P3 General Studies: Home Electricity Safety (Paper 3 (challenge))

2026-11-13

Tutor Prep Guide · conductors and insulators, power and electricity cost, series and parallel circuits, fuses and earth protection, and safe habits at home

Paper three adds reverse work and comparisons: current from power, kWh against joules, mixed series and parallel resistance, air conditioner cost and LED savings.

Direct answers　List the known quantities, write the formula that applies, and finally check that the units and the size of the answer make sense.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: P3 General Studies: Home Electricity Safety
Level	parts of a closed circuit, conductors against insulators, series against parallel, adding up battery voltage, and electrical safety

02　Questions (10 items)

FoundationA 1000 W appliance is connected to 220 V. What current does it take?
FoundationHow many joules are there in one unit of electricity?
FoundationAn appliance label reads 220 V 50 Hz. What do the two numbers mean?
IntermediateTwo 6 ohm resistors in parallel are joined in series with a 4 ohm resistor. What is the total resistance?
IntermediateAn 8 ohm resistor is connected to a 12 V supply. What current flows?
IntermediateTwo 60 W lamps are connected in parallel to 220 V. What is the total current?
ChallengeA 1500 W air conditioner runs 8 hours a day for 30 days. How many units and what cost at $1.2 per unit?
ChallengeA 2000 W heater and a 1000 W hairdryer share a 13 A socket at 220 V. Is that an overload?
ChallengeA 10 W LED lamp replaces a 60 W filament lamp for 5 hours a day. How much energy and money is saved in a year at $1.2 per unit?
Error redoSomeone changes a bulb barefoot on a wet floor with wet hands. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	4.5 A	Current is power divided by voltage: one thousand divided by two hundred twenty is about 4.5 A.
2	3600000 J	One kWh is 1000 W times 3600 s, which gives 3600000 J.
3	The rated voltage is 220 V and the alternating current frequency is 50 Hz.	The voltage must match the Hong Kong supply, and the frequency means the current changes direction fifty times a second.
4	7 ohms	The parallel pair of six and six gives 3 ohms, and adding 4 ohms gives 7 ohms.
5	1.50 A	Current is voltage divided by resistance: twelve divided by eight gives 1.5 A.
6	0.55 A	The total power is 120 W, and one hundred twenty divided by two hundred twenty is about 0.55 A.
7	360.0 units, $432.0	Fifteen hundred times eight times thirty gives 360 units, and times 1.2 makes 432.0 dollars.
8	Yes: the total is 3000 W, which is 104.9 percent of the 2860 W socket rating.	The rating is 220 V times 13 A, which gives 2860 W, and 3000 W is above it.
9	91.25 units, $109.5	Fifty watts are saved each hour, and fifty times five times three hundred sixty five gives 91250 Wh, that is 91.25 units, worth 109.5 dollars.
10	Wet hands and a wet floor make a conducting path, and the body is earthed directly, so the shock can be severe.	The main switch should be off first, and both the hands and the floor should be dry.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Paper four clears the conductor, insulator and battery direction misconceptions.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.