ONACD - Editable curriculum designed for teachers by teachers

Efficiency

Where does the energy go?

ONACD –Editable Curriculum designed for teachers by teachers

LED bulbs are more efficient than any other light bulb

ONACD - Editable curriculum designed for teachers by teachers

Depending on the information given you can use either of the power equations to determine power consumption.

  or  How much energy does a 1500W hair dryer being used for

60 seconds use?E = Pt

E = (1500)(60)E = 90 000 Joules

 Was all of the heat energy from the hair dryer transferred to

the hair it was drying?No, most of it was lost to heating the air surrounding the hair.

P IVEPt

EPt

ONACD - Editable curriculum designed for teachers by teachers

Heat loss to the surroundings often causes devices to be inefficient. Energy is transferred from an input source and transformed to output energy at the load.

 For example, in one second a 60W bulb uses an input of

60J of energy to output 3J of light energy.

ONACD - Editable curriculum designed for teachers by teachers

EfficiencyThe efficiency of the 60W bulb in the previous example is:

Efficiency = 0.05 or 5%Examples:1. An old computer monitor operates at 1.3A. How many

kilowatts does it use? P = IV P = (1.3)(110) P = 143W P = 0.143kW

2. At $0.12 per kWh how much does it cost to run for 4 hours?Cost = kW x hr x rateCost = (0.143)(4)(0.12); Cost = $0.069 or 7 cents 

360JefficiencyJ

ONACD - Editable curriculum designed for teachers by teachers

3. Consider a 300 W hotplate heats 400 mL of water for 6 minutes (360 seconds). The temperature raised 25 oC.

a) How many joules of energy were used? 

b) It requires 1675J to heat 400 mL of water 1oC. How much energy should have been used to heat the water 25 oC?

25 times 1675 = 41875J c) How much energy was “lost”?

108000 - 41875 = 66125J

108000E Joules300360E

EPt

ONACD - Editable curriculum designed for teachers by teachers

d) Where did this energy go?Heat was lost to the surroundings around the edges of the

element and off the surface of the water and container.

 e) What is the efficiency of the hotplate set-up?

 f) What could be done to improve the efficiency?Use a container that covers the entire element, use a lid, use a

better insulating container.

outputefficiencyinput

41875108000

efficiency 39%efficiency