P2 Investigating Resistance (page 185) Flashcards
You saw on page 180 how the length of the wire affects its resistance. Now its time to do an experiment to see how placing resistors in series or in parallel can affect the resistance of the whole circuit.
What is the process for investagating ‘Adding Resistors in Series…’?
1) first you’ll need to find at least four identical resistors.
2) then build the circuit shown on the right (see diagram 1 on page 185), using one of the resistors. Make a note of the potential difference of the battery (V).
3) Measure the current through the circuit using the ammeter. Use this to calculate the resistance of the circuit using R = V ÷ I.
4) Add another resistor, in series with the first (see diagram 2 on page 185).
5) Again, measure the current through the circuit and use this and the potential difference for the battery to calculate the overall resistance of the circuit.
6) Repeat steps 4 and 5 until you’ve added all your resistors.
7) Plot a graph of the number of resistors against the total resistance of the circuit (see graph on page 185).
What is the process for investigating Resistors in Parallel?
1) using the same equipment as you would for investigating in series (so the experiment is a fair test), build the same initial circuit.
2) Measure the total current through the circuit and calculate the resistance of the circuit using R = V ÷ I (again, V is the potential difference of the battery).
3) Next, add another resistor, in parallel with the first, see diagram 3 on page 185.
4) Measure the total current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit.
5) Repeat steps 3 and 4 until you’ve added all your resistors.
6) Plot a graph of the number of resistors in the circuit against the total resistance.
What is the process for investigating Resistors in Parallel?
1) using the same equipment as you would for investigating in series (so the experiment is a fair test), build the same initial circuit.
2) Measure the total current through the circuit and calculate the resistance of the circuit using R = V ÷ I (again, V is the potential difference of the battery).
3) Next, add another resistor, in parallel with the first, see diagram 3 on page 185.
4) Measure the total current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit.
5) Repeat steps 3 and 4 until you’ve added all your resistors.
6) Plot a graph of the number of resistors in the circuit against the total resistance.
Your results should match the resistance rules, which are what?
1) you should find that adding resistors in series increases the total resistance of the circuit (adding a resistor decreases the total current through the circuit).
2) The more resistors you add, the larger the resistance of the whole circuit.
3) When you add resistors in parallel, the total current through the circuit increases - so the total resistance of the circuit has decreased.
4) The more resistors you add, the smaller the overall resistance becomes - as shown by the graph on page 185.
5) These results agree with what you learnt about resistane in series and parallel circuits on pages 183 and 184.
Draw a diagram of a single circuit that could be used to investigate the effect of adding resistors in parallel. Your circuit should include switches (1 mark)
See diagram on page 247, p. 185, Q1
(1 mark for a circuit with several resistors connected in parallel and switches allowing one resistor to be added at a time).
