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Resistance

Resistance. Challenge. Try and increase the temperature of 200 ml of tap water as much as possible in five minutes using the equipment provided. Looking down into an electric jug. Some examples of incandescent lights.

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Resistance

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  1. Resistance

  2. Challenge Try and increase the temperature of 200 ml of tap water as much as possible in five minutes using the equipment provided.

  3. Looking down into an electric jug

  4. Some examples of incandescent lights • For each part of the household light globe labelled in the diagram explain what its function is.

  5. Explain the role resistance plays in … • conductors • insulators

  6. In your prac report use the following questions to help discuss your results. • Come up with some reasons to explain why thin wire is wrapped in coils around the ceramic support, and why the element heats the water so quickly. • Why do you think the jug’s body is ceramic and not metallic?

  7. Activity • pHET Activity Handout • http://phet.colorado.edu/en/simulation/resistance-in-a-wire • Also on wiki

  8. Resistance • Resistance of an electronic component refers to how difficult it is for electrons to flow through it. • The unit of resistance is the: ohm.

  9. Resistance • Components can have widely differing resistance values • Eg. metal wires – no (or very little) resistance • Light globe – resistance of 40 ohms • Volt meter – resistance of 5 million ohms • There are two main types of resistors: • Fixed resistors • Variable resistors

  10. Fixed Resistors (ohmic conductors) • Made from a solidified carbon mixture • Very useful in electronic circuits because they maintain their set resistance value • Deliberately placed to ‘use up’ a given amount of voltage in a circuit, reducing the voltage available to other components in the circuit • Coloured bands are used to indicate the value of its resistance • Because the resistor is so small the 1st three bands represent the resistors value, the 4th band is used to indicate the accuracy of the stated resistance value

  11. Resistor Bands

  12. Worksheet • Reading resistors

  13. Resistance in series • In a series circuit the total resistance can be calculated by adding the resistance from each component RT = R1 + R2 + R3 .....

  14. Resistance in parallel • In a parallel circuit the total resistance can be calculated by using the following formula ......

  15. Practical work • Complete: • Experiment 3.3 Resistors in series – the voltage divider page 90 Text • Experiment 3.4 Resistors in parallel page 91 Text • Use the digital version to verify your results

  16. Ohm’s Law • Georg Ohm discovered that some items (called Ohmic conductors) will maintain a set resistance value regardless of the voltage that is applied to them. • The resistance of ohmic conductors can be calculated using Ohm’s Law: Where: • R = resistance (ohms - ) • I = current in amps (A) • V = voltage across the components (volts - V) R= V I

  17. Ohm’s Law • However, many items are non-ohmic, (e.g. light bulbs) which means their resistance alters with variations in the voltage that is applied to them. • This is because the item has different resistances at different operating temperatures (largely decided by the applied voltage).

  18. Ohm’s Law • Example: • 4 amps current runs through a kettle’s heating element. The voltage supplied is 240V. What is the resistance of the element? • I = 4 amps • V = 240 V • R = ? • V = IR • R = V/I • R = 240V/4 amps • R = 60 • Draw a colour coded fixed resistor to match this element.

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