Question

do the math | energy efficiency the ratio of useful energy to the overall input energy in a system is known as efficiency. incandescent light bulbs are not efficient. they transform a large portion of electrical energy into thermal energy instead of light. to save energy, these bulbs have been replaced with compact fluorescent lamps (cfls) and light - emitting diodes (leds). cfls and leds use less electrical energy to produce the same amount of light.
the flow of energy through a system can be expressed in watts (w). one watt is equal to the flow of one joule of energy for one second. this table shows the wattage of each type of bulb with equivalent brightness. electrical energy is often measured in kilowatt - hours (kw·h), or the amount of energy used in one hour at the rate of 1,000 w.
energy used by light bulbs

bulb type	watts (j/s)	energy used over 2,000 hours (kw·h)
cfl	14	28
led	10	20

1. suppose the price of electricity is 12.75 cents per kw·h. what is the difference in cost in dollars between the use of an incandescent bulb for 2,000 hours and an led bulb for the same amount of time?
2. how much more energy (in j) does an incandescent bulb use than a cfl in one minute?

describe efficient energy use
laptops and other electronic devices become warm when they are in use. some of the chemical energy from the battery is transformed into thermal energy instead of electrical energy. much of the electrical energy produced by the battery becomes thermal energy as it flows through the computer parts. the electrical energy is useful, but the thermal energy is not. the battery loses chemical energy in both useful and non - useful transformations as it is used.

1. discuss hand - cranked flashlights transform mechanical energy into other forms of energy. which energy transformations are useful, and which are not? explain why most hand - cranked flashlights are made with leds.

Explanation:

20.

Step1: Calculate cost of incandescent bulb

Cost of incandescent bulb = Energy used × Price per kWh. Energy used is 120 kWh and price is 12.75 cents per kWh = 0.1275 dollars per kWh. So cost = 120×0.1275 = 15.3 dollars.

Step2: Calculate cost of LED bulb

Cost of LED bulb = Energy used × Price per kWh. Energy used is 20 kWh and price is 0.1275 dollars per kWh. So cost = 20×0.1275 = 2.55 dollars.

Step3: Calculate cost difference

Cost difference = Cost of incandescent - Cost of LED = 15.3 - 2.55 = 12.75 dollars.

Step1: Calculate energy used by incandescent in 1 minute

Power of incandescent bulb is 60 W. Energy = Power×Time. Time = 1 minute = 60 s. So energy used by incandescent, $E_{i}=60\times60 = 3600$ J.

Step2: Calculate energy used by CFL in 1 minute

Power of CFL bulb is 14 W. Energy = Power×Time. Time = 60 s. So energy used by CFL, $E_{c}=14\times60 = 840$ J.

Step3: Calculate energy difference

Energy difference = $E_{i}-E_{c}=3600 - 840=2760$ J.

Hand - cranked flashlights transform mechanical energy into electrical energy (useful) and then into light energy (useful). Some mechanical energy is also transformed into thermal energy (not useful) due to friction. LEDs are used because they are more energy - efficient, converting a higher proportion of electrical energy into light energy and less into thermal energy compared to other light sources.

Answer:

12.75

21.