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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.

6
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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?

11
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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.

16
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17
  • 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?

29
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30
  • 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.

31
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32
  • When X is a filament lamp, the graph is a curve,
    as shown

33
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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

50
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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)

68
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