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IB DemystifiedMYP Sciences
Electrical power and electricity in the home
A kettle has the biggest power in most homes, yet the fridge may cost more to run. Power, time and a few safety devices explain the bill on your wall, and why a fault does not kill you.
Recommended for MYP 5 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
Figure 1. How an appliance with a metal case is wired to the mains.
calculate power using power = voltage × current, and energy using energy = power × time
calculate energy in kilowatt-hours and the cost of electricity
describe mains electricity and the difference between a.c. and d.c.
explain how fuses, earth wires, circuit breakers and double insulation protect people
choose a suitable fuse and calculate the efficiency of appliances
evaluate choices about energy use in homes and communities
Before you start
You will use these skills. If any feel shaky, review them first.
current, voltage and resistance, and voltage = current × resistance
series and parallel circuits
energy stores and transfers, and efficiency
Key vocabulary
Power
The rate of energy transfer, in watts (W). 1 W = 1 J per second.
Kilowatt-hour (kWh)
The energy transferred by a 1 kW appliance in 1 hour: 3 600 000 J. Electricity bills are charged per kWh.
Alternating current (a.c.)
Current that keeps reversing direction, as in mains electricity. Direct current (d.c.) flows one way, as from a battery.
Live, neutral and earth
The live wire (brown) is at a high voltage; the neutral wire (blue) completes the circuit; the earth wire (green and yellow) is a safety wire.
Fuse
A thin wire that melts if the current is too large, breaking the circuit.
Double insulation
An appliance with a plastic case and extra insulation, which cannot become live and needs no earth wire.
Understanding electrical power
What is it?
Electrical power is how quickly an appliance transfers energy. A 2000 W heater transfers 2000 J every second.
power = voltage × current energy = power × time
Why does it happen?
Voltage is the energy given to each unit of charge, and current is how much charge flows each second. Multiply them and you get energy per second: power. The longer an appliance runs, the more energy it transfers, which is why both power and time decide the cost.
How do we know?
Ammeters and voltmeters measure current and voltage, and the meter in every home measures the total energy in kWh. Appliances carry a power rating on their label, which you can check with a plug-in energy meter.
Why does it matter?
Understanding power helps families cut bills and emissions, lets engineers design safe wiring, and helps communities choose between energy sources. In many countries, mains electricity is 230 V at 50 Hz; in the USA it is 120 V at 60 Hz.
What does it connect to?
This topic links to circuits and resistance, energy resources and generating electricity, electromagnetism and transformers, and the climate impact of energy use. In mathematics it uses rearranging formulae, unit conversion and percentages.
Paying for electricity and staying safe
Cost: energy (kWh) = power (kW) × time (h), then cost = kWh × price per kWh. Always convert watts to kilowatts first.
Safety devices (Figure 1):
Fuse (A) in the live wire: melts if the current is too large. Choose the smallest rating just above the normal current.
Earth wire (D) connects a metal case to earth. If the live wire touches the case, a large current flows to earth and blows the fuse, so the case never stays live.
Circuit breakers do the same job as fuses, act faster and can be reset.
Double insulation: appliances with plastic cases need no earth wire.
Electricity in the real world
Electricians size cables and fuses for each circuit in a building: lighting circuits carry small currents, while cookers and showers need their own thick cables. Energy labels on appliances help shoppers compare running costs. Grid operators plan for the evening peak, when millions of kettles and cookers switch on at once.
Worked examples
Example 1: current and fuse
A 1500 W microwave runs on 230 V. Find the current and choose a fuse from 3 A, 5 A and 13 A.
current = power ÷ voltage = 1500 ÷ 230 = 6.5 A.
The fuse must be above 6.5 A, so 3 A and 5 A would blow in normal use.
Choose the 13 A fuse.
Example 2: cost of use
A 3 kW oven is used for 90 minutes. Electricity costs 30 cents per kWh. Find the cost.
Convert the time: 90 minutes = 1.5 h.
energy = 3 kW × 1.5 h = 4.5 kWh.
cost = 4.5 × 30 = 135 cents.
In the eAssessment
Electricity questions reward careful units and clear safety explanations. Expect:
Calculate: chains such as power → current → fuse, or power → kWh → cost. Show each step and convert W to kW, and minutes to hours.
Explain: how an earth wire and fuse work together, in sequence: fault, large current, fuse melts, supply disconnected.
Interpret and evaluate: tables of appliances or bulbs, and current–voltage data for components such as filament lamps.
Discuss: energy choices such as smart meters or solar home systems, weighing social, economic, environmental and ethical points.
Common ways to lose marks: confusing power with energy; forgetting to convert to kW or hours; choosing a fuse below the normal current; and saying the earth wire “absorbs” electricity.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: how does a lamp's power change with voltage?
Partially guided investigation · about 50 minutes · pairs
Research question
How do the current through a 12 V filament lamp, and its power, change as the voltage across it is increased from 2 V to 12 V?
Scientific background
Power = voltage × current. As more current flows, the filament gets hotter and glows more brightly; the resistance of a metal changes with temperature.
Hypothesis
Write your own hypothesis about how the current and power will change, with a justification.
Variables
Identify the independent and dependent variables, and at least three control variables, explaining how and why you will control each.
Apparatus
Variable low-voltage d.c. supply (0–12 V), 12 V filament lamp in a holder, ammeter, voltmeter, connecting leads.
Method
Connect the lamp in series with the ammeter; connect the voltmeter in parallel across the lamp.
Set the supply to 2 V and record the voltage and current.
Increase the voltage in 2 V steps up to 12 V, recording both readings each time.
Repeat the whole set, and calculate power = voltage × current and resistance = voltage ÷ current.
Plot current against voltage.
Safety. Use only a low-voltage supply, never the mains. The lamp becomes hot: do not touch it until it has cooled. Switch off between readings to avoid overheating.
Results from one pair are used in the Criterion C questions below. Then evaluate: why switch off between readings?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
