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Resistance

GCSE Physics
Resistance

• Book Reference
Resistance
- Pg 123
Ohm’s Law (the statement and
  equation)
- page 126
Learning Intentions

• Describe the relationship between
  voltage and current
• State what is meant by the resistance of
  a circuit
• State Ohm’s Law
• Recognise how a device obeys Ohm’s Law
• Recall how to measure the resistance
  of a device in a circuit
Voltage and Current

• For many electrical devices
  there is a relationship between
  the current flowing through it
  and the voltage across it

• As the current flows through it
  faster, what happens to the
  voltage?

• Why?
Resistance

• What other words can we use to
  explain resistance?


      Impede
                       Obstruct
       Slow down


    Oppose              Challenge
Resistance
         Resistance is anything that will
       RESIST a current. It is measured
        in Ohms, a unit named after me.

 The resistance of a component can be
 calculated using the equation of Ohm’s         Georg Simon Ohm
 Law:                                              1789-1854
                          Voltage
        Resistance =
                          Current

                            V                      V
Symbol           R    =
                            I
                           Volt, V
Unit       Ohm, Ω =                         I          R
                          Amp, A
Resistance
Give it to me straight…
                                           If the resistance
                                   V         is constant the
                   R       =
                                   I       voltage increases
                                                 at the same
Re-arranged    V       =       R   X   I   proportion as the
                                                     current

   • Thecurrent in a metallic conductor is
    directly proportional to the voltage
    between its ends (provided that the
    temperature and other physical conditions are
    the same).
For a component in a circuit which obeys Ohm’s
law, measuring the current as the voltage
increases will give a graph with the shape shown
below…


    Change in V
                                       If we measure the
                                       gradient, we get
                                       the resistance.
                  Change in I
                                           V
                                           I = R


                                 Notice that the graph
                                goes through the origin
An example question:
                                          Ammeter
                                          reads 2A
                                A




             1) What is the resistance across this bulb?
      V
                       R = V / I = 10 / 2 = 5Ω
             2) Assuming all the bulbs are the same what
                is the total resistance in this circuit?
Voltmeter
                         Total Voltage = 10 x 3 = 30 V
reads 10V
                            R = V / I = 30 / 2 = 15Ω
Questions from the book

• Pg 126 and 127
• Questions 17 - 22
Learning Intentions

• State how the resistance of a wire or
  device can change
• Plot and interpret a graph for the
  voltage and current passing through a
  device
Ohm’s Law




The Stony Tube with water…
Resistance
 In an electrical circuit RESISTANCE is the
    measure of how the material opposes or
         impedes the flow of electrons
• Glass and synthetic polymers have great
  resistance because so many of their
  electrons are not free to move from
  atom to atom
• Silver and copper atoms offer only slight
  resistance to an electric current because
  many of their electrons are free to
  move from atom to atom.
What affects resistance?
                          Here we have a piece of
-                         electrical wire with a set EMF
                       + pushing the electrons (large
                          spheres) through it.
  The small spheres are the atoms making up the
  material
 1. Cross sectional Area
                           If the thickness of the wire
                           is reduced then the
                           resistance increases.
   The electrons obviously don’t have as much space to
   flow down the wire.
What affects resistance?

2. Length of the material




Comparing the ‘journey’ of an electron down a longer piece
of wire, we can see that it has to deal with more
friction from the sides of the wire.
Therefore the longer the wire the higher the
resistance of the material.
What affects resistance?
 Temperature



When the temperature of a material is turned up it
becomes hotter. This is because the atoms are vibrating
more vigorously and bouncing off each other and giving
off heat energy.

