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 TV draws 120 W for 5 h a day and 2 W the other hours. Over 30 days at $0.15 per kWh, what does it cost Ari?

   $2.87
2. 2.

   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
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 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
   • About 207 s
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 heater, since no device beats 100% efficiency
   • The heat pump, which uses 3.3 kWh costing $5.00
   • They cost the same, since both give 10 kWh of heat
   • 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 3.33 kWh, subtracting 6 points from 36%
   • About 0.34 kWh, multiplying 1.0 × 0.36 × 0.94
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 resistive heater, since it costs $3,200 less to buy
   • The heat pump, by $14,800 over 15 years
   • The heat pump, by $14,000, leaving out the heater price
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 0.5 kV, from I = 6,000 ÷ 5.0
   • About 1.7 kV
   • About 17 kV
   • About 35 kV, reading 35 A as 35 kV
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?

   • 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

9. Read this first

   Night meter readings (example data)

   time	reading (kWh)
   11 pm	61480.0
   6 am	61482.1

10. 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 260 kWh a year
    • About 0.30 kW, or 2,100 kWh a year
    • About 0.30 kW, or 2,600 kWh a year
11. 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.

    • Per 100 km the EV costs $5.00 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 $4.05 and the gas car $7.00
    • The EV costs $5.00 and the gas car $70