Download Electromotive force

Electromotive force
Learning Objectives
• (a) recall and use appropriate circuit symbols
as set out in SI Units, Signs, Symbols and
Abbreviations (ASE, 1981) and Signs, Symbols
and Systematics (ASE, 1995);
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(a) define potential difference (p.d.);
(b) select and use the equation W = VQ;
(c) define the volt;
(d) describe how a voltmeter may be used to
determine the p.d. across a component;
(e) define electromotive force (e.m.f.) of a source
such as a cell or a power supply;
(f) describe the difference between e.m.f. and
p.d. in terms of energy transfer.
What is the point of a circuit?
• Circuit are there to deliver electrical energy to
a device.
• At GCSE you learnt that Power = Energy / time
• (you may not remember it but you did!)
Definition time
• Electromotive force is the energy transferred per
unit charge when one type of energy is converted
into electrical energy
• This makes more sense with examples: a standard
cell works by turning chemical energy into
electrical energy so is producing an emf
• A dynamo turns kinetic energy into electrical
energy so is producing an emf
Calculation time
• Electromotive force is the energy transferred per
unit charge when one type of energy is converted
into electrical energy
• Energy per unit charge is the same as energy
divided by charge so
• Electomotive force = Electrical energy transferred
(e.m.f.)
charge
Units time
• Electomotive force = Electrical energy transferred
(e.m.f.)
charge
• Energy is measured in Joules (J)
• Charge is measured in Coulombs (C)
• So e.m.f is measured in JC-1
• So e.m.f is measured in JC-1
• But this is given it’s own unit called the Volt
• Definition
• 1 volt is 1 Joule per Coulomb
What does this mean?
• So a 1.5V cell will provide each Coulomb of
charge with 1.5J
• If you needed 6V to make a device work . Each
1.5V cell would provide 1.5J to every Coulomb
so 4 of the 1.5V cells would provide 1.5J each
to every Coulomb meaning that every
Coulomb was carrying a total of 6J of electrical
energy.
How about UK mains
• Well the e.m.f for UK mains is 230V so this
means that every Coulomb is carrying 230J of
energy with it (which is why it is much more
dangerous)
This will annoy you!
• In equations to do with electricity we give
e.m.f the letter E
• Therefore we need to give energy a letter so
we give it W
Potential Difference
• For this part we are going to assume that all of
the electical energy made is used. For most
devices this is true but in A2 Capacitors are
introduced which store some of the electrical
energy. Let’s not worry about them for now!
Potential difference
• Definition time
• Potential difference is the electrical energy per
unit charge when electrical energy is
converted to another form of energy
• (Like our definition for e.m.f. but in reverse)
So imagine……
• A battery of cells has an e.m.f of 6V. It is
connected to one bulb. Therefore each
coulomb is supplied with 6J of energy. All of
that energy is used in the bulb to light up and
make heat etc so it has a potential difference
of 6V
The Voltmeter
• P.d is what you have referred to as voltage up
to now.
• A voltmeter measures p.d. by measuring the
difference in electrical potentials
• (think of this as comparing the energy per
coulomb before entering a device and then
after leaving a device)
• Voltmeters are connected in parallel (i.e.
across the component)
Equations can be mixed and matched
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1 ampere= 1 coulomb per second
That means I= Q/t so Q = It
1 volt = 1 joule per coulomb
That means V=W/Q so W=QV
1 watt = 1 joule per second
That means P=W/t so W=Pt
We can combine these
• Q=It;
W= QV;
W=Pt
• So W = VIt
• P=W/t = QV/t = VIt/t = VI
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a) define resistance;
(b) select and use the equation for resistance R= V/I
(c) define the ohm;
(d) state and use Ohm’s law;
(e) describe the I–V characteristics of a resistor at
constant temperature, filament lamp and light-emitting
diode (LED);
• (f) describe an experiment to obtain the I–V
characteristics of a resistor at constant temperature,
filament lamp and light-emitting diode (LED);
• (g) describe the uses and benefits of using light
emitting diodes (LEDs).
Resistance and Ohm’s Law
• Definition
• The current through a conductor is
proportional to the potential difference across
it provided physical conditions, such as
temperature remain constant
Resistance
• Resistance = Potential difference/current
• Therefore it is measured in volts per ampere
which is known as the ohm
Resistance
• A resistor of resistance 4.6Ω needs a p.d of
4.6V to allow the current to be 1 ampere
The filament lamp
• As the p.d. increases the lamp becomes hot.
• This in turn increases the lamp’s resistance
The diode and therefore the LED
• Diodes allow current to pass only in one direction
• LEDs do the same but emit visible light when a
current goes through them
• Advantages of an LED are;
• they switch on instantly
• Are very robust
• Are very versatile
• Operate on low p.ds
• Have a long working life