The quicker the atoms are moving the more
difficult it is for the electrons to get to where
they want to go.
The resistance increases.
Website Help

• http://micro.magnet.fsu.edu/electro
  mag/electricity/resistance.html
Questions

• Page 128 Questions 23 and 24

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Resistance

  • 2. Resistance • Book Reference Resistance - Pg 123 Ohm’s Law (the statement and equation) - page 126
  • 3. Learning Intentions • Describe the relationship between voltage and current • State what is meant by the resistance of a circuit • State Ohm’s Law • Recognise how a device obeys Ohm’s Law • Recall how to measure the resistance of a device in a circuit
  • 4. Voltage and Current • For many electrical devices there is a relationship between the current flowing through it and the voltage across it • As the current flows through it faster, what happens to the voltage? • Why?
  • 5. Resistance • What other words can we use to explain resistance? Impede Obstruct Slow down Oppose Challenge
  • 6. Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using the equation of Ohm’s Georg Simon Ohm Law: 1789-1854 Voltage Resistance = Current V V Symbol R = I Volt, V Unit Ohm, Ω = I R Amp, A
  • 7. Resistance Give it to me straight… If the resistance V is constant the R = I voltage increases at the same Re-arranged V = R X I proportion as the current • Thecurrent in a metallic conductor is directly proportional to the voltage between its ends (provided that the temperature and other physical conditions are the same).
  • 8. For a component in a circuit which obeys Ohm’s law, measuring the current as the voltage increases will give a graph with the shape shown below… Change in V If we measure the gradient, we get the resistance. Change in I V I = R Notice that the graph goes through the origin
  • 9. An example question: Ammeter reads 2A A 1) What is the resistance across this bulb? V R = V / I = 10 / 2 = 5Ω 2) Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter Total Voltage = 10 x 3 = 30 V reads 10V R = V / I = 30 / 2 = 15Ω
  • 10. Questions from the book • Pg 126 and 127 • Questions 17 - 22
  • 11. Learning Intentions • State how the resistance of a wire or device can change • Plot and interpret a graph for the voltage and current passing through a device
  • 12. Ohm’s Law The Stony Tube with water…
  • 13.
  • 14. Resistance In an electrical circuit RESISTANCE is the measure of how the material opposes or impedes the flow of electrons • Glass and synthetic polymers have great resistance because so many of their electrons are not free to move from atom to atom • Silver and copper atoms offer only slight resistance to an electric current because many of their electrons are free to move from atom to atom.
  • 15. What affects resistance? Here we have a piece of - electrical wire with a set EMF + pushing the electrons (large spheres) through it. The small spheres are the atoms making up the material 1. Cross sectional Area If the thickness of the wire is reduced then the resistance increases. The electrons obviously don’t have as much space to flow down the wire.
  • 16. What affects resistance? 2. Length of the material Comparing the ‘journey’ of an electron down a longer piece of wire, we can see that it has to deal with more friction from the sides of the wire. Therefore the longer the wire the higher the resistance of the material.
  • 17. What affects resistance? Temperature When the temperature of a material is turned up it becomes hotter. This is because the atoms are vibrating more vigorously and bouncing off each other and giving off heat energy. The quicker the atoms are moving the more difficult it is for the electrons to get to where they want to go. The resistance increases.
  • 18. Website Help • http://micro.magnet.fsu.edu/electro mag/electricity/resistance.html
  • 19. Questions • Page 128 Questions 23 and 24

Editor's Notes

  1. Current is the flow of the charge carriers provided by the attraction of the electrons to the positive terminal, the attraction is bigger when the voltage is increased (more energy given to the electrons to move)
  2. The resistance of the electrical component is what connects voltage and current – for an ohmic device, the current will always increase by the same proportion as what the voltage has increased by. The factor that links current and voltage is resistance.
  3. As the Voltage increases, each Coulomb of charge has more attraction to the battery’s terminal and will move faster. As the Current is increased, there are more Coulombs of charge passing through the wire (the component receives more Coulombs with energy attached to them – so more energy transferred per second) The relationship between these two aspects of electricity is the resistance http://micro.magnet.fsu.edu/electromag/java/filamentresistance/index.html