Electric power and the energy a home uses

Answer Key - Grade 11 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   Ari runs a 1500 W oven for 40 min and a 900 W microwave for 10 min. How much energy is used, in kWh?

   1.15 kWh

2. Read this first

   Two water heaters (example data); electricity costs $0.15 per kWh

   model	price ($)	energy use (kWh per year)
   X	600	3000
   Y	1400	1000

3. 2.

   Using the water-heater table, which model costs less to buy and run for 10 years, and by how much, Jun?

   • Model Y, by $3,000: the energy saving alone
   • Model X, since it costs $800 less to buy
   • Model Y, by $800: the price difference
   • Model Y, by $2,200 over 10 years

4. Two bulbs (example data); electricity costs $0.15 per kWh

   bulb	power (W)	light (lm)	price ($)	life (h)
   Incandescent	60	800	1.00	1000
   LED	9	800	4.00	15000

5. 3.

   Using the bulb table, what does 15,000 h of light cost from each bulb type, buying bulbs as needed, Kira?

   • Incandescent about $150; LED about $20, leaving out the LED price
   • Incandescent about $135; LED about $20, counting just the energy
   • Incandescent about $15; LED about $4, counting just the bulbs
   • Incandescent about $150; LED about $24
6. 4.

   A 2.4 kW dryer runs 1.5 h per load, 200 loads a year. Peak power costs $0.18 per kWh, off-peak $0.09. What does moving every load off-peak save a year, Luz?

   • About $0.32 a year, for a single load
   • About $6.50 a year
   • About $130 a year, using the whole peak price
   • About $65 a year
7. 5.

   A 15 A circuit at 120 V feeds one outlet strip. Which set of appliances can run together without tripping the breaker, Milo?

   • Kettle 1,500 W and coffee maker 900 W: 2,400 W is under 15 A
   • Coffee maker 900 W and microwave 1,200 W: each is under 1,800 W
   • Kettle 1,500 W and fan 100 W: 1,600 W is under 1,800 W
   • Toaster 1,100 W and microwave 1,200 W: each is under 1,800 W
8. 6.

   A line of 2.0 Ω carries 100 kW. At 2,000 V the current is 50 A; at 20,000 V it is 5.0 A. How does the heating loss in the line change, Noah?

   • It rises, since the voltage is higher
   • It falls from 5,000 W to 500 W, one tenth
   • It stays the same, since the power sent is 100 kW
   • It falls from 5,000 W to 50 W, one hundredth
9. 7.

   A kettle is 90% efficient. Heating 1.0 kg of water by 80 °C needs 334,880 J. Electricity costs $0.15 per kWh. What does one boil cost, Pia?

   • About 16 cents
   • About 1.3 cents, multiplying by 0.90
   • About 1.4 cents, ignoring the 90% efficiency
   • About 1.6 cents
10. 8.

    A window air conditioner removes 8,000 BTU/h and has an EER of 10 (BTU/h per watt). It runs 6 h a day for 120 days at $0.15 per kWh. What does it cost Ren?

    • $8,640, multiplying by the EER
    • $86.40, from 576 kWh
    • $8.64
    • $864, treating 8,000 BTU/h as 8,000 W
11. 9.

    With every appliance off, a meter shows a steady 85 W draw. At $0.15 per kWh, what does that standby load cost in a year, Tala?

    • About $112 a year
    • About $1,117 a year
    • About $11 a year
    • About $0.31 a year, the cost of a single day

12. Two refrigerators (example data); electricity costs $0.15 per kWh

    model	price ($)	energy use (kWh per year)
    A	900	410
    B	810	520

13. 10.

    Using the same table, a refrigerator lasts 12 years. Which model costs less to buy and run for 12 years, and by how much, Zee?

    • Model A, by $16.50, one year's saving
    • Model A, by $198, adding the price gap to the saving
    • Model B, by $90, comparing the purchase prices alone
    • Model A, by $108