Download Electric Current, Potential Difference and Resistance

8.3.2 Electrical Energy in the Home
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The unit of electric charge is the Coulomb
(abbreviated C).
Ordinary matter is made up of atoms which have
positively charged nuclei and negatively charged
electrons surrounding them.
Charge is quantized as a multiple of the electron
or proton charge:
The influence of charges is characterized in
terms of the forces between them (Coulomb's
law) and the electric field and voltage produced
by them.
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Electrons are very small.
You can have a million electrons flowing around
a circuit without noticing, they won't light a lamp.
In physics, we take a very, very large number of
electrons as 1 unit of charge - called a Coulomb.
Charge is given the symbol Q.
1 Coulomb = 6.2 x 1018 electrons.
(This is 6.2 million million million electrons).
Such a large number of electrons can do useful
things(like light a lamp).
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A charge in an electric field experiences an
electric force.
Work is done by the electric field if the
electric force acting on the charge causes it
to move from one point to another. These
two points differ in their electric potential.
The magnitude of the work done on the
charge by the electric field is a measure of
the difference in potential.
Lines of
equipotential
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Introductory Video – Voltage and Current
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The electric potential difference (V) is the
work done per unit charge as a charge is
moved between two points in an electric field.
V=W
q
Where W = work in Joules
q = charge in Coulombs
The volt (V) is the unit used to measure
electric potential difference.
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Do question 13 p117 of your text.
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Students frequently confuse the symbol V
used to express the unit (volt) with the term
used in the expression for electric potential
difference.
It should be emphasized that sometimes the
same symbol can be used in different ways to
express different things.
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Voltage is electric potential energy per unit
charge, measured in joules per coulomb ( =
volts).
Energy is measured in Joules.
The power supply (the cell or battery)
gives an amount of energy to each Coulomb
going around an electric circuit.
1 Volt = 1 Joule per Coulomb.
The word "per" means "divided by",
so Voltage = Energy ÷ Charge.
This can be rearranged to give
Energy = Voltage x Charge. W (Energy) = V x q.
A 6 Volt cell gives 6 Joules of energy to each
Coulomb going around the circuit.
Voltage (which is also called potential difference,
or p.d.) is measured using a voltmeter.
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Voltage (which is also called
potential difference, or p.d.)
is measured using a
voltmeter.
The voltmeter, shown as a
circle with the letter V
inside,
is always connected in
parallel with the
component.
(The voltmeter is said to be
connected across the
component, where the word
"across" means "in parallel
with").
The circuit on the left would
show the voltage of the cell.
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The circuit on the right shows the
voltmeter connected across a lamp.
This will tell you how many Joules of
energy are being converted
from electrical energy into light
energy (+heat)for each Coulomb
which passes through it.
A reading of 6 Volts tells you that 6
Joules of energy are being
converted for each Coulomb
passing through the lamp.
A reading of 10 Volts tells you that
10 Joules of energy are being
converted for each Coulomb
passing through the lamp.
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When a voltage is generated by a battery, or
by the magnetic force according to Faraday's
Law (next year), this generated voltage has
been traditionally called an "electromotive
force" or emf.
The emf represents energy per unit charge
(voltage) which has been made available by
the generating mechanism and is not a
"force".
The term emf is retained for historical
reasons.
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Think of Coulombs as though they are buses,
taking a large number of electrons (like
passengers)from one side of the cell,
through all the components in the circuit,
and back to the other side of the cell.
This is called direct current.
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Electric current is the rate of charge (Q or q)
flow past a given point in an electric circuit
under the influence of an electric field.
It is measured in Coulombs/second which is
named Amperes.
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The "rate of flow of coulombs" (called
"current") around an electric circuit
is measured in amps.
1 Amp = 1 Coulomb per second.
The word "per" means "divided by",
so current = charge ÷ time
Current, which is given the symbol I, is
shown using an ammeter.
I = Q/t
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The ammeter, shown as a
circle with the letter A inside,
is always connected in series
with a component.
If the ammeter reads 1 Amp,
then the current (I) = 1 Amp
at that point in the circuit.
I = 1 Amp = 1 Coulomb per
second.
If the ammeter reads 6 Amps,
then I = 6 Amps = 6
Coulombs per second.
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Charge, as we saw
earlier is given the
symbol Q and is
measured in Coulombs.
So current = charge ÷
time.
I=Q÷t
This can be rearranged
to give
Q = I x t,
or, charge = current x
time
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Although it is electrons which are the mobile
charge carriers which are responsible for
electric current in conductors such as wires, it
has long been the convention to take the
direction of electric current as if it were the
positive charges which are moving.
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Voltage - Current