4 3 electric circuits and electrical safety

A group of cells is called a battery.

The symbol for a variable resistor has an arrow going through it, whereas the symbol for a fixed resistor does not.

A relay uses a small current in one circuit to switch on a much larger current in another circuit.

A thermistor is a temperature-dependent resistor. This means its resistance changes depending on its temperature.

When the temperature of a thermistor is increased, its resistance decreases.

LDR stands for light-dependent resistor.

When light intensity on an LDR is increased, its resistance decreases. (Remember: Light Decreases Resistance - LDR)

False. Current is the same everywhere in a series circuit.

True. In a parallel circuit, the current from the power source is larger than the current in each branch.

True. The amount of current flowing in any circuit depends on the potential difference and resistance in the circuit.

If the e.m.f. (potential difference) of the power source is increased, but everything else in the circuit remains constant, the current will increase.

Adding components to a circuit increases the total resistance of the circuit, so the current would decrease (for a constant e.m.f.).

True. One advantage of a parallel circuit is that each bulb can be arranged in its own branch with its own switch.

False. An advantage of parallel circuits is that if components in one branch fail, the components in other branches will still work. If one component fails in a series circuit, then the circuit is broken and the current cannot flow.

True. Current is conserved at a junction in a parallel circuit because charge is always conserved.

False. Current will only split equally at a junction if the resistance in each branch is equal.

False. When several cells are connected in series, their combined e.m.f. is the sum of their individual e.m.f.s.

The total potential difference across the components in a series circuit is equal to the sum of the individual potential differences across each component.

The potential difference across an arrangement of parallel resistances is the same as the potential difference across one branch in the arrangement of the parallel resistances.

When two or more resistors are connected in series, the combined resistance is the sum of the individual resistances.

True. The combined resistance of two resistors in parallel is less than that of either resistor by itself.

The equation for the combined resistance of two resistors connected in parallel is Where:

A potential divider splits the potential difference of a power source between two components.

True. When a potential divider is used in a circuit, the resistor with the largest resistance will have a greater potential difference across it.

A potentiometer is a type of variable resistor consisting of a coil of wire and sliding contact.

Examples of applications of potential dividers include:

The potential divider equation is Where:

Exposed wires can cause electric shocks if touched.

Overheating of cables could lead to melting of the insulation and exposing the wires, or fires.

Moisture can conduct electricity, so using electric circuits in damp conditions could lead to electric shocks, or short circuits which may damage the appliance or even start a fire.

If plug sockets are overloaded it can lead to overheating which can cause melting or even fires.

True. Mains circuits do consist of a live wire, a neutral wire and an earth wire.

A switch must be connected to the live wire in order for the circuit to be turned off safely.

The casing of an electrical appliance must be either non-conducting or earthed to prevent the casing from becoming electrified and causing an electric shock to anyone who touches it.

A fuse is a safety device designed to break the circuit if the current exceeds a certain amount.

A trip switch is a switch on the consumer unit that will break the circuit if the current exceeds a certain amount.

True. A fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance.