Electricity powers almost everything in an HDB flat: the lights, the kettle, the ceiling fan, a phone charger. The topic starts with static electricity, which is charge that stays put. It then builds up to current in circuits, resistance, the cost of electricity and using the 230 V mains supply safely.
Students learn the two kinds of charge and how objects are charged by rubbing and induction. They also learn how to measure current and draw circuit diagrams, how to read I–V graphs, and how to work out power, energy and an electricity bill. Electrical safety appears in almost every exam paper on electricity, and questions ask students to explain each safety feature, not just name it.
What these notes cover
- •Two kinds of charge
- •Electric fields and field lines
- •Charging by rubbing
- •Charging by induction
- •Electrostatic hazards and uses
- •Current and charge flow
- •Circuit symbols, series and parallel circuits
- •Resistance and the rules for wires
- •I–V graphs
- •Power, energy and the kilowatt-hour
- •Cost of electricity
- •Electrical safety at home
Key definitions
- Electric field
- A region in which an electric charge experiences a force.
- Current
- How fast charge flows past a point in a circuit, measured in amperes (A) with an ammeter connected in series.
- Conventional current
- Current that flows from the positive (+) terminal of the cell, through the outside circuit, to the negative (−) terminal. This is the direction drawn on circuit diagrams.
- Resistance
- How much a component opposes the flow of current, measured in ohms (Ω).
- Kilowatt-hour (kW h)
- The energy used by a 1 kW appliance running for 1 hour. It is the unit used on the electricity bill.
Key formulas
Current and charge
I = Q ÷ t, so Q = It
Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). Convert minutes to seconds first.
Resistance
R = V ÷ I
V is the voltage across the component in volts (V), I is the current through it in amperes (A) and R is the resistance in ohms (Ω).
Power
P = VI
P is power in watts (W). Rearranged as I = P ÷ V, it gives the normal operating current used to choose a fuse.
Electrical energy
E = VIt
E is energy in joules (J), with t in seconds. This is the same as energy = power × time.
Energy in kilowatt-hours
E (kW h) = power in kW × time in hours
Before multiplying, divide watts by 1000 to get kW and change the time to hours (30 minutes = 0.5 h).
Cost of electricity
cost = energy used in kW h × cost per kW h
The cost per kW h is the tariff. In Singapore it is about 30 cents per kW h and changes every three months.
Worked example
A family boils water with a 2 kW kettle for 0.5 h every day. The tariff is 30 cents per kW h. Find the cost for 30 days.
- 1Energy per day: E = 2 kW × 0.5 h = 1 kW h per day.
- 2Energy for 30 days: E = 1 × 30 = 30 kW h.
- 3Cost: cost = 30 × 30 cents = 900 cents = $9.
Watch out: Convert power to kilowatts and time to hours before multiplying. A 2000 W kettle is 2 kW, not 2000 kW, and 30 minutes is 0.5 h.
Common mistakes
Saying an object gained protons
Wrong: The ruler became positive because it gained protons.
Right: It became positive because it lost electrons. Protons never move during charging; only electrons transfer.
Giving the electron-flow direction for current
Wrong: Current flows from − to +.
Right: Conventional current flows out of the + terminal, round the circuit and into the − terminal. Flow from − to + is electron flow.
Blaming high voltage for overloading
Wrong: Overloading means too many volts from the socket.
Right: The supply stays at 230 V. Overloading means the appliances draw too much total current from one socket, so the cable overheats.
Questions students ask
how to charge a sphere by induction using earthing
Bring the charged rod near the sphere without touching it, then touch the far side with a finger so that electrons can flow to or from the earth. Remove the finger first, then the rod, and the sphere is left with a charge opposite to the rod's. If the rod is removed first, electrons flow back and the sphere ends up neutral.
difference between conventional current and electron flow
Conventional current flows from the positive (+) terminal of the cell, through the outside circuit, to the negative (−) terminal. Electron flow is the actual movement of electrons, in the opposite direction, from − to +. Arrows on a circuit diagram show conventional current unless the question says otherwise.
why does the I–V graph of a filament lamp curve
As the voltage increases, the current heats the thin filament, and heating a metal increases its resistance. Each extra volt then pushes through less extra current than before. The graph starts steep and then flattens towards the voltage axis.
how to choose the correct fuse rating for an appliance
Find the normal operating current using I = P ÷ V, then choose the standard fuse rating just above it. If the rating is too low, the fuse blows during normal use. If it is too high, a dangerously large current can flow without the fuse ever blowing.
In the full PDF
- •Labelled diagrams of electric field lines, both induction methods and the inside of a three-pin plug
- •I–V graphs for an ohmic conductor, a filament lamp and a semiconductor diode
- •Further worked examples on charging, Q = It, resistance, power, energy, fuse choice and earthing faults
- •The remaining common mistakes for every section
- •The circuit symbol table and the rules for drawing circuit diagrams
- •A summary at the end of each section, from static charge to electrical safety