Download ph1 elec #4

Electric Power [Text: p.67-74]
[4]
Electrical Energy
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The electrical potential energy transformed by a circuit element is …
E = qV = VIt
where E = energy [E]; q = charge [C]; I = current [A];
V = potential difference [V]; t = time [s]
Electric Power
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Power [P] is the rate of tranfer of energy and is measured in watt,W …
1 watt = 1 joule/second
P = E/t = VI
where P = electric power [W]; V = potential difference [V];
I = electric current [I]
Kilowatt-hours
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The energy used by an appliance depends on the power it is using and how long it
has been switched on … E = Pt When dealing with domestic appliances, times are
usually measured in hours and power in kilowatt. Instead of talking “joules” for
electric energy use in the home, the unit “kilowatt-hour” has been introduced …
1 kWh = 3.6x106 J
To convert joule to kWh … divide by 3.6x106
To convert kWh to joule … multiply by 3.6x106
AC Power
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Batteries and solar cells produce DC or direct current electricity … the DC current
flows in one direction, from the (+) terminal to the (-) terminal.
Power produced by an electrical generator is AC power. Alternating current [AC]
changes direction periodically … in Victorian household power, the current changes
direction 50 times every second [50 Hz]
Victorian AC electricity varies between +340 V and -340 V … but supplies the
equivalent power of a 240 V DC supply.
AC Power Transmission
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Although power lines are good conductors, with a small resistance, their long length
means some of the electricity in them is lost as heat.
Power loss in transmission lines [ Ploss = I2R ] depends significantly on the current
flowing in them … minimising current = minimising power loss.
Losses can be reduced by transmitting the electricity at a very high voltage
and comparatively low current.
Transformers at power stations “step up” to a very high voltage [and much lower
current] before transmission. Substations and power pole transformers “step
down the voltage to the 240 Ac supply as it enters the household.
In a transformer working efficiently, the power input = power output … so as
voltages are increased by the transformer, currents are reduced.
Problems p.75 Q. 1, 2, 3, 4, 5, 6, 7, 8 and 10
U1 Homework – Electricity No.4
Complete and submit the following homework task on A4 loose leaf, clearly showing
your name, all working and reasoning.
A 240 V washing machine has a power rating of 1200 W and is used for an average of
45 minutes per day. Electricity is charged at 15 cents per kWh.
1. What current is drawn by the machine when it is operating?
2. How much electrical potential energy does the machine use in 45 minutes?
[ in J and kWh ]
3. With average use, how much does it cost to run the machine each week?
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U1 Homework – Electricity No.4
Complete and submit the following homework task on A4 loose leaf, clearly showing
your name, all working and reasoning.
A 240 V washing machine has a power rating of 1200 W and is used for an average of
45 minutes per day. Electricity is charged at 15 cents per kWh.
1. What current is drawn by the machine when it is operating?
2. How much electrical potential energy does the machine use in 45 minutes?
[ in J and kWh ]
3. With average use, how much does it cost to run the machine each week?
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U1 Homework – Electricity No.4
Complete and submit the following homework task on A4 loose leaf, clearly showing
your name, all working and reasoning.
A 240 V washing machine has a power rating of 1200 W and is used for an average of
45 minutes per day. Electricity is charged at 15 cents per kWh.
1. What current is drawn by the machine when it is operating?
2. How much electrical potential energy does the machine use in 45 minutes?
[ in J and kWh ]
3. With average use, how much does it cost to run the machine each week?
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Problems: Electric Power
------------------------------------------------------------------------1. A heating element has a potential difference across it of 120 V and a
current through it of 0.56 A.
How much heat energy does it release in 2.4 minutes?
------------------------------------------------------------------------2. A 100 W light globe is on for 2 hours.
How much energy does it use [in joule and kWh]?
------------------------------------------------------------------------3. An 850 W petrol powered portable generator produces a current of
3.6 A when operating at its peak.
What is the EMF of the generator?
------------------------------------------------------------------------3. A 240 V, 600 W LCD TV is on 4 hours a day for a week. The tariff for
electrical power is 15 cents per kWh?
(a) What current does the TV draw?
(b) How much electrical potential energy [in J] does the TV use per day?
(c) What is the cost of running the TV 4 hours per day for 1 week?
------------------------------------------------------------------------4. A 100 MW power station generates electricity at 20 kV. A step-up
transformer is then used to bump the voltage to 200 kV before it is
transmitted to a nearby city in transmission lines having a total
resistance of 10 .
(a) What electric current is generated at the power station?
(b) What electric current is in the transmission lines?
(c) What is the power loss in the transmission lines? [use Ploss = I2R]
(d) What would have been the power loss [use Ploss = I2R] in the
transmission line if the step up transformer had not been used to
reduce the current?
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