Summary
Highlights
Components have resistance, which impedes charge flow but is necessary for their function (e.g., a bulb lights up, a resistor produces heat). For an ohmic resistor, PD and current are directly proportional (V ∝ I), resulting in a straight line graph. Ohm's Law states V = IR (Voltage = Current × Resistance). The steeper the V-I graph, the lower the resistance. Metals, like those in a light bulb's filament, are non-ohmic; their resistance changes with temperature and current, causing a curved V-I graph as higher temperatures increase atomic vibrations, impeding electron flow.
A diode only allows current to flow in one direction, exhibiting very high resistance in one direction and very low in the other. LEDs (light-emitting diodes) work similarly but also emit light. The resistance of a metal wire is directly proportional to its length. Thermistors are resistors whose resistance decreases with increasing temperature, while Light Dependent Resistors (LDRs) have resistance that decreases with increasing light intensity, making them useful in sensors.
In series circuits, total PD is shared among components, current is the same throughout, and total resistance is the sum of individual resistances. A potential divider circuit shares total potential difference. In parallel circuits, PD is the same across each branch, current is shared among branches, and adding more resistors in parallel decreases the total resistance as it offers more paths for current.
Power is the rate of energy transfer, calculated by P = VI (Power = Voltage × Current). Substituting Ohm's Law (V=IR) yields an alternative equation: P = I²R.
Direct current (DC) flows in one direction (e.g., from a battery). Alternating current (AC) and PD rapidly change direction (e.g., mains electricity). In mains circuits, the neutral wire is at zero volts, and the live wire alternates its potential, averaging to 230 volts at 50 Hz. Plugs have a live wire (brown), neutral wire (blue), and an earth wire (yellow/green) for safety, connected to the appliance's metal casing to prevent electric shock.
Fuses are safety devices connected to the live wire, designed to melt and break the circuit if the current exceeds a certain limit (e.g., 3, 5, or 13 amps). This protects the appliance and user. To determine the correct fuse, calculate the normal operating current using P=VI and choose the next highest standard fuse.
The National Grid transmits electricity across the country. To minimize energy loss as heat during transmission, transformers are used. Step-up transformers significantly increase voltage (and decrease current) near power stations. This reduces energy loss (P = I²R). Step-down transformers then reduce the voltage to a safe level (230V) for homes and businesses. For an ideal transformer, power in equals power out (V_primary * I_primary = V_secondary * I_secondary), meaning voltage and current are inversely proportional. The ratio of turns in the coils (N_p/N_s) is equal to the ratio of voltages (V_p/V_s). Transformers work by AC in the primary coil creating a changing magnetic field in the iron core, which induces current and voltage in the secondary coil.
Static electricity occurs when insulating materials are rubbed, transferring electrons and leaving objects charged. Objects with opposite charges attract, while those with like charges repel. Electric fields surrounding charged objects are represented by lines showing the direction of force on a positive charge (positive to negative). The strength of an electric field decreases with distance from the charged object.
Electricity is the flow of charge (electrons) carrying energy from a source to a component. A circuit requires complete loops for charges to flow. Batteries (or cells) convert chemical energy into electrical energy, transferring it to electrons. Electrons move from the positive to the negative terminal. As electrons pass through a component like a light bulb, their energy converts to light and heat. Electrons are refilled with energy in the battery and continuously flow around the circuit.
Potential difference (PD) or voltage indicates the energy transferred per coulomb of electrons. One volt means one joule of energy per coulomb. Voltmeters measure PD and are connected in parallel across the component. In a series circuit, electrons lose all their energy passing through components. The formula for PD is V = E/Q (Voltage = Energy/Charge), or rearranged as E = QV.
Current measures the rate of charge flow (how fast charge moves). The formula for current is I = Q/T (Current = Charge/Time), or rearranged as Q = IT. Current is symbolized by 'I'. Ammeters measure current and must be connected in series with the component.