Year 9 Science · Unit 3 · Lesson 23
Challenge Worksheet
Learning Goals
1. (a) Why can’t the star ratings on a large fridge and a small fridge tell you which one costs less to run? (b) What is the difference between a valid investigation and a reliable one?
Scenario: relighting the school hall
The school hall has 40 light fittings, each holding two 28 W fluorescent tubes. The lights are on for 10 hours a day, 200 days a year, and the school pays 32 cents for each kWh. Each tube can be swapped for a 14 W LED tube that gives the same light, at 12 dollars per tube, installed.
2. Calculate (a) the power of all the hall lights before and after the swap, (b) the energy and money saved each year, and (c) the payback time, to two decimal places.
(a) Before W
(a) After W
(b) Energy kWh
(b) Saving dollars
(c) Payback years
Investigation: which part of the school is the most energy-hungry?
A class read the school’s sub-meters to record the electricity used by five zones over four weeks, and measured the floor area of each zone.
| Zone | Electricity used (kWh) | Floor area (m²) | kWh per m² |
|---|---|---|---|
| Gym | 1,500 | 1,000 | |
| Science labs | 840 | 120 | |
| Library | 1,260 | 420 | |
| Canteen | 1,080 | 90 | |
| Administration | 960 | 240 |
Source: sample class results.
3. Complete the last column of Table 1 by dividing each zone’s kWh by its floor area. Show one calculation here.
4. Which zone uses the most electricity in total, and which uses the most per square metre? Suggest a reason for the canteen’s result, and say which figure is more useful for deciding where to look for savings.
Find the flaws: a student’s investigation plan
“I wanted to find out which classroom at my school uses the most electricity. I measured the power of the air conditioner in Room 4 on one Monday in April using the label on the back. I recorded the air conditioner running for 5 hours that day. I then multiplied by 200 school days to estimate annual energy use. I did this for five different classrooms. I found Room 4 uses the most electricity because it has the oldest air conditioner.”
5. Identify four flaws in the plan. For each, say whether it affects validity or reliability, and how to fix it.
| Flaw | Validity or reliability, and why | How to fix it |
|---|---|---|
Scenario: spending the energy budget
The principal says: “The gym uses the most electricity, so the whole energy budget should be spent on the gym.”
6. Evaluate the principal’s plan. Use Table 1, your calculations and what you know about energy audits, and recommend how the budget should be spent.