Transcript Slide 1 - divaparekh

AQA GCSE 1a-3
Electrical Energy
AQA GCSE Physics pages 50 to 61
AQA GCSE Science pages 254 to 265
January 10th 2011
AQA GCSE Specification
ELECTRICAL POWER, COST & DISTRIBUTION
11.3 Why are electrical devices so useful?
Using skills, knowledge and understanding of how science works:
• to compare and contrast the particular advantages and disadvantages of using different electrical
devices for a particular application.
• to calculate the amount of energy transferred from the mains using,
energy transferred = power × time
(kilowatt-hour, kWh) (kilowatt, kW) (hour, h)
• to calculate the cost of energy transferred from the mains using,
total cost = number of kilowatt-hours x cost per kilowatt-hour
Skills, knowledge and understanding of how science works set in the context of:
• Examples of energy transformations that everyday electrical devices are designed to bring about.
• Examples of everyday electrical devices designed to bring about particular energy transformations.
• The amount of electrical energy a device transforms depends on how long the appliance is switched
on and the rate at which the device transforms energy.
• The power of an appliance is measured in watts (W) or kilowatts (kW).
• Energy is normally measured in joules (J).
• Electricity is transferred from power station to consumers along the National Grid.
• The uses of step-up and step-down transformers in the National Grid.
• Increasing voltage (potential difference) reduces current, and hence reduces energy losses in the
cables.
Electrical energy
Electrical energy is useful because it is easy
to transform into useful forms of energy.
Electrical devices
How many electrical devices can you spot in this picture?
Some uses of electrical energy
Device
Useful energy
Wasted energy
Light bulb
Light
Heat
Electric heater
Heat
Light
Electric motor Kinetic & potential
iPod
Hairdryer
Heat & sound
Sound
Heat
Heat and kinetic
Sound
Electrical devices
Notes questions from pages 50/254 & 51/255
1. Why is electrical energy useful?
2. Copy four items from the table on page
50/254.
3. Copy and answer questions (a), (b) and (c) on
page 51/255.
4. Copy the Key Points on page 51/255.
5. Answer the summary questions on page
51/255.
Electrical devices
ANSWERS
In text questions:
Summary questions:
(a) Electrical energy
1. Electrical, thermal,
transforms to kinetic energy
light
of the brush, thermal
2. 1C, 2C, 3A, 4B
energy due to friction
between the moving parts
and resistance, and sound
(b) (i) microphone
(ii) a loudspeaker
(a) An amplifier
Electrical power
The electrical power of a device is a measure of how
quickly the device uses electrical energy.
electrical power = electrical energy
time
electrical power is measured in watts (W)
1 watt means I joule per second
also:
1 kilowatt (kW) = 1 000 watts
1 megawatt (MW) = 1 000 000 watts
Electrical power ratings
These are always
shown on an electrical
device along with
voltage and frequency
requirements.
Power comparisons
Device
Torch
Electric kettle
Car
Power rating
1W
2 000 W or 2 kW
50 000 W or 50 kW
Space rocket
100 000 000 W or 100 MW
Power station
10 000 MW
The Sun
100 000 000 000 000 000 000 MW
Question 1
Calculate the power of a light bulb that uses
2400 joules of electrical energy in 60 seconds.
electrical power = electrical energy
time
= 2400 J
60 s
electrical power = 40 watts
Question 2
Calculate the energy used in joules by a toaster of power
2kW in 5 minutes.
electrical power = electrical energy
time
becomes:
electrical energy = power x time
= 2 kW x 5 minutes
= 2000 W x 300 seconds
electrical energy used = 600 000 joules (or 600 kJ)
Answers
Complete:
Electrical
energy used
600 J
Time
Power
30 s
20 W
10 000 J
20 s
500 W
600 J
40 s
15 W
90 kJ
5 minutes
300 W
Choose appropriate words to fill in the gaps below:
Electrical energy is convenient to use as it is
transferred
easily____________
into useful forms of energy. Electrical
joules
energy is measured in ________,
symbol J.
power
The electrical _________
of a device is equal to the rate at
electrical
which a device transforms ___________
energy to other
forms of energy.
watts
Power is measured in _________,
symbol W. A one kilowatt
thousand
device uses one ____________
joules of electrical energy
second
every __________.
