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3 e electronics 301110

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Ohm's law and introduction to resistance

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3 e electronics 301110

  1. 1. Electricity 3E: Tuesday 30th November
  2. 2. Resistance <ul><li>Learning Objectives: </li></ul><ul><li>Know what resistance is and what units we measure it in </li></ul><ul><li>Starter </li></ul><ul><li>Write down as many meanings for the word resistance as you can in 1 minute! </li></ul>
  3. 3. Experimental setup <ul><li>Set up the circuit as shown </li></ul><ul><li>Turn on the power supply and adjust the supply voltage to 6V </li></ul><ul><li>Take readings of current, I (A), through, and the voltage, V (V) across the resistor and record in a table in your workbook – use a ruler! </li></ul><ul><li>Repeat this for several different values (6V down to 0V) of the voltage and record both voltage and current through the resistor in your table of results </li></ul><ul><li>Plot a graph of your results (voltage, V (V) on y-axis; current, I (A) on x-axis) using a best fit line – use a ruler! </li></ul>Battery Pack A V Resistor
  4. 4. Analysing results: Voltage, V (V) Current, I (A) 0V 0A I (A) V (V)
  5. 5. Gradients <ul><li>How do we find a gradient of a graph in maths? </li></ul><ul><li>Instead of y we have Voltage, V, instead of x we have Current, I </li></ul><ul><li>If we have a straight line, the gradient is constant </li></ul><ul><li>Gradient, m = V/I = constant: RESISTANCE </li></ul>
  6. 6. Definition of Resistance <ul><li>Electrical Resistance of an object is a measure of its opposition to the passage of an electric current </li></ul><ul><li>Resistance is measured in Ohms ( Ω ) </li></ul><ul><li>Resistance = voltage / current </li></ul><ul><li>R=V/I V=IR I=V/R </li></ul>George Simon Ohm German Physicist V I R
  7. 7. Ohm’s Law questions <ul><li>V = IR </li></ul><ul><li>Complete the questions on the handout </li></ul>
  8. 8. Resistors <ul><li>Resistors are simple components used to control the current and voltage in a circuit </li></ul>
  9. 9. Resistive heating <ul><li>For a given component, power P = IV where I is the current through that component and V is the voltage across that component </li></ul><ul><li>Substituting from V=IR we get: </li></ul><ul><ul><li>P = I 2 R = V 2 /R = IV </li></ul></ul><ul><ul><li>Power is measured in Watts </li></ul></ul>
  10. 10. Resistors in series <ul><li>If we join resistors in series we increase the resistance of the circuit. The current will decrease </li></ul><ul><li>The total resistance in series is equal to the sum of the individual resistances: </li></ul><ul><li>R s = R 1 + R 2 + R 3 </li></ul><ul><li>where R s = equivalent series resistance </li></ul>
  11. 11. Resistors in parallel <ul><li>If we join components in parallel we decrease the resistance of the circuit. The current will increase . </li></ul><ul><li>The combined resistance in parallel is calculated using the formula: </li></ul><ul><li>1/R P = 1/R 1 + 1/R 2 + 1/R 3 </li></ul><ul><li>where R P = equivalent parallel resistance </li></ul>

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