P1 Revision Booklet

Cost of electricity

Medium (Grade C)

Q1.          The diagram shows how one type of electric storage heater is constructed. The heater has ceramic bricks inside. The electric elements heat the ceramic bricks during the night. Later, during the daytime, the ceramic bricks transfer the stored energy to the room.

(a)     In winter, the electricity supply to a 2.6 kW storage heater is switched on each day between midnight and 7 am. Between these hours, electricity costs 5 p per kilowatt-hour.
Calculate the daily cost of using the storage heater.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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                                              Cost = .................................................. p
(3)

(b)     Homes with electric storage heaters have a separate meter to measure the electricity supplied between midnight and 7 am. Another meter measures the electricity supplied at other times. This electricity supplied at other times costs 15 p per kilowatt-hour.
Electricity companies encourage people to use electricity between midnight and 7 am by selling the electricity at a lower cost.
Suggest why.
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(1)

(c)     By 7 am, the temperature at the centre of the ceramic bricks is about 800 °C.
The temperature of the outside metal casing is about 80 °C.
The ceramic bricks are surrounded by ‘super-efficient’ insulation.
Explain why.
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(2)

(d)     At 7 am, the electricity supply switches off and the temperature of the ceramic bricks starts to fall. The temperature of the bricks falls by 100 °C over the next four hours. During this time, 9 000 000 J of energy are transferred from the bricks.
Calculate the total mass of ceramic bricks inside the heater.
Specific heat capacity of the ceramic bricks = 750 J/kg °C.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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                                                     Mass = .................................................. kg
(2)
(Total 8 marks)

 

Q2.          The information plate on a hairdrier is shown.

          (a)     What is the power rating of the hairdrier?
..................................................................................................................................
(1)

          (b)     (i)      Write down the equation which links current, power and voltage.
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(1)

(ii)     Calculate the current in amperes, when the hairdrier is being used. Show clearly how you work out your answer.
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Current = .............................. amperes
 (2)

(iii)     Which one of the following fuses, 3A, 5A or 13A, should you use with this hairdrier?
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(1)

(c)     The hairdrier transfers electrical energy to heat energy and kinetic energy.

          Use the following equation to calculate the efficiency of the hairdrier in transferring electrical energy into heat energy.
efficiency = 
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Efficiency = ..............................
(2)

(d)     One kilowatt-hour of electricity costs 6p. Use the following equation to calculate how much it will cost to use the hairdrier for 10 minutes.
cost of electricity = energy transferred × price per unit
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......................................................................................................................................
......................................................................................................................................
Cost = .............................
(2)
(Total 9 marks)

 

Q3.          (a)     The picture below shows the bathroom in a house.

          Describe three examples of dangerous practice in the use of mains electricity in this bathroom.
1. ................................................................................................................................
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2. ................................................................................................................................
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3. ................................................................................................................................
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(3)

(b)     In the table below three electrical appliances are listed with their power ratings and the number of hours they are used each week.

ELECTRICAL APPLIANCE	POWER RATINGS
(W)	TIME USED EACH WEEK
(h)	k Wh USED
EACH WEEK	
TV	200	35	 	
Kettle	2000	2	 	
Toaster	1000	1	 	
Cooker	11 500	7	 	

(i)      Complete the table by inserting the number of kWh used by each appliance each week.

(ii)     Which appliance would cost the least to run per week?
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(iii)     The cost of running a toaster is 8p per week. How much does it cost to run the kettle each week?
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(6)
(Total 9 marks)
 
Q4.          The diagram shows a fan heater.

(a)     Complete this sentence.
          The fan heater is designed to transfer electrical energy as ........................................
          energy and ........................................ energy.
(2)

(b)     The fan heater is connected to the mains by a three core cable.
(i)      Why are the wires in the cable made out of copper?
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(ii)     Why are the wires in the cable covered by plastic?
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(2)

(c)     
You may find this equation useful when answering this part of the question
 
energy transferred (kWh)  =  power (kilowatt, kW)  ×  time (hour, h)	

(i)      The power of the fan heater is 2.75 kW.
Calculate how many kilowatt hours (kWh) of energy are transferred when the fan heater is used for 6 hours.
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Number of kilowatt hours .........................
(2)

(ii)     How much will it cost to use the fan heater for 6 hours if one Unit of electricity costs 7p?
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Cost ..................... p
(2)

(d)     A fault caused a much higher than normal current to flow in the heater. 
Describe what happened to the wire in the fuse.
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(2)
(Total 10 marks)
 Q5.          The diagram shows the label from a new freezer.

