Electric Power and Energy Auestions

[Psyche]

1. An electric space heater draws 15.0 A from a 120 V source. It is operated, on average, for 5.0h (hours) each day. a) How much power does the heater use and b) At $.10 per kWh, what does it cost to operate the heater for 30 days?

2. Suppose each person in the United States uses 12,000 kWh of electricity a year and the power plants using coal to generate electricity are 40% efficient. If 1.0 kg of burned coal releases 33.0 mJ a) how much coal will be burned during a year for a family of four b) how large a pile of coal would that be (using 2620 kg/ m^3) c) how much heat is given off to the environment

3. A light bulb uses 65 watts (used for 6 hours a day) and a digital clock uses 5 watts (used for 24hrs a day). Find the cost of operating each one for 30 days. Assume the price is $.10 per kWh

 

The equations given=

power= current x voltage, energy= power x time, cost= energy x price

 

 

 

I can find a) at 1800 w (watts), but I have no idea where to start on part b.

For 2, I'm unsure of where to star. I know the family of four would use 48,000 kWh a year, but after that, I'm lost

[swansont]

You have the equations. Most of the work, after that, is unit conversion. For example, you have a light bulb that uses 100W = 100J/s. Running for an hour the energy is E=Pt, which is 100J/s*3600s = 360,000J (you have to have the time in seconds for the units to work out), or 100W = 0.1kW so E = 0.1 kW*1h = 0.1 kWh

 

Which one you use depends on what the question is asking.

Edited May 29, 2012 by swansont
fix unit typo

[Enthalpy]

1.0 kg of burned coal releases 33 MJ (millions of Joules) not 33 mJ (thousandth of Joule).

 

It may all look easy, but even after years of unit conversions, engineers and physicists still forget to convert hours into seconds, kilograms into Newtons... Maybe the most frequent source of errors.

[ACUV]

................................ E = 0.1 kWh*1h = 0.1 kWh ................................

 

swansont, no doubt, had in mind the following,

 

................................ E = 0.1 kW*1h = 0.1 kWh ................................

 

Elementary, my dear swansont !