Cambridge IGCSE Combined Science 0653 Physics P4 connects charge flow with voltage, resistance, energy transfer, circuit behaviour and mains safety. A successful solution separates what flows, what transfers energy and what opposes current.
P4.1 Charge and current
There are positive and negative charges. Like charges repel and unlike charges attract. Conductors allow charge to move readily; insulators do not. Metals conduct through mobile delocalised electrons.
Electric current is charge passing a point per unit time:
I=tQ
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Current is measured in amperes with an ammeter connected in series. Conventional current is from positive to negative, opposite to electron flow in a metal. Direct current has one direction; alternating current repeatedly reverses direction.
Electromotive force and potential difference
The source voltage causes current. Electromotive force is electrical work done by a source per unit charge around a complete circuit. Potential difference is work done per unit charge between two points. Both are measured in volts, but they describe different positions in the energy-transfer chain.
A voltmeter is connected in parallel across the component or source. Choose a range that avoids overload while giving useful resolution. In a series circuit, source voltage is shared among components.
Resistance
Resistance links potential difference and current:
R=IV
For a metallic conductor of the same material, resistance is directly proportional to length and inversely proportional to cross-sectional area. A long thin wire therefore has greater resistance than a short thick wire. Use measured values rather than assuming every component has constant resistance under every condition.
Electrical power, energy and cost
Circuits transfer energy from a cell or mains source to components and then to surrounding stores. Electrical power and energy are:
P=IV
E=IVt
One kilowatt-hour is the energy transferred by a 1kW appliance operating for one hour. For cost, calculate energy in kilowatt-hours and multiply by price per kilowatt-hour. Do not convert to joules if the tariff is quoted per kilowatt-hour.
P4.2 Components and diagrams
Know the symbols and behaviour of cells, batteries, power supplies, switches, fixed and variable resistors, heaters, lamps, motors, ammeters, voltmeters and fuses. Supplement diagrams may include generators and light-emitting diodes. An LED conducts in its forward direction and must be oriented correctly.
Draw connecting wires with a ruler, use accepted symbols and avoid pictures of apparatus. Place ammeters in series and voltmeters in parallel.
Series circuits
Current is the same at every point in a series circuit. The total potential difference is the sum across the components. Series resistances add:
Rtotal=R1+R2+⋯
Adding a series resistor increases total resistance and usually reduces source current for a fixed supply voltage.
Parallel circuits
Each parallel branch has the same potential difference as the whole arrangement. Current entering a junction equals current leaving it. Source current is the sum of branch currents.
The combined resistance of parallel resistors is less than either individual resistance. For two resistors:
Rtotal1=R11+R21
Parallel lamps receive the full supply voltage and operate independently, so one lamp failing does not necessarily switch off the others.
P4.3 Electrical safety
Current heats wires. Damaged insulation exposes conductors; damp conditions reduce resistance through unintended paths; overloaded plugs or extension leads cause excessive current and heating.
A fuse melts and breaks the circuit when current exceeds its rating. A trip switch opens the circuit and can be reset. Select the smallest rating above the appliance's normal operating current. A conductive casing is earthed so a fault current produces rapid disconnection. A double-insulated appliance instead uses a non-conducting outer casing and does not require an earth connection.
The syllabus does not require detailed residual-current-device knowledge here.
P4.4 Energy-changing devices
Cells and batteries transfer chemical energy into electrical energy. Generators transfer kinetic energy into electrical energy. Motors transfer electrical energy into kinetic energy. Name both starting and receiving stores or pathways rather than saying a device creates energy.
Worked application: appliance current and cost
A 2.0kW heater operates from a 230V supply. Its current is I=P/V=2000/230=8.7A to two significant figures. A 10A fuse is suitable because it is just above the normal current; a 5A fuse would melt during normal operation. If the heater runs for 3.0h, it transfers 2.0×3.0=6.0kW⋅h. At a tariff of 30 cents per kilowatt-hour, the cost is 180 cents. The kilowatt-hour calculation uses kilowatts and hours directly.
Common misconceptions and corrections
Current is used up. Charge flow is conserved at junctions; components transfer energy.
Voltage flows through a wire. Current is charge flow; voltage is work per unit charge.
An ammeter goes across a component. It is connected in series.
Parallel resistance is found by ordinary addition. Adding branches lowers combined resistance.
A larger fuse is always safer. An oversized fuse may not disconnect before wiring overheats.
Earthing prevents all faults. It provides a low-resistance fault path that helps protective devices operate.
A battery creates energy. It transfers chemical energy into electrical energy.
Kilowatt-hour measures power. It is a unit of energy.
Assessment guidance
Write the governing equation before substitution and convert prefixes carefully. For circuit explanations, state the correct current and potential-difference rules before calculating. Label meters and component symbols clearly. In safety questions, link hazard, excess or unintended current, heating or shock, and the protective response. Cost questions require energy in the tariff's unit. When comparing wires, hold material and temperature constant before using length and area relationships.
Check whether a question describes the source, a component or the whole circuit before assigning a voltage. Give calculated answers with a unit and a sensible precision. For fuse selection, show the normal operating current and justify the next suitable rating rather than selecting a value by inspection.
Retrieval practice
Explain the difference between electron flow and conventional current.
Calculate the charge passing when I=0.40A for 30s.
State the current and voltage rules for series and parallel circuits.
Explain why adding a parallel branch increases source current.
Compare the operation of a fuse, trip switch, earth wire and double insulation.
