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Electric Currents

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Electric Currents Topic 5.1 Electric potential difference, current and resistance – PowerPoint PPT presentation

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Title: Electric Currents

1
Electric Currents
• Topic 5.1 Electric potential difference, current
and resistance

2
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.

3
• When you move a charge in an electric field its
potential energy changes.
• This is like moving a mass in a gravitational
field.

4
• 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.

5
• 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.

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7
• 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.

8
• 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.

9
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.

10
• 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?

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12
• 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

13
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 x p.d.., V
• (joules) (coulombs) (volts)

14
The Electronvolt
• 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

15
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.

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• 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.

18
What happens when a battery is attached to the
copper wire?
• 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.

19
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.

20
• 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.

21
• In fact

Or I ?q/ ?t Current is the rate of flow of
charge
22
• Which way do the electrons move?
• 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.

23
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.

24
• 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.

25
• Resistance is measured in ohms (O) and is defined
in the following way
• The resistance of a conductor is the ratio of the
p.d. applied across it, to the current passing
through it.
• In fact

26
Resistors
• Resistors are components that are made to have a
certain resistance.
• They can be made of a length of nichrome wire.
• Nichrome wire is a nickel-chromium mixture.

27
Ohms 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.
• Materials that obey Ohm's law are called ohmic
conductors.

28
Ohmic and Non-Ohmic Behavior
• 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

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34
• 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.

35
• The graphs for the wire and the lamp are
symmetrical.
• The current-voltage characteristic looks the
same, regardless of the direction of the current.

36
Power Dissipation
37
Electric Circuits
• Topic 5.2 Electric Circuits

38
Electromotive Force
• Defining potential difference
• The coulombs entering a lamp have electrical
potential energy
• those leaving have very little potential energy.
• There is a potential difference (or p.d.) across
the lamp, because the potential energy of each
coulomb has been transferred to heat and light
within the lamp.
• p.d. is measured in volts (V) and is often called
voltage.

39
• The p.d. between two points is the electrical
potential energy transferred to other forms, per
coulomb of charge that passes between the two
points.

40
• Resistors and bulbs transfer electrical energy to
other forms, but which components provide
electrical energy?
• A dry cell, a dynamo and a solar cell are some
examples.
• Any component that supplies electrical energy is
a source of electromotive force or e.m.f.
• It is measured in volts.
• The e.m.f. of a dry cell is 1.5 V, that of a car
battery is 12 V

41
• A battery transfers chemical energy to electrical
energy, so that as each coulomb moves through the
battery it gains electrical potential energy.
• The greater the e.m.f. of a source, the more
energy is transferred per coulomb. In fact
• The e.m.f of a source is the electrical potential
energy transferred from other forms, per coulomb
of charge that passes through the source.
• Compare this definition with the definition of
p.d. and make sure you know the difference
between them.

42
Internal Resistance
43
• The cell gives 1.5 joules of electrical energy to
each coulomb that passes through it,
• but the electrical energy transferred in the
resistor is less than 1.5 joules per coulomb and
can vary.
• The circuit seems to be losing energy - can you
think where?

44
• The cell itself has some resistance, its internal
resistance.
• Each coulomb gains energy as it travels through
the cell, but some of this energy is wasted or
lost' as the coulombs move against the
resistance of the cell itself.
• So, the energy delivered by each coulomb to the
circuit is less than the energy supplied to each
coulomb by the cell.

45
• Very often the internal resistance is small and
can be ignored.
• Dry cells, however, have a significant internal
resistance.
• This is why a battery can become hot when
supplying electric current.
• The wasted energy is dissipated as heat.

46
Resistance Combinations
47
Resistors in series
48
• The diagram shows three resistors connected in
series
• There are 3 facts that you should know for a
series circuit
• the current through each resistor in series is
the same
• the total p.d., V across the resistors is the sum
of the p.d.s across the separate resistors, so V
Vl V2 V3
• the combined resistance R in the circuit is the
sum of the separate resistors

49
• R Rl R2 R3
• Suppose we replace the 3 resistors with one
resistor R that will take the same current I when
the same p.d. V is placed across it

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51
• This is shown in the diagram. Let's calculate R.
• We know that for the resistors in series
• V Vl V2 V3
• But for any resistor p.d. current x resistance
(V I R).
• If we apply this to each of our resistors, and
remember that the current through each resistor
is the same and equal to I, we get
• IR IRlIR2IR3
• If we now divide each term in the equation by I,
• we get
• R R1 R2 R3

52
Resistors in parallel
53
• We now have three resistors connected in
parallel
• There are 3 facts that you should know for a
parallel circuit
• the p.d. across each resistor in parallel is the
same
• the current in the main circuit is the sum of the
currents in each of the parallel branches, so
• I I1 I2 I3
• the combined resistance R is calculated from the
equation

54
• Suppose we replace the 3 resistors with one
resistor R that takes the same total current I
when the same p.d. V is placed across it.

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56
• This is shown in the diagram. Now let's
calculate R.
• We know that for the resistors in parallel
• I I1I2I3
• But for any resistor, current p.d. resistance
(I V/R ).
• If we apply this to each of our resistors, and
remember that the
• p.d. across each resistor is the same and equal
to V,
• we getV/RV/R1 V/R2 V/R3
• Now we divide each term by V, to get

57
• You will find that the total resistance R is
always less than the smallest resistance in the
parallel combination.

58
Circuit Diagrams
• You need to be able to recognize and use the
accepted circuit symbols included in the Physics
Data Booklet

59
Ammeters and Voltmeters
• In order to measure the current, an ammeter is
placed in series, in the circuit.
• What effect might this have on the size of the
current?
• The ideal ammeter has zero resistance, so that
placing it in the circuit does not make the
current smaller.
• Real ammeters do have very small resistances -
around 0.01 O.

60
• A voltmeter is connected in parallel with a
component, in order to measure the p.d. across
it.
• Why can this increase the current in the circuit?
• Since the voltmeter is in parallel with the
component, their combined resistance is less than
the component's resistance.
• The ideal voltmeter has infinite resistance and
takes no current.
• Digital voltmeters have very high resistances,
around 10 MO, and so they have little effect on
the circuit they are placed in.

61
Potential dividers
• A potential divider is a device or a circuit that
uses two (or more) resistors or a variable
resistor (potentiometer) to provide a fraction of
the available voltage (p.d.) from the supply.

62
• The p.d. from the supply is divided across the
resistors in direct proportion to their
individual resistances.

63
• Take the fixed resistance circuit - this is a
series circuit therefore the current in the same
at all points.
• Isupply I1 I2
• Where I1 current through R1
• I2 current through R2

64
• Using Ohms Law

65
Example
66
With sensors
• A thermistor is a device which will usually
decrease in resistance with increasing
temperature.
• A light dependent resistor, LDR, will decrease in
resistance with increasing light intensity.
(Light Decreases its Resistance).

67
Example
• Calculate the readings on the meters shown below
when the thermistor has a resistance of
• a) 1 kW (warm conditions) and b) 16 kW. (cold
conditions)

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