WORD SELECTION:
second
transferred
watts
joules
power
thousand
electrical
Electrical power
Notes questions from pages 52/256 & 53/257
1.
2.
3.
4.
5.
6.
7.
What is meant by ‘power’?
Copy the equation for power on page 52/256 stating
clearly the units used for each quantity in the equation.
Copy and answer question (a) on page 53/257.
Copy the table on page 53/257.
Copy and answer questions (b) and (c) on page
53/257.
Copy the Key Points on page 53/257.
Try summary questions 1, 2 & 4 on page 53/257.
Electrical power
ANSWERS
In text questions:
(a) 5000W (5kW)
(b) 100
(c) Probably not, as
100W would keep
one or two light bulbs
on, but only when
you pedal
Summary questions:
1. (a) (i) a mains filament lamp
(ii) A 10 000W electric
cooker
(b) 6 million kilowatts
2. (a) 100 000J
(b) 0.12 (or 12%)
4. 12 000 kJ
Paying for electricity
An electricity meter is used
to measure the usage of
electrical energy.
The meter measures in
kilowatt-hours (kWh)
A kilowatt-hour is the
electrical energy used by
a device of power one
kilowatt in one hour.
Question
Calculate the energy used in joules by a hairdryer of power
1kW in 1 hour.
electrical power = electrical energy
time
becomes:
electrical energy = power x time
= 1 kW x 1 hour
= 1000 W x 3 600 seconds
electrical energy used = 3 600 000 joules (or 3.6 MJ)
Calculating cost
1 . Calculate kilowatt-hours used from:
kilowatt-hours = kilowatts x hours
2 . Calculate cost using:
cost in pence = kilowatt-hours x cost per kWh
Electricity currently costs about 12p per kWh
Question 1
Calculate the cost of using an electric heater of
power 2kW for 5 hours if each kWh costs 12p.
kilowatt-hours = kilowatts x hours
= 2kW x 5 hours
= 10 kWh
cost in pence = kilowatt-hours x cost per kWh
= 10 kWh x 12p
= 120p
cost of using the heater = £1.20
Question 2
Calculate the cost of using a mobile phone charger
power 10W for 6 hours if each kWh costs 12p.
kilowatt-hours = kilowatts x hours
= 10W x 6 hours
= 0.01 kW x 6 hours
= 0.06 kWh
cost in pence = kilowatt-hours x cost per kWh
= 0.06 kWh x 12p
cost of using the heater = 0.72p
Electricity bill
Calculate the cost of the electricity that
you use over a three month period (90
days).
Typical power values:
energy efficient light bulb – 15 W
desk-top computer – 300 W
hairdryer – 2 kW
television – 100 W
charger – 10 W
Example: light bulb used for 4 hours per day:
kWh = (0.015 x 4 x 90) = 5.4 kWh;
cost = 5.4 x 12p = 64.8p
Simulations
Electricity Bill - eChalk
BBC Bitesize Revision:
The cost of using electricity - includes energy cost
calculation applet
Using electrical energy
Notes questions from pages 54/258 & 55/259
1.
2.
3.
4.
5.
6.
7.
8.
9.
Define what is meant by the ‘kilowatt-hour (kWh)’
Explain why a 2 kW heater uses 6 kWh when used continuously
for three hours.
Copy and answer question (a) on page 54/258.
How many joules of energy are there in 1 kWh?
What does a domestic electricity meter measure?
Copy and answer questions (b) and (c) on page 55/259.
Electricity has become more expensive recently. Repeat question
(c) this time for units costing 12p each.
Copy the Key Points on page 55/259.
Answer the summary questions on page 55/259.
