Transcript 5.1

Electric Currents
Topic 5.1 Electric potential
difference, current and resistance
Electric Potential Energy
If you want to move a charge closer to a
charged sphere you have to push against
the repulsive force
 You do work and the charge gains electric
potential energy.
 If you let go of the charge it will move
away from the sphere, losing electric
potential energy, but gaining kinetic
energy.

When you move a charge in an electric
field its potential energy changes.
 This is like moving a mass in a
gravitational field.

The electric potential V at any point in an
electric field is the potential energy that
each coulomb of positive charge would
have if placed at that point in the field.
 The unit for electric potential is the joule
per coulomb (J C-1), or the volt (V).
 Like gravitational potential it is a scalar
quantity.

In the next figure, a charge +q moves
between points A and B through a
distance x in a uniform electric field.
 The positive plate has a high potential
and the negative plate a low potential.
 Positive charges of their own accord,
move from a place of high electric
potential to a place of low electric
potential.
 Electrons move the other way, from low
potential to high potential.

In moving from point A to point B in the
diagram, the positive charge +q is moving
from a low electric potential to a high
electric potential.
 The electric potential is therefore different
at both points.

In order to move a charge from point A
to point B, a force must be applied to
the charge equal to qE
 (F = qE).
 Since the force is applied through a
distance x, then work has to be done to
move the charge, and there is an
electric potential difference between
the two points.
 Remember that the work done is
equivalent to the energy gained or lost
in moving the charge through the
electric field.
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Electric Potential Difference
Potential difference
 We often need to know the difference in
potential between two points in an electric
field
 The potential difference or p.d. is the
energy transferred when one coulomb of
charge passes from one point to the other
point.
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The diagram shows some values of the
electric potential at points in the electric
field of a positively-charged sphere
 What is the p.d. between points A and B in
the diagram?

When one coulomb moves from A to B it
gains 15 J of energy.
 If 2 C move from A to B then 30 J of
energy are transferred. In fact:

Change in Energy
Energy transferred,
 This could be equal to the amount of
electric potential energy gained or to the
amount of kinetic energy gained

W
=charge, q
(joules) (coulombs)

x p.d.., V
(volts)
The Electronvolt
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One electron volt (1 eV) is defined as the energy
acquired by an electron as a result of moving
through a potential difference of one volt.
Since W = q x V
And the charge on an electron or proton is 1.6 x
10-19C
Then W = 1.6 x 10-19C x 1V
W = 1.6 x 10-19 J
Therefore 1 eV = 1.6 x 10-19 J
Conduction in Metals
A copper wire consists of millions of
copper atoms.
 Most of the electrons are held tightly to
their atoms, but each copper atom has
one or two electrons which are loosely
held.
 Since the electrons are negatively
charged, an atom that loses an electron is
left with a positive charge and is called an
ion.

The diagram shows that the copper wire is
made up of a lattice of positive ions,
surrounded by free' electrons:
 The ions can only vibrate about their fixed
positions, but the electrons are free to
move randomly from one ion to another
through the lattice.
 All metals have a structure like this.
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What happens when a battery is
attached to the copper wire?
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The free electrons are repelled by the negative
terminal and attracted to the positive one.
They still have a random movement, but in
addition they all now move slowly in the same
direction through the wire with a steady drift
velocity.
We now have a flow of charge - we have electric
current.
Electric Current
Current is measured in amperes (A) using
an ammeter.
 The ampere is a fundamental unit.
 The ammeter is placed in the circuit so
that the electrons pass through it.
 Therefore it is placed in series.
 The more electrons that pass through the
ammeter in one second, the higher the
current reading in amps.

1 amp is a flow of about 6 x 1018 electrons
in each second!
 The electron is too small to be used as the
basic unit of charge, so instead we use a
much bigger unit called the coulomb (C).
 The charge on 1 electron is
only 1.6 x 10-19 C.

 In
fact:
Or I = Δq/ Δt
Current is the rate of flow of charge
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Which way do the electrons move?
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At first, scientists thought that a current was made up of
positive charges moving from positive to negative.
We now know that electrons really flow the opposite way, but
unfortunately the convention has stuck.
Diagrams usually show the direction of `conventional current'
going from positive to negative, but you must remember that
the electrons are really flowing the opposite way.
Resistance
A tungsten filament lamp has a high
resistance, but connecting wires have a
low resistance.
 What does this mean?
 The greater the resistance of a
component, the more difficult it is for
charge to flow through it.
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The electrons make many collisions with
the tungsten ions as they move through
the filament.
 But the electrons move more easily
through the copper connecting wires
because they make fewer collisions with
the copper ions.
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Resistance is measured in ohms (Ω) and is defined
in the following way:
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The resistance of a conductor is the ratio of the p.d.
applied across it, to the current passing through it.
In fact:
Resistors
Resistors are components that are made
to have a certain resistance.
 They can be made of a length of nichrome
wire.
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Ohm’s Law
The current through a metal wire is
directly proportional to the p.d. across
it (providing the temperature remains
constant).
 This is Ohm's law.
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Materials that obey Ohm's law are called
ohmic conductors.
Ohmic and Non-Ohmic
Behaviour
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What do the current-voltage graphs tell
us?
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When X is a metal resistance wire the
graph is a straight line passing through the
origin: (if the temperature is constant)
This shows that: I is directly proportional to
V.
If you double the voltage, the current is
doubled and so the value of V/I is always the
same.
Since resistance R =V/I, the wire has a
constant resistance.
The gradient is the resistance on a V against
I graph, and 1/resistance in a I against V
graph.
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When X is a filament lamp, the graph is a
curve, as shown:
Doubling the voltage produces less than
double the current.
 This means that the value of V/I rises as
the current increases.
 As the current increases, the metal
filament gets hotter and the resistance of
the lamp rises.
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The graphs for the wire and the lamp are
symmetrical.
 The current-voltage characteristic looks
the same, regardless of the direction of the
current.
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Power Dissipation