# Physics homework

## Energy Assignment II

1. What releases more heat: combustion of 10 grams of coal or 10 grams of gasoline? (1 point)

1. If 1-kg of methane (CH4) is burned every second, how much “thermal power” is available? (2 points)
2. How many BTUs of heat energy are needed to raise the temperature of 12 gallons of water from 500F to 1300F? (2 points)

1. What is the efficiency of an engine that has 3000 J of heat added during combustion and loses 2200 J of heat in the exhaust? (2 points)

1. What is the change in electric potential energy when 3 Coulomb charge flows from 55 Volts to 40 Volts? Is this a gain or loss of energy? (2 points)

1. A light bulb draws a current of 2 Amps when connected to a wall outlet (120 Volts). What is the resistance and wattage of the bulb? (3 points)
2. A household circuit at 120 V is fused for 20 A. How many 100-Watt light bulbs can be simultaneously used in this circuit? (2 points)

1. Suppose a family of four is considering purchasing a solar electric device for their home. Annually, they currently consume ~ 10,000 kWh of electricity and pay \$0.1/kWh.  Assume a 2.5 kW solar electric device operates perfectly for 4,000 hours a year and costs \$15,000 (installed).  Approximately how many years will it take to break even on this investment without any incentives or rebates?  (3 points)
2. Suppose you have a total of five (5) “vampire loads” in your house, each of which requires 10 W of power as long as they are plugged in (examples of “vampire loads” are cell phone chargers and plasma TV’s on standby – always on, often in dark corners, sucking electric power). If operated every day for one year, calculate the annual cost of electricity to power these loads if charged \$0.2/kWh.  (3 points)

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