Electricity flashcards

Key points

• Current is the rate of flow of electric charge, measured in amps (A).
• Charge is measured in coulombs (C). In a metal wire it is carried by electrons.
• One amp is one coulomb of charge passing a point every second.

Formula

0.5 A flowing for 2 minutes

Key points

• Charge is not used up as it flows round the circuit.
• With only one path, all the charge passes every point, so an ammeter reads the same anywhere in the loop.
• What a component takes from the charge is energy, not charge.

Key points

• Potential difference (p.d.) is the energy transferred per unit of charge passing between two points.
• It is measured in volts (V): 1 volt is 1 joule per coulomb.
• It is often called voltage. The supply gives the charge energy, and the components transfer it.

Formula

2 C of charge through a 6 V lamp

Key points

• Resistance is how strongly a component opposes the current, measured in ohms (Ω).
• For the same potential difference, a bigger resistance means a smaller current.
• At constant resistance, current is directly proportional to p.d.: double V and I doubles.

Formulae

12 V across a 4 Ω resistor

Key points

• An ammeter measures current, so it goes in series, in the loop with the component.
• A voltmeter measures potential difference, so it goes in parallel, across the component.
• Work out the resistance from the two readings: R = V ÷ I.
• Required practical: for a wire at constant temperature, the resistance rises in proportion to its length.

Key points

• The current is the same through every component.
• The supply's p.d. is shared: the p.d.s across the components add up to the supply's.
• The resistances add up, and one break stops the current everywhere.

Formula

4 Ω and 6 Ω in series on 12 V

Key points

• Each branch gets the full supply p.d.
• The current splits at a junction: the branch currents add up to the current from the supply.
• The branch with the smaller resistance takes the bigger current.
• The total resistance is less than the smallest resistor on its own.
• House lights are in parallel: each gets 230 V and switches on or off on its own.

6 Ω and 3 Ω in parallel on 6 V

Key points

• Fixed resistor at constant temperature: a straight line through the origin, so its resistance is constant.
• Filament lamp: a curve that gets flatter, because the filament heats up and its resistance rises.
• The hotter the filament, the more its ions vibrate and the more often electrons collide with them.
• Diode: current flows one way only. In reverse its resistance is very high, so the graph is flat.
• Required practical: change the p.d. with a variable resistor, record I and V, then reverse the supply.

Key points

• Thermistor: its resistance falls as its temperature rises.
• LDR (light-dependent resistor): its resistance falls as the light on it gets brighter.
• That makes them sensors: thermistors in thermostats, LDRs in lights that switch on at dusk.
• In series with a resistor, as its resistance falls, its share of the p.d. falls too.

Key points

• Power is the energy transferred each second, in watts (W): 1 W is 1 J/s.
• P = I²R comes from putting V = IR into P = VI.
• The current is squared, so doubling it makes four times the heating.

Formulae

A kettle on 230 V draws 10 A

Key points

• Energy transferred = power × time, with the time in seconds.
• A current heats a resistor: electrons collide with its ions and make them vibrate more.
• That heating is useful in kettles and toasters, but wasted in cables.

Formula

A 60 W lamp on for 5 minutes

Key points

• Direct current (d.c.) always flows the same way. Cells, batteries and solar cells give d.c.
• Alternating current (a.c.) keeps reversing its direction.
• UK mains is a.c. at about 230 V and 50 Hz.
• 50 Hz means the current goes one way and back again 50 times every second.

Key points

• Live (brown): carries the alternating p.d. from the supply, at about 230 V.
• Neutral (blue): completes the circuit, at or near 0 V.
• Earth (green and yellow): a safety wire at 0 V that only carries a current if there is a fault.
• Touching the live wire is dangerous even with the switch off: your body is at 0 V, so a current flows through you.

Key points

• The National Grid is the system of cables and transformers linking power stations to homes.
• Step-up transformers raise the p.d. as electricity leaves the power station, up to 400 000 V.
• For the same power (P = VI), a high p.d. means a small current, so less energy heats the cables.
• Step-down transformers lower the p.d. to about 230 V for safe use in homes.