- 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…
- 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

- 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)
- 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.
- 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