(a)     An old freezer has an energy consumption per year of 350 kWh.
          Use the equation in the box to calculate the extra cost of using the old freezer for one year compared with using a new ‘A’ rated freezer.
total cost = number of kilowatt-hours × cost per kilowatt-hour	

Assume 1 kilowatt-hour (kWh) of energy costs 12 p.
Show clearly how you work out your answer.
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Extra cost per year = £ ..............................
(2)

(b)     The price of the new freezer was reduced in a sale.
          Reducing the price reduces the payback time for replacing the old freezer from 12 years to 9 years.
Calculate, in pounds, how much the new freezer was reduced in the sale.
Show clearly how you work out your answer.
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Price reduced by = £ ........................................
(2)

(c)     An advertisement in a shop claims that:
‘Replacing an old freezer with a new ‘A’ rated freezer will benefit the environment.’
Do you agree that replacing the freezer will benefit the environment?
Answer yes or no. ..............................
Explain the reasons for your answer.
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(2)
(Total 6 marks
 Q6.          (a)     The diagram shows the energy transformations produced by a TV.

(i)      Use the information in the diagram and the equation in the box to calculate the efficiency of the TV.
 
efficiency = 	

         Show clearly how you work out your answer.
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Efficiency = ..............................
(2)

(ii)     What eventually happens to the useful energy transferred by the TV?
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(1)

(b)     Electrical appliances left on standby use energy.
          The bar chart shows the power for the appliances that one family leaves on standby when they go on holiday.

          The family is on holiday for a total of 175 hours.

(i)      Use the information in the bar chart and the equation in the box to calculate the energy wasted by leaving the compact stereo on standby while the family is on holiday.
 
energy transferred
(kilowatt-hour, kWh)	=	power
(kilowatt, kW)	×	time
(hour, h)	

         Show clearly how you work out your answer.
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Energy wasted = .............................. kilowatt-hours
(2)

(ii)     Electricity costs 12 p per kilowatt-hour.
         Use the equation in the box to calculate the cost of leaving the compact stereo on standby while the family is on holiday.
 
total cost = number of kilowatt-hours × cost per kilowatt-hour	

         Show clearly how you work out your answer.
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Cost = .............................. p
(1)

(c)     A headline from a recent newspaper article is shown below.
                               
          Explain why leaving appliances on standby damages the environment.
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(2)
(Total 8 marks)

 

Q7.          The data included in the diagrams gives the power of the electrical appliances.

TV
160 W	Radiant heater
1.0 kW	Hairdryer
1100 W	

			
Sandwich toaster
1.1 kW	Food processor
0.4 kW	Table lamp
40 W	

(a)     (i)      Which of the appliances are designed to transform electrical energy to kinetic energy?
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(1)

(ii)     Which of the appliances waste energy as heat?
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(1)

(b)     Leaving the radiant heater switched on is likely to lead to more carbon dioxide being emitted into the atmosphere than leaving the table lamp on for the same length of time.
Explain why.
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(2)

(c)     A homeowner decides to monitor the amount of electrical energy used in his home. He can do this by using the home’s electricity meter or by using a separate electronic device.
The table gives some information about each method.
 
Electricity meter	Electronic device	
Records to the nearest
kilowatt-hour	Records to the nearest
1/100th kilowatt-hour	
Homeowner takes
readings at regular
intervals	Energy use recorded
continuously and
stored for one year	
 	Displays a graph
showing energy use
over a period of time	

(i)      Complete the following sentence.
The reading given by the electronic device is more .................................................. than the reading given by the electricity meter.
(1)

(ii)     Suggest how data collected and displayed by the electronic device could be useful to the homeowner.
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(3)
(Total 8 marks)

 Q8.           A student uses an electric motor to lift a load.

In the motor, the electrical energy is transferred into other types of energy. Some of this energy is useful and the rest of the energy is wasted. 
(a)     (i)     Name the useful energy output from the electric motor.
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(1)

(ii)      What eventually happens to the wasted energy?
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(1)

(b)     The graph shows the input energy the motor needs to lift different loads by one metre.

