Efficiency and Sankey Diagrams

Efficiency

·         What two things does efficiency compare?

·         Write down the formula for efficiency.

·         What is efficiency measured in?

·         Can it ever be equal to, or more than 100%? Give a reason for this.

·         Calculate the efficiency of a bulb if Energy in = 100 J and Useful energy out = 10J

·         An equivalent energy saving bulb gives off the same amount of light (100W) but “uses” 25 W of electricity. Calculate its efficiency

·         Calculate the efficiency of a car if Energy in = 5000 J and Energy out = 1250 J

Energy

• List the nine forms of energy (in 2 columns)
• Use the example of the falling parachutists to show that energy isn’t made just transferred from one form to another.
• State the law of conservation of energy.
• Use the example of the rollercoaster to show that in every energy transfer there is always some heat loss.
• What does this mean for any energy transfer?
• Why is an energy transfer diagram useful?

Prep find out why energy transfer diagrams are called Sankey Diagrams.

Saving energy

Saving Energy

• What has happened to the price on energy over the last decade?
• What is different between the price rise in 2006 and 2007?
• Do you think energy prices may fall in the future?
• What are EU Energy labels?
• How much energy does an A grade Fridge freezer consumes in a year?
• You have a fridge freezer that uses 500 kWh per year.
• You replace it with a band A freezer.
• How much do you save?
• Write a formula to calculate payback time
• Copy this table and fill it in for the house on page 267

Energy saving method	Cost (£)	Savings per year (£)	Payback time (years)

• Do question 7
• Now check you have covered the questions on page 266 – 267
  
  
  
  
  
  
  
  
  
  
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Paying for electricity worksheet

The table below shows some household appliances with their power ratings and how long they are used for in a typical day. Complete the table by working out how many units of electricity are used in each situation and the cost if one Unit (1 kilowatt-hour) is 12p.

Appliance	Power rating (in kW)	X	Time used (in hours)	=	Number of kW h used	X 12 p	Total cost (p)
Tumble dryer	3.5	X	1	=	3.5	X 12	4.2
Electric fire	2	X	3	=	6	X 12	7.2
Electric shower	10	X	0.2	=	2	X 12	24
Hall light	0.1	X	5	=	0.5	X 12	6
Radio	0.05	X	4	=	0.2	X 12	2.4
Electric kettle	3	X	0.4	=	1.2	X 12	14.4
Television	0.16	X	5	=	0.8	X 12	9.6
Fridge-freezer	0.1	X	24	=	2.4	X 12	28.8
Oven	3	X	0.45	=	1.35	X 12	16.2

 

1       A laptop computer has an electricity supply of 20 V and takes a current of 4  A. What is its power?   

P = I x V = 4 A x 20 V = 80 W

2       A light bulb uses the mains supply of 230 V. The current through the bulb is 0.1 A. What is the power of the light bulb?

P = I x V = 0.1 A x 230 V = 23 W

3       The light bulb in question 2 is switched on for 10 minutes. How much energy does it use? (Hint: use energy = power × time and remember to convert the time into seconds.)

E = P x t = 23 W x (10 x 60 s) = 13 800 J

4       A 3 kW electric fire is switched on for 2 hours. Electricity costs 12p per kW h. How much does it cost to use the fire?

kWh used = kWh x h = 3 kW x 2 hrs = 6 kWh          Cost = 6kWh x 12p = 72p

5     A 2 kW electric heater is used for an hour.

      a    What is the power of the heater in watts?      2000 W

      b    How many seconds are there in 2 hours?      2hr x 60 min x 60 s = 72 000 s

      c    How much energy (in Joules) does the heater use while it is switched on? 

      E = P x t = 2000 W x 72 000 s = 144 000 000 J = 144 MJ

      d    How much energy does it use in kilowatt-hours?       E = 2kW x 2 hr = 4kWh