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Student worksheet

3. Practical Electricity

Name:

Class: ______ Date: ________

Open the simulation and work through each chapter. Write your prediction before you run the test.

a. Electrical power

Predict

A 2200 W kettle element (24 Ω) is replaced by a 12 Ω element on the same 230 V supply. Predict the new power.

My prediction (and why)

What happened

Was I right? If not, what was wrong with my thinking?

Test: recording table

Appliance	P / W	I = P/V / A	R = V²/P / Ω

Pattern I found:

Challenge

Choose the kettle element so that it heats 1.0 kg of water from 30 °C to 100 °C in 2.0 minutes (294 000 J needed). Assume no energy is lost.

My solution

Exam practice

A rice cooker draws 3.0 A from the 230 V mains. Calculate its power. [2]
A 1000 W iron is connected to 230 V mains. Calculate the current in it. [2]
A heating element of resistance 50 Ω carries a current of 4.0 A. Calculate the power transferred. [2]

b. Energy and cost

Predict

Which costs more per month: 1.0 kW air-con for 8 h/day, or 3.0 kW water heater for 0.5 h/day?

My prediction (and why)

What happened

Was I right? If not, what was wrong with my thinking?

Test: recording table

Appliance	Power / kW	Hours per day	kWh per month	Cost per month

Pattern I found:

Challenge

Change the hours of use so that the household’s monthly bill (30 days, before GST) is below S$100.00 (generic mode: the equivalent of 350 kWh). The refrigerator must stay on 24 hours a day.

My solution

Exam practice

A 2.2 kW kettle is used for 15 minutes each day. Calculate the energy it uses in 30 days, in kWh. [2]
Electricity costs 28.59 cents per kWh. Calculate the cost of running a 150 W refrigerator for 24 hours a day for 30 days, in dollars. [3]
How many joules are there in 1 kWh? [1]

c. Mains wiring and hazards

Predict

Which single wire is most dangerous to touch: live, neutral or earth? Why?

My prediction (and why)

What happened

Was I right? If not, what was wrong with my thinking?

Test: recording table

Wire	Colour	Letter	Plug terminal	p.d. to earth / V

Pattern I found:

Challenge

Wire the plug for a 2200 W kettle correctly and fit the correct fuse. (Use Real view to wire the plug.)

My solution

Exam practice

What is the colour of the insulation on the earth wire? [1]
A Brown
B Blue
C Green and yellow stripes
D Red
Answer: ____
Which wire is at a high alternating p.d. relative to the ground? [1]
A Live
B Neutral
C Earth
D None of them
Answer: ____
Why is it dangerous to use a hair dryer with wet hands? [1]
A Water increases the mains p.d.
B Wet skin has a lower resistance, so a larger current can flow through the body
C Water makes the fuse blow
D The hair dryer uses more power
Answer: ____

d. Fuses and circuit breakers

Predict

2200 W kettle on a 13 A fused extension; an 1800 W hair dryer is added. Predict what happens.

My prediction (and why)

What happened

Was I right? If not, what was wrong with my thinking?

Test: recording table

Appliance	Power / W	Current / A	Fuse chosen / A

Pattern I found:

Challenge

Choose the correct plug fuse for each appliance (230 V mains).

My solution

Exam practice

A 230 V hair dryer is rated 1800 W. Calculate its normal current. [2]
Which fuse should be fitted to the plug of the 1800 W hair dryer? [1]
A 3 A
B 5 A
C 13 A
D 30 A
Answer: ____
Why are fuses fitted in the live wire rather than the neutral wire? [1]
A The live wire carries more current
B So that when the fuse blows the appliance is disconnected from the high p.d.
C The neutral wire is too thin
D So that the fuse blows more easily
Answer: ____

e. Earthing and faults

Predict

Fault 1: earthed metal kettle + 13 A fuse, live touches casing. Fault 2: same but earth wire missing. Predict what happens in each.

My prediction (and why)

What happened

Was I right? If not, what was wrong with my thinking?

Test: recording table

Earth	Fuse	Casing	Casing p.d. / V	Person current / mA	Safe?

Pattern I found:

Challenge

With the fault switched on, find two different arrangements that keep the person safe (no current through them) and leave nothing overheating.

My solution

Exam practice

What is the main purpose of the earth wire? [1]
A To carry the normal current back to the supply
B To provide a low-resistance path to the ground for a fault current
C To make the fuse blow during normal use
D To reduce the p.d. of the mains
Answer: ____
A hair dryer has a plastic casing and the symbol ⧈. Why does it not need an earth wire? [1]
A It uses very little current
B It is double insulated: the casing is an insulator so it cannot become live
C Its fuse is in the neutral wire
D Plastic conducts the current to earth
Answer: ____
In a fault, the live wire touches an earthed metal casing. The total resistance of the fault path is 0.80 Ω. Calculate the fault current from 230 V mains. [2]