What can you conclude from the graph about the relationship between the load lifted and the input energy needed?
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(2)

 
(c)	A shop uses escalators to lift customers to different floor levels. The escalators use electric motors. When the shop is not busy some escalators are turned off. A sign tells the customers that the escalators are turned off to save energy.		

(i)      Each escalator has one motor with an average power of 4000 W. The motor is turned on for an average of 8 hours each day, 6 days each week. Electricity costs 15 pence per kilowatt-hour.
Calculate the cost of the electricity used in an average week to run one escalator.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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                                           Cost = ............................................. pence
(3)

(ii)     Give one environmental advantage to turning off electrical appliances when they are not being used.
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(1)
(Total 8 marks)

 

Q9.          The picture shows a new washing machine. When the door is closed and the machine switched on, an electric motor rotates the drum and washing.

(a)     Complete the following sentences.
(i)      An electric motor is designed to transform electrical energy into
............................................... energy.
(1)

(ii)     Some of the electrical energy supplied to the motor is wasted as
....................................... energy and ...................................... energy.
(1)

(b)     What happens to the energy wasted by the electric motor?
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(1)

(c)     The diagram shows the label from the new washing machine.

An ‘A’ rated washing machine is more energy efficient than a ‘C’ rated washing machine.
Explain what being more energy efficient means.
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(2)

(d)     The graph shows that washing clothes at a lower temperature uses less energy than washing them at a higher temperature. Using less energy will save money.

(i)      Electricity costs 12 p per kilowatt-hour (kWh).
The temperature setting is turned down from 40 °C to 30 °C.
Use the graph and equation in the box to calculate the money saved each wash cycle.
 
total cost   =   number of kilowatt-hours   ×   cost per kilowatt-hour	

Show clearly how you work out your answer.
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                                 Money saved = .................................................. p
(2)

(ii)     Suggest why reducing the amount of energy used by washing machines could reduce the amount of carbon dioxide emitted into the atmosphere.
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(1)
(Total 8 marks)
 Q10.          (a)     The diagram shows two switches on a room heater. The heater has three power settings. The power produced by two of the settings is given in the table.
 
	Setting	Power in watts	
	Low	700	
	Medium	1400	
	High	 	

(i)      When both switches are on, the heater works at the high power setting.
What is the power of the heater, in kilowatts, when it is switched to the high power setting? 
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                        Power = ......................................................... kilowatts
(1)

(ii)     The heater is used on the high power setting. It is switched on for 1½ hours.
Use the equation in the box to work out the energy transferred from the mains to the heater in 1½ hours.
 
energy transferred    =    power    ×    time	

Show clearly how you work out your answer and give the unit.
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                         Energy transferred = ....................................................
(3)

(iii)    This type of heater is a very efficient device.
What is meant by a device being very efficient?
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(1)

(b)     The graph shows how the temperature of a room changes during the 1½ hours that the heater is used.

After 1 hour, the temperature of the room has become constant, even though the heater is still switched on.
Explain why.
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(2)
(Total 7 marks)
Cost of electricity-Mark scheme
M1.        (a)      E = P × t
91 (p)
an answer £0.91 gains 3 marks
an answer 0.91 gains 2 marks
allow 2 marks for energy transferred = 18.2 (kWh)
or
substitution into 2 equations combined, ie 2.6 × 7 × 5
allow 1 mark for correct substitution into E = P × t, ie E = 2.6 × 7
or
allow 1 mark for multiplying and correctly calculating an incorrect energy transfer value by 5
3

(b)     answers should be in terms of supply exceeding demand
accept there is a surplus / excess of electricity (at night)
1

(c)     reduce (rate of) energy transfer (from ceramic bricks)
accept heat for energy
do not accept no energy / heat escapes
do not accept answers in terms of lost / losing heat if this implies heat is wasted energy
1

so keeping the (ceramic) bricks hot for longer
accept increase time that energy is transferred to the room
accept keep room warm for longer
or
to stop the casing getting too hot
accept so you do not get burnt (on the casing)
1

(d)     E = m × c × θ
120
allow 1 mark for correct substitution
ie 9 000 000 = m × 750 × 100
2
[8]

