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Most pupils only remember that electricity is dangerous, and cannot say when. This lesson makes them argue with power and current: how many appliances overload one socket, and why wet hands are far worse.
Objectives — Pupils can: (1) compute units and cost with power times time divided by 1000, and state that one unit is one kilowatt-hour; (2) state the 2,860 watt ceiling of a 13 amp socket at 220 volts and judge an extension board by adding watts; (3) explain with resistance why wet hands are worse, quoting 2.2 milliamps against 220 milliamps; (4) state five safety rules and give a reason for each.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Multiplication and unit conversion | How many kilowatts is 1000 watts | One kilowatt, moving the decimal point three places |
| Basic circuit idea | What three things does a lit bulb need | A source, the bulb, and wires in a complete loop |
| Conductors and insulators | Which conducts, copper or plastic | Copper conducts, plastic insulates |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0 to 5 minutes | Opening question | Hold up an extension board and ask how many appliances it can take, writing guesses on the board | Say how many their board at home carries and why they guessed that |
| 5 to 15 minutes | Power and units | Teach power times time divided by 1000, working an air conditioner at 2000 watts for 5 hours | Repeat the calculation for a 60 watt fan over 10 hours, getting 0.6 units |
| 15 to 22 minutes | Cost | Write an example price of 1.20 per unit and multiply 10.6 units by it | Compute 12.72 and say that cost depends on both power and time |
| 22 to 32 minutes | Socket ceiling | Write 220 times 13 equals 2860 and add the air conditioner, fan and kettle one at a time | Say still safe or overloaded after each addition, and by how many watts |
| 32 to 40 minutes | Why shock happens | Compare 100 kilo-ohms dry skin with 1 kilo-ohm wet skin, writing 2.2 and 220 milliamps | Say how many times larger the wet current is and which range affects the heart |
| 40 to 45 minutes | Five rules and exit check | Ask why after each rule and write the reason beside it | State the five rules, each with one reason |
03 Teaching script (read it out)
Scene 1: Units are energy, not power
Separate two ideas first. Watts tell you how fast, units tell you how much. A 1000 watt appliance for one hour uses one unit. So a 2000 watt air conditioner for 5 hours is 2000 times 5 divided by 1000, which is 10 units. Your 60 watt fan for 10 hours is only 0.6 units, because it is slow and the time is short.
Pupils restate that units equal power times time divided by 1000, so high watts alone do not mean high use.
Scene 2: The socket limit is current, not holes
A 13 amp socket means 13 amps at most. Hong Kong runs at 220 volts, and 220 times 13 is 2,860 watts. An air conditioner at 2,000 watts plus an 800 watt fan is 2,800 watts, still safe with only 60 watts spare. Add a 1,000 watt kettle and you have 3,800 watts, needing 17.27 amps, beyond 13 amps, so the board heats up. The danger is the total watts, not the number of plugs.
Pupils restate that the socket limit is about total watts, and 13 amps means 2,860 watts.
Scene 3: Why wet skin is far worse
Skin has resistance. Dry hands are about 100 kilo-ohms, so 220 volts divided by 100 kilo-ohms gives only 2.2 milliamps, which usually just stings. Wet hands carry impurities, so resistance drops to about 1 kilo-ohm, and the same voltage drives 220 milliamps, a hundred times more, enough to disturb the heart rhythm. The rule about wet hands is arithmetic, not superstition.
Pupils restate that water lowers skin resistance, so the same voltage drives a much larger current.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Compute units and cost | A 1500 watt heater runs 4 hours at 1.20 per unit. What is the cost | (1) 1500 times 4 equals 6000; (2) divide by 1000 for 6 units; (3) multiply by 1.20 | 6 units, cost 7.20 |
| Judge an overloaded board | A 13 amp board holds a 2000 watt air conditioner and an 800 watt fan. Can it take a 300 watt stereo | (1) ceiling is 220 times 13 equals 2860; (2) total is 2000 plus 800 plus 300 equals 3100; (3) compare 3100 with 2860 | No, it is 240 watts over, needing 14.09 amps |
| Explain the shock difference | Why is a wet hand more dangerous than a dry hand on the same appliance | (1) state resistance: high when dry, low when wet; (2) note the voltage is unchanged; (3) finish with current equals voltage divided by resistance | Wet skin has low resistance, so current rises from milliamps to a lethal level |
| Match a rule to a reason | Give the reason for never plugging in with wet hands | (1) water lowers skin resistance; (2) current rises; (3) describe the effect on the heart | Water lowers resistance, current reaches 220 milliamps and can cause ventricular fibrillation |
Sanity check (teach this): Check the arithmetic afterwards: units must always be smaller than the wattage because you divided by 1000, so an answer of 2000 units means the division was skipped; cost must equal units times price, and a cost smaller than the unit count means a wrong price; for an overload, the total watts divided by 220 must exceed 13 before you call it unsafe.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| An 800 watt vacuum runs 30 minutes, four times a week. How many units in a four-week month | 0.8 units times 4 is 3.2 per week, times 4 weeks is 12.8 units | Whether they converted 30 minutes to 0.5 hours before multiplying |
| A 13 amp board holds a 1200 watt rice cooker, a 900 watt microwave and a 700 watt kettle. Is it safe | Total 2800 watts, under 2860, safe with only 60 watts spare | Whether they found the ceiling first, then added, rather than counting plugs |
| A phone charger outputs 5 volts at 2 amps. What is the power, and why does it not shock you | 5 times 2 is 10 watts; at 5 volts the current through a body is tiny | Whether they separated low power from low voltage as two reasons |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Treating watts as units | The 2000 watt air conditioner for 5 hours uses 2000 units | Watts tell you how fast, units tell you how much; 2000 watts for 5 hours is 10 units, so multiply by the time. |
| Counting plug holes | A board has six holes, so six appliances are safe | Do not count holes, add watts; 13 amps means 2,860 watts, and going over is the danger. |
| Thinking wet hands only tingle | Wet hands just sting a little, nothing happens | Dry skin gives 2.2 milliamps, wet skin 220 milliamps, a hundred times more, enough to stop the heart. |
| Pulling a casualty by hand | When someone is shocked, pull them away immediately | Switch off the supply or move the source with a dry wooden stick first, or you will be shocked too. |
07 Homework (paper, section and question numbers)
- Primary 5 General Studies paper, Section C Science and Technology, item 8 — Section C item 8: the three things a lit bulb needs; then ask for one sentence on why the loop must be complete
- Primary 5 General Studies paper, Section C Science and Technology, item 7 — Section C item 7: the three changes a force can make, used to compare with an appliance heating up
- Primary General Studies chapter index — Link from the science and technology chapter to circuits and conductors
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| A 2000 watt air conditioner runs 3 hours. How many units | 6 units | An answer of 2000 means the division by 1000 was skipped, so reteach unit conversion |
| What is the maximum wattage of a 13 amp socket | 2,860 watts, from 220 times 13 | If missing, reteach the ceiling and make them multiply once |
| Why is a wet hand more dangerous | Water lowers skin resistance, so current rises to 220 milliamps | Answering only because it is wet means resistance and current were not taught |
09 Tutor FAQ
Which year is this for
Written for Primary 5. For Primary 4, teach units and the five rules only and leave resistance for later.
Must pupils memorise the tariff
No. The lesson uses 1.20 as an example price and says so; the point is units times price.
Is there a matching paper
The Primary 5 General Studies paper has circuit items in Section C with an answer sheet, and the homework block names the section and item.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.