Using electrical energy
ANSWERS
In text questions:
(a) 2.4 kWh
(b) Checked
(c) £100.94
(12p units = £173.04)
Summary questions:
1. (a) kilowatt;
(b) kilowatt-hours;
(c) kilowatt-hour,
kilowatt, hour
2. (a) (i) 1.5 kWh;
(ii) 0.5 kWh; (iii) 0.8 kWh
(b) 19.6 p
3. 4 kW
The National Grid
The National Grid is the
network of cables
connecting power stations
to our homes and other
buildings.
The advantage of such a
grid of cables is that more
than one power station can
be used to supply our
electricity.
Voltage levels
Power stations produce electricity at 25 000 V.
This is increased to up to 400 000V by step-up
transformers.
The voltage is decreased near our homes at local
sub-stations to 230V by step-down transformers.
Why high voltage levels are used.
Electrical cables have resistance (like friction) which
causes electrical energy to be transformed into waste heat
energy when an electric current flows.
Energy wastage to heat decreases when the voltage
used is increased because the electric current is
decreased.
However, the higher the voltage
the greater is the danger and the
larger the pylons that have to be
used to carry the cables.
Underground or overground cables?
Advantages of underground cables:
– They would not look unsightly.
– They would not be subject to weather for
example wind and ice.
– They would not be a hazard to aircraft
– They would be less accessible to
vandalism or terrorism.
Advantages of overground cables:
– They are easier to put up than burying
cables below ground or under rivers and
canals.
– They are easier to maintain and repair.
Repairing ‘live’
power lines
Choose appropriate words to fill in the gaps below:
cables that connects
The National Grid is a network of ______
stations to electricity users.
power _______
25 kV which is
Power stations produce electricity at _______
step-up transformers to up to 400 kV.
increased by _______
Near electricity users, __________
step-down transformers are used to
decrease the voltage to down to _______.
230 V
High voltages __________
energy loss to heat due to the
decrease
current
resistance of the power cables to electric _________.
WORD SELECTION:
stations
230 V
decrease
cables
step-up
step-down
current
25 kV
Simulations
The National Grid - includes animated power
station diagram
The National Grid
Notes questions from pages 56/260 & 57/261
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is the ‘National Grid’?
Copy figure 2 on page 56/260.
What is the purpose in the National Grid of: (a) step-up
transformers and (b) step-down transformers.
What is the reason why a very high voltage is used on power
lines?
Copy and answer question (a) on page 57/261.
List the advantages and disadvantages of burying cables
underground.
Copy and answer question (b) on page 57/261.
Copy the Key Points on page 57/261.
Answer summary questions 1 & 3 on page 57/261.
The National Grid
ANSWERS
In text questions:
(a) Electrical energy would
be wasted in the cables.
Less electrical power
would be supplied to the
consumers.
(b) Faults would be harder
to find. The ground
would need to be dug up
to make the repair.
Summary questions:
1. (a) Up, bigger
(b) Down, smaller
3. (a) The higher the voltage, the
smaller the current for the same
power delivered. The power
wasted due to the heating of the
cables is much less for a smaller
current.
(b) The grid voltage has to be
stepped-down for use in homes.
Step-down transformers are
needed at the sub-stations.
Essential electricity issues
Notes questions from pages 58/262 & 59/263
1. Answer questions 1, 2, 3 and 5 on pages
58/262 and 59/263.
Essential electricity issues
ANSWERS
1. Too much current would be pushed
through it and it would burn out.
2. (a) 1600 kWh (b) £112
3. Tubs and wooden paddles.
5. (a) Powergreen (b) Power Co.
How Science Works
ANSWERS
(a) Correctly labelled graph with efficiency on the Y-axis
and mass lifted on the X-axis. The line of best fit
should be a CURVE.
(b) As the mass increases, so does the efficiency, up to a
point. Then the efficiency decreases with increasing
mass.
(c) Partial support.
(d) Repeat measurements.
(e) They have not been checked.
(f) Having another person check the results / Using data
from other sources / Using a different method.