M2.          (a)     800 (W)
accept 0.8kW but this answer must have the unit
1

(b)     (i)      power = voltage × current
accept the equation rearranged
accept P = VI
do not accept C for current
do not accept P = VA 
do not accept power = VA
do not accept

unless subsequent calculation shows understanding
1

(ii)     3.5 (A)
accept a larger number of d.p. but you must be able to round to 3.5
allow 1 mark for
current = 
 
or (I =) 
2

(iii)     5 (A)
independent of (ii) unless e.c.f from part (b)(ii)
1

(c)     0.95 or 95 (%)
allow I mark if useful energy output is given as 760 ignore any incorrect unit
2

(d)     0.8(p) or 0.799(p)
unit not essential unless converted to £
allow subsequent rounding up to 1p
allow 1 mark for correct conversion to kW (0.8) or
allow 1 mark for correct conversion to hours (1/6 or 0. 16)
2
[9]

 

M3.          (a)     Mains socket – once only
Shower cable can get wet
Trailing cable to fire (not heater unless fire clearly identified)
Use of fire
Free running cable from ceiling
Appliance on side of bath
Use of ordinary light switch
Free cable to sink light
any 3 each for1 mark
3

(b)     (i)      7, 4, 1, 80.5
Four right – 2
Three right – 1
All right in W – 1
2

(ii)     Toaster
1

(iii)     32p
gets 3 marks
Else 8 × 4 
gets 2 marks
Else unit cost = 8p 
gets 1 mark
3
[9]
 M4.          (a)     heat / thermal
kinetic / movement
each for 1 mark
2
(b)     (i)      its a good (electrical) conductor
for 1 mark
1

(ii)     its a good (electrical) insulator / very poor conductor
for 1 mark
1

(c)     (i)      2.75 × 6
gains 1 marks
         but
16.5
gains 2 marks
2

(ii)     (c)(i) × 7 or no. of kW h × cost/kW h
gains 1 marks
         but
115.5 or e.c.f if correct
gains 2 marks
2

(d)     it would heat and melts / blows / burns out / breaks circuit
any two for 1 mark each (fuse wire just breaks – gains 1)
(blows up – gets 0)
(fuse causing wire to melt gets 1)
2
[10]
 
M5.          (a)     £15
allow 1 mark for use of 125 (kWh)
allow 1 mark for an answer 1500
allow both marks for 1500 pence / p
allow 1 mark for correct calculation of annual cost for either freezer (£27 and £42)
2

(b)     £45
          or their (a) × 3
allow 1 mark for correct use of 3
allow 1 mark for 12 – 9 = 3
2

(c)     any two from:
the marks are for the explanation
          yes plus explanation
•        less electricity / energy needed / used
accept less energy wasted
•        less (fossil) fuels burned
accept a named fossil fuel
do not accept conserving (fossil) fuels
•        less polluting gases emitted
accept a named polluting gas / greenhouse gases / carbon emissions / reduce global warming
accept an answer in terms of nuclear fuel
eg    less nuclear fuel required (1)
       less nuclear waste (1)
2
          or no plus explanation
•        old freezer must be disposed of
•        hazardous chemicals inside freezer
accept CFC gases
•        (lot of) energy used in producing new freezer
[6]
 
M6.          (a)     (i)      0.6
accept 60 %
allow 1 mark for useful energy = 480
answer 0.6 with any unit or 60 gains 1 mark only
2

(ii)     transferred to surroundings
accept goes into the air
accept heats the surroundings up
accept gets spread out
accept transferred into heat (only)
do not accept wasted / lost unless qualified
destroyed negates mark
transferred into light / sound negates mark
1

(b)     (i)      1.75
allow 1 mark for converting to kW
answers of 0.7, 0.525, 0.35, 0.875, 1.05, 5.25 gains 1 mark
answers of 1750 or 17.5 gains 1 mark
2

(ii)     21p or £0.21 or their (b)(i) × 12
1

(c)     any two from:
•        (more) electricity needs to be generated
(more) electricity is being used
•        (more) power stations needed
•        (more) fossil fuels burnt
accept named fossil fuel
•        (more) pollutant gases emitted
accept named gas
accept harmful for pollutant
accept greenhouse gases
accept atmospheric pollution
accept answer in terms of any form of electricity generation and an associated environmental problem
2
[8]
 M7.         (a)      (i)     food processor
hairdryer
both required and no other
either order
1

