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## Resistance

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**Challenge**Try and increase the temperature of 200 ml of tap water as much as possible in five minutes using the equipment provided.**Some examples of incandescent lights**• For each part of the household light globe labelled in the diagram explain what its function is.**Explain the role resistance plays in …**• conductors • insulators**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?**Activity**• pHET Activity Handout • http://phet.colorado.edu/en/simulation/resistance-in-a-wire • Also on wiki**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.**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**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**Worksheet**• Reading resistors**Resistance in series**• In a series circuit the total resistance can be calculated by adding the resistance from each component RT = R1 + R2 + R3 .....**Resistance in parallel**• In a parallel circuit the total resistance can be calculated by using the following formula ......**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**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**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).**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.