secondary turns and reduces voltage. In an ideal transformer, increasing voltage decreases current proportionally because input and output power are equal.

Electricity and magnetism are important in radiotherapy equipment. High voltage accelerates electrons, radiofrequency electromagnetic fields accelerate electrons in a LINAC, and bending magnets can steer electron beams. Electrical circuits also support monitoring, control and safety systems.