Current Electricity

Current

Is the rate of flow of charge

$$I=\frac{Q}{t}$$

An ammeter

Is a device that measures current. An ammeter must be connected in series in a circuit.

AC and DC

Direct current (DC) always flows in one direction, whereas alternating current (AC) constantly changes direction.

Voltage or potential difference (V)

Between two points is the work done when 1 coulomb of charge is moved from one point to the other

$V=\frac{W}{Q}$

Electromotive force (emf)

The potential difference between terminals of a battery when no current is being drawn from the battery

A voltmeter

Is a device that measures potential difference across a component. A voltmeter must be connected in parallel or across the component in a circuit.

Resistance

The resistance of a conductor is the ratio of the voltage across it to the current flowing through it. Unit: ohm

Ohm

Unit of resistance. A conductor has the resistive value of 1 ohm if a current of 1A passes through it when a potential difference of 1V is applied across the conductor

Ohms law

States that the current flowing through a conductor is directly proportional to the potential difference across it, if the temperature remains constant.

$$V=RI$$

Resistivity

Of a material is the resistance of a cube of side 1m of the material

Joules law

States that the rate of heat produced in a conductor is proportional to the square of the current flowing through it

Ion

An atom or group of atoms that has lost or gained one or more electrons

Electrolysis

The passing of electric current through a liquid causing a chemical reaction

A fuse

Is a safety device consisting of a piece of wire which melts when a current above a pre set value passes through it

A miniature circuit breaker (MCB)

Is a safety device which breaks the circuit if it detects a current above a pre set value

A residual current device (RCD)

A safety device that breaks the circuit if it detects a current difference between live and neutral above a pre set value

A radial circuit

Is where separate live and neutral wire are connected from the distribution box (power supply) to an appliance which uses a large current. (E.g. electric cooker, electric shower, immersion heater)

A ring circuit

Is where three terminals of each socket (live, neutral, earth,) are connected to three corresponding wires arranged in a ring or loop with each end of the ring connected to the distribution box

Bonding

Is a safety precaution whereby all metal pipes, taps, and tanks are earthed.

Kilowatt-hour

The amount of energy used by a 1000W appliance in 1 hour

A semiconductor

Is a substance whose resistivity is between that of a good conductor and a good insulator. The resistivity of a semiconductor decreases as the temperature increases.

Holes

Are positivity charged gaps or spaces in a substance which are created when electrons break free from covalent bonds

Doping

The addition of impurities to a semiconductor to increase its conductivity

Intrinsic conduction

Occurs in a pure semiconductor when there is an equal number of electrons moving from negative to positive and holes moving in the opposite direction

Extrinsic conduction

Is an increased rate of conduction due to the addition of impurities

Thermistor

A semiconductor whose resistance decreases as temperature increases

Light dependent resistor (LDR)

Is a semiconductor whose resistance decreases as light intensity increases