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 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
2. 2.

   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 167 s, from 334,880 × 0.90 ÷ 1,800
   • About 186 s, ignoring the 90% efficiency
   • About 258 s, using 100 °C as the temperature rise
3. 3.

   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.

   • 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
   • The heat pump, which uses 3.3 kWh costing $5.00
4. 4.

   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%
5. 5.

   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
6. 6.

   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 35 kV, reading 35 A as 35 kV
   • About 17 kV
   • About 0.5 kV, from I = 6,000 ÷ 5.0
7. 7.

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

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

8. Read this first

   Meter readings at 8 am (example data)

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

9. 8.

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

   • 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
   • Average about 1.5 kW; it cannot show when power peaks

10. Night meter readings (example data)

    time	reading (kWh)
    11 pm	61480.0
    6 am	61482.1

11. 9.

    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 2.1 kW, from 2.1 kWh overnight
    • About 0.30 kW, or 2,600 kWh a year
    • About 0.30 kW, or 260 kWh a year
    • About 0.30 kW, or 2,100 kWh a year
12. 10.

    An EV uses 0.30 kWh per km at the battery through a 90%-efficient charger. Electricity is $0.15 per kWh. A gas car uses 7.0 L per 100 km at $1.00 per L. Compare costs, Zee.

    • The EV costs $4.05 and the gas car $7.00
    • The EV costs $4.50 and the gas car $7.00, leaving out the charger loss
    • The EV costs $5.00 and the gas car $70
    • Per 100 km the EV costs $5.00 and the gas car $7.00