Electric power and the energy a home uses

Answer Key - Grade 11 Science Worksheet - Challenge Me

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

1. 1.

   A game console draws 160 W for 3 h a day and 10 W the other hours. Over 30 days at $0.16 per kWh, what does it cost Bo?

   $3.31
2. 2.

   A desktop draws 200 W for 6 h a day and 5 W the other hours. Over 28 days at $0.14 per kWh, what does it cost Esme?

   $5.06
3. 3.

   Jun's 1,800 W kettle is 90% efficient. Water needs 4,186 J per kg per °C. How long to heat 1.0 kg from 20 °C to 100 °C?

   • About 207 s
   • About 186 s, ignoring the 90% efficiency
   • About 167 s, from 334,880 × 0.90 ÷ 1,800
   • About 258 s, using 100 °C as the temperature rise
4. 4.

   Electricity costs $0.15 per kWh. For 10 kWh of heat, a resistive heater needs 10 kWh; a heat pump giving 3 kWh of heat per kWh needs less. Compare, Kira.

   • The heat pump, which uses 3.3 kWh costing $5.00
   • They cost the same, since both give 10 kWh of heat
   • The heater, since no device beats 100% efficiency
   • The heat pump uses 3.3 kWh, costing $0.50 against $1.50
5. 5.

   A plant turns fuel into electricity at 36% efficiency, and 94% reaches the home. How much fuel energy gives 1.0 kWh at the home, Luz?

   • About 2.78 kWh, leaving out the transmission loss
   • About 2.96 kWh
   • About 0.34 kWh, multiplying 1.0 × 0.36 × 0.94
   • About 3.33 kWh, subtracting 6 points from 36%
6. 6.

   A home needs 12,000 kWh of heat a year for 15 years. A resistive heater costs $800; a heat pump (COP 3) costs $4,000. Electricity costs $0.15 per kWh. Which is cheaper, Milo?

   • The heat pump, by $18,000: the energy saving alone
   • The heat pump, by $14,000, leaving out the heater price
   • The resistive heater, since it costs $3,200 less to buy
   • The heat pump, by $14,800 over 15 years
7. 7.

   A 5.0 Ω line delivers 600 kW. To waste at most 1% (6.0 kW) as heat, what is the lowest transmission voltage, Noah?

   • About 1.7 kV
   • About 17 kV
   • About 35 kV, reading 35 A as 35 kV
   • About 0.5 kV, from I = 6,000 ÷ 5.0
8. 8.

   A 20 A, 120 V circuit may carry 80% of its rating for hours. Which set of appliances can run together for hours, Pia?

   • Microwave 1,200 W and toaster 1,100 W: 2,300 W is under 2,400 W
   • Heater 1,500 W, lamp 100 W, TV 150 W and laptop 65 W: 1,815 W
   • Two 1,500 W heaters: 3,000 W is fine on a 20 A circuit
   • Kettle 1,500 W and toaster 1,100 W: 2,600 W is about 22 A, near enough

9. Read this first

   Meter readings at 8 am (example data)

   day	reading (kWh)
   Mon	60000
   Tue	60036
   Wed	60071
   Thu	60108
   Fri	60143

10. 9.

    Using the meter table, what does the data imply about the home's average power, and what can it not show, Ren?

    • Average about 1.5 kW; it cannot show when power peaks
    • Average about 36 kW, from 36 kWh in a day
    • Average about 1.5 kW; the load was steady throughout each day
    • Average about 1.5 kW; it shows that peak power is 1.5 kW

11. Night meter readings (example data)

    time	reading (kWh)
    11 pm	61480.0
    6 am	61482.1

12. 10.

    Only the refrigerator and standby loads run overnight. From the meter table, what is the base load, and about how many kWh a year, Tala?

    • About 0.30 kW, or 2,600 kWh a year
    • About 0.30 kW, or 260 kWh a year
    • About 2.1 kW, from 2.1 kWh overnight
    • About 0.30 kW, or 2,100 kWh a year