(ii)     TV
Table lamp
Food processor
all required and no other
any order
1

(b)     any two from:
•        transfers / requires / uses more energy / power
accept more electricity used
accept higher power
•        more electricity needs to be generated
•        more (fossil) fuels (likely) to be burnt
accept a named fossil fuel
2

(c)     (i)      precise
this answer only
1

(ii)     any three from:
•    can look for trends / patterns
•    help reduce energy use / consumption
•    reduce bills
accept save money
•    identify appliances which use a lot of energy
•    replace appliances with more efficient ones
•    see effect of leaving appliances on (standby)
to monitor usage is insufficient
answers in terms of environment are insufficient
3
[8]
 M8.         (a)    (i)        kinetic (energy)
allow gravitational potential (energy) / gpe
movement is insufficient
1

(ii)     dissipates into the surroundings
allow warms up the surroundings / air / motor
accept lost to the surroundings
accept lost as heat
ignore reference to sound
it is lost is insufficient
1

(b)     energy (required) increases with load
accept positive correlation
do not accept (directly) proportional
1

further amplification eg increases slowly at first (or up to 4 / 5 N), then increases rapidly
simply quoting figures is insufficient
an answer that only describes the shape of the line gains no marks
1

(c)     (i)      E = P × t
2880
accept £28.80 for all 3 marks
an answer £2880 gains 2 marks
allow 1 mark for obtaining 48 h or converting to kW
allow 2 marks for correct substitution
ie 4 × 48 × 15
note: this substitution may be shown as two steps
an answer 2 880 000 gains 2 marks
an answer £4.80 / 480 gains 2 marks
an answer of 192 (ie calculation of energy without subsequent calculation of cost) gains 1 mark)
3

(ii)     any sensible suggestion eg
conserves fossil fuels
less (fossil) fuels burned
less pollutant gas (produced)
accept a named pollutant gas
less greenhouse gas (produced)
saves energy is insufficient
1
[8]
 M9.         (a)      (i)     kinetic 
do not accept movement
1

(ii)     thermal
accept heat for thermal
sound
do not accept noise for sound
both answers required in either order
1

(b)     transferred to surroundings / surrounding molecules / atmosphere
‘it escapes’ is insufficient
or
becomes dissipated / spread out
accept warms the surroundings
accept degraded / diluted
accept a correct description for
surroundings eg to the washing machine
do not accept transformed into heat on its own
1

(c)     a smaller proportion / percentage of the energy supplied is wasted
owtte
accept a statement such as ‘less energy is wasted’ for 1 mark
do not accept costs less to run
ignore references to uses less energy
2

(d)     (i)      2.4 (p) 
accept 2 p if it is clear from the working out this is rounded from 2.4 p
allow 1 mark for correct substitution of correct values
ie 0.2 × 12
allow 1 mark for calculating cost at 40 °C (13.2 p)
or
cost at 30 °C (10.8 p)
2

(ii)     any one from:
•        less electricity needed
ignore answers in terms of the washing machine releasing less energy
an answer in terms of the washing machine releasing CO2 negates the mark
do not accept less energy is produced
•        fewer power stations needed
•        less fuel is burned
accept a correctly named fuel
do not accept less fuel is needed
1
[8]
 
M10.         (a)      (i)     2.1 
correct answer only
1

(ii)     3.15
or
their (a)(i) × 1.5 correctly calculated
allow 1 mark for correct substitution
ie 2.1 × 1.5
or
their (a)(i) × 1.5
2

kilowatt-hour
accept kWh
or
a substitution 2100 × 5400 scores 1 mark
2100 × 5400 incorrectly calculated with answer in joules scores 2 marks
an answer of 11 340 000 scores 2 marks
an answer of 11 340 000 J scores 3 marks
1

(iii)    most (input) energy is usefully transformed
accept does not waste a lot of energy
accept most of the output / energy is useful
do not accept it does not waste energy
1

(b)     the room is losing energy / heat
1

at the same rate as the heater supplies it
this mark only scores if the first is scored
do not accept heater reaches same temperature as room / surroundings
rate of heat gain = rate of heat loss scores both marks
1
[7]