Download Electromotive Force and Potential difference

Electromotive Force and Potential difference
Remember
W = VQ
Electrical Power, Potential difference and Current
We have seen that
Power is defined as the rate at which work is done or
Power is the rate at which energy is converted from one form to another.
Most energy conversions in this section involve electrical energy converting to heat energy.
𝑃𝑜𝑤𝑒𝑟 =
𝑊𝑜𝑟𝑘
𝑇𝑖𝑚𝑒
𝑃=
𝑊
𝑡
Remember 1 watt = 1 joule per second
Now if we take W = VQ and divide both sides by time (t), we get P = VI (because W/t = P, and Q/t = I)
P = VI
Measuring Potential Difference
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Voltages in Series:
VTotal = V1 + V2+V3
Voltages in parallel are the same. i.e V1=V2=V3
A Voltmeter is used to measure Potential Difference.
A Voltmeter is always connected in parallel with whatever it is measuring.
Electromotive Force (emf)
A Voltage when applied to a full circuit is called an emf*.
The unit of emf is also the Volt
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Some Sources of Electromotive Force (emf)
Mains
Simple Cell
Lead-acid accumulator
Dry batteries
Thermocouple
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Exam Questions
1. [2003]
Explain the term emf
2. [2008 OL]
Name a source of potential difference.
3. [2002]
The ESB supplies electrical energy at a rate of 2 MW to an industrial park from a local power station, whose
output voltage is 10 kV. Calculate the current.
4. [2004]
A table lamp has a power rating of 100 W. What is the most suitable fuse for the lamp (assuming mains voltage)?
5. [2003 OL]
The kettle has a power rating of 2 kW when connected to the ESB mains voltage of 230 V.
Calculate the current that flows when the kettle is first plugged in.
6. [2004 OL]
An electric heater has a power rating of 2 kW when connected to the ESB mains supply of 230 V.
Calculate the current that flows through the heater.
7. [2003 OL]
The fuse in the previous question is a 5 A fuse.
This current will only flow for a very short time. Explain why.
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