Energy Conversion and Efficiency

Tracing energy through everyday conversion systems, and the two formulas Science 24 uses: percent efficiency and E = Pt.

11 questions, free, marked as you go with a worked explanation on every one.

What this covers

An electric motor is supplied with 800 J of electrical energy and delivers 240 J of useful mechanical energy. What is its percent efficiency?
For that same motor, supplied with 800 J and delivering 240 J of useful energy, how many joules are wasted, mostly as heat and sound?
A 1 500 W kettle is used for a total of 4 hours over a week. Using E = Pt, how many kilowatt hours of energy does it use?
A 1 500 W kettle used for a total of 4 hours over a week uses 6 kWh of energy. Electricity costs 8.5 cents per kilowatt hour. What does it cost, in cents, to run the kettle that week?
A gas-powered generator runs a light bulb. Which sequence best describes the energy conversions?
A gas furnace takes in 12 000 J of energy from its fuel and delivers 9 600 J of useful heat to the house. What is its percent efficiency?
A 2 000 W space heater is left running and uses 5 kWh of energy. Using E = Pt, for how many hours was it running?
Two light bulbs give out the same amount of light. Bulb A draws 60 W and bulb B draws 9 W. If both run for 1 000 hours, how many kilowatt hours does the household save by choosing bulb B?
Use the table. What is the car engine's efficiency, as a percent?
An LED light bulb receives 10 W of electrical power and produces 9 W of useful light output. Find its efficiency.
An incandescent bulb is only 40% efficient at producing light. Where does the other 60% of its input energy go?