Interactive Lesson Notes
P4.2.1 Electrical charge
- There are positive and negative charges.
- Like charges repel. Unlike charges attract.
- Electrostatic charging by friction involves a transfer of electrons.
- Conductors allow charge to move easily. Insulators do not.
- Charge is measured in coulombs (C).
- An electric field is a region where a charge experiences a force.
| Conductors | Insulators |
|---|---|
| Copper, aluminium, graphite | Plastic, rubber, glass, dry wood |
P4.2.2 Electric current
- Current is related to the flow of charge.
- In metals, current is related to the flow of electrons.
- Ammeters are used to measure current and are connected in series.
- Direct current (d.c.) flows in one direction. Alternating current (a.c.) changes direction repeatedly.
- Conventional current goes from positive to negative, but electrons move from negative to positive.
I = Q / t
P4.2.3 Voltage, e.m.f. and p.d.
- Voltage from a source causes current to flow in a circuit.
- In a series circuit, the source voltage is shared between components.
- Voltmeters are used to measure voltage and are connected in parallel.
- Electromotive force (e.m.f.) is the work done by a source in moving a unit charge around a complete circuit.
- Potential difference (p.d.) is the work done by a unit charge passing between two points in a circuit.
- Both e.m.f. and p.d. are measured in volts (V).
P4.2.4 Resistance
- Resistance tells us how much a component opposes current.
- A resistor of constant resistance gives a straight-line current-voltage graph through the origin.
- Resistance can be determined using an ammeter and a voltmeter.
- For metallic conductors, resistance increases with length and decreases with cross-sectional area.
R = V / I
P4.2.5 Electrical energy and electrical power
- Circuits transfer energy from a source to components and then to the surroundings.
- Electrical power is the rate at which energy is transferred.
- Electrical energy depends on power and time.
- The kilowatt-hour (kWh) is the energy used by a 1000 W appliance in 1 hour.
P = IV
E = IVt
P4.3.2 Series and parallel circuits
- In a series circuit, the current is the same at every point.
- In a series circuit, total p.d. = sum of individual p.d.s.
- In a parallel circuit, the current from the source is larger than the current in each branch.
- At a junction, current entering = current leaving.
- The p.d. across each branch of a parallel circuit is the same as the p.d. across the whole arrangement.
- For resistors in series: total resistance is the sum.
- For resistors in parallel: total resistance is less than either resistor alone.
- For resistors in parallel: Combined resistance is 1/R_total = 1/R1 + 1/R2 + 1/R3 + ......
- Lamps in parallel stay bright and can work independently if one lamp fails.
P4.4 Electrical safety
- Current can produce a heating effect.
- Hazards include damaged insulation, overheating cables, damp conditions, and overloading plugs or extension leads.
- Fuses and trip switches protect circuits by breaking the circuit when current becomes too large.
- Outer casings should be double-insulated or earthed for safety.