FT - 14 Electricity
Q1.(a)
A student used the apparatus drawn below to investigate the heating effect of an electric
heater.
(i)
Before starting the experiment, the student drew Graph A.
Graph A shows how the student expected the temperature of the metal block to
change after the heater was switched on.
Describe the pattern shown in Graph A.
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(2)
(ii)
The student measured the room temperature. He then switched the heater on and
measured the temperature of the metal block every 50 seconds.
The student calculated the increase in temperature of the metal block and
plottedGraph B.
After 300 seconds, Graph B shows the increase in temperature of the metal block is
lower than the increase in temperature expected from Graph A.
Suggest one reason why.
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(1)
(iii)
The power of the electric heater is 50 watts.
Calculate the energy transferred to the heater from the electricity supply in
300 seconds.
Use the correct equation from the Physics Equations Sheet.
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Energy transferred = ........................................... J
(2)
(b)
The student uses the same heater to heat blocks of different metals. Each time the heater
is switched on for 300 seconds.
Each block of metal has the same mass but a different specific heat capacity.
Metal
Specific heat capacity in J/kg°C
Aluminium
900
Iron
450
Lead
130
Which one of the metals will heat up the most?
Draw a ring around the correct answer.
aluminium
iron
lead
Give, in terms of the amount of energy needed to heat the metal blocks, a reason for your
answer.
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(2)
(c)
A homeowner uses an electric immersion heater to heat the water in his hot water tank.
The hot water tank has no insulation.
(i)
Draw a ring around the correct answer to complete each sentence.
conduction.
Energy is transferred through the water
by
convection.
evaporation.
conduction.
Energy is transferred through the copper wall of the hot water tank
by
convection.
evaporation.
(2)
(ii)
To keep the water in the tank hot for longer, the homeowner fits an insulating jacket
around the tank. The insulating jacket costs £12 to buy.
The homeowner expects to save £16 each year from reduced energy bills.
Calculate the pay-back time for the insulating jacket.
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Pay-back time = ........................................... years
(2)
(Total 11 marks)
HT - 14 Electricity
Q2.A householder wants to reduce his electricity bill. He records the power rating of four electrical
appliances and how much time each appliance is used in one day.
(a)
Appliance
Power rating
Time in use in
one day in hours
Energy transferred
in one day in kWh
Shower
10 kW
0.50
5.0
Fridge–freezer
160 W
24
Plasma TV
400 W
6.0
2.4
Kettle
3000 W
0.25
0.75
(i)
Complete the table by calculating the energy transferred in one day by the fridgefreezer.
Use the correct equation from the Physics Equations Sheet.
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Energy transferred = .................................................... kWh
(2)
(ii)
Which appliance would cost the least to use each day?
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(1)
(b)
The householder decides to change his 10 kW electric shower. A gas boiler heats the
water for the new shower.
The electric shower transfers 5 kWh of energy to heat the water each day.
The cost of electricity is 12 pence per kWh. The cost of gas is 5 pence per kWh.
Calculate the saving the householder will make if the water is heated using gas.
Assume that the efficiency of the two methods of heating water is the same.
Use the correct equation from the Physics Equations Sheet.
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Saving = ........................................
(3)
(c)
As an alternative method to heat water for his shower, the householder installs solar
panels on his roof. The householder could use solar panels on the roof to heat all the
water for his house.
The Sun transfers energy to the solar panels at a rate of 8400 J/s. The mass of water in
the pipes is 20 kg. The water has a specific heat capacity of 4200 J/kg°C.
Calculate the temperature rise of the water per minute.
Use the correct equation from the Physics Equations Sheet.
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Temperature rise per minute = ........................................°C
(3)
(Total 9 marks)
M1.(a)
(i)
temperature (increase) and time switched on are directly proportional
accept the idea of equal increases in time giving equal increases
in temperature answers such as:
•
as time increases, temperature increases
•
positive correlation
•
linear relationship
•
temperature and time are proportional
score 1 mark 2
(ii)
(iii)
(b)
any one from:
“it” refers to the metal block
•
energy transfer (from the block) to the surroundings
accept lost for transfer
accept air for surroundings
•
(some) energy used to warm the heater / thermometer (itself)
accept takes time for heater to warm up
•
(metal) block is not insulated
1
15 000
allow 1 mark for correct substitution, ie 50 × 300 provided no
subsequent step shown 2
lead
reason only scores if lead is chosen 1
needs least energy to raise temperature by 1°C
accept needs less energy to heat it (by the same amount)
lowest specific heat capacity is insufficient 1
(c)
(i)
convection
correct order only
1
Conduction 1
(ii)
3 / 4 (year)
Or allow 1 mark for correct method, ie
shown
0.75 or
9 months or
274 days
2
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M2.(a)
(i)
3.84 (kWh)
allow 3.8
correct substitution
(E = P x t =) 0.16 × 24
gains 1 mark provided no subsequent step shown
or
an answer of 3840 or 3800 gains 1 mark
2
(ii)
kettle
allow ecf from (a)(i)
1
(b)
(saving =) 35 p or £0.35 (per day)
for full marks the correct unit must be given
correct calculation of cost of gas or electricity is worth 1 mark for
each calculation
electric = (5kWh × 12 =) 60 (p)
accept £0.60
gas = (5kWh × 5 =) 25(p)
accept £0.25
calculation of saving per kWh
(12 - 5 =) 7 (p)
is worth 1 mark
3
(c)
6 (°C)
allow max 2 marks for calculation of temperature rise per second
0.1 (°C)
allow correct substitution for 1 mark provided no subsequent step
shown
i.e. 8400 = 20 x 4200 x θ
or
8400 x 60 = 20 x 4200 x θ
or
8400 = 84000 x θ
or
θ=
or
θ=
or
θ=
3
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