3.3.1 Practical Circuits
3.3.1 Investigating Resistors
Aims
In this experiment you will investigate different combinations and arrangements of resistors in an electrical circuit. You will measure the current and pd in a circuit where identical resistors are arranged in series and in parallel. This will enable you to learn about the relationships between current and pd in series and parallel circuits.
Equipment, materials and method
1 For this experiment you will require electrical leads, 3 × 4.7 resistors, a 1.5V cell, a voltmeter and a milliammeter (or two multimeters).
2 Set up a simple circuit with one resistor and the cell. Measure the current in the circuit and the pd across the cell. Draw your circuit and add your measurements to your diagram.
3 Add a second resistor in series. Draw your circuit. Measure the current flowing in the circuit, the pd across each individual resistor and the total pd across both resistors. Record your measurements on your diagram.
4 Repeat for three resistors in series.
5 Set up a circuit with the cell and two resistors in parallel. Draw your circuit and record the current through each resistor and the pd across it. Also measure the current through the cell.
6 Repeat the experiment for three resistors in parallel. Draw your circuit and record your measurements.
Questions
1 What happens to the total current in the circuit as you add resistors in series? Hence, suggest what happens to the total resistance as you add resistors in series.
2 What happens to the total current in the circuit as you add resistors in parallel? Hence, suggest what happens to the total resistance as you add resistors in parallel.
Results
Sketch all of your circuits and add your measurements to your diagrams.
Discussion
1 What pattern (or patterns) can you identify for the current and the pd in a circuit where the number of resistors in series is increasing?
2 What pattern (or patterns) can you identify for the current and the pd in a circuit where the number of resistors in parallel is increasing?
3 (a) Use your measurements to calculate:
(i) the average resistance of a single resistor
(ii) the resistance of each combination.
(b) Show that your results in 3(a) are consistent with the appropriate resistance combination rule.
Investigating cell combinations
Aims
In this experiment you will investigate different combinations and arrangements of cells in an electrical circuit. You will measure the current and potential difference in a circuit where identical cells are arranged in series and in parallel. This will enable you to learn about how to combine cells to use them as effectively as possible. You will also be asked to think carefully about the way you record and present your data.
Equipment, materials and method
1. For this experiment you will require 10 leads, a 4.7Ω resistor, 3 × 1.5V cells, a voltmeter and a milliammeter (or two multimeters), and a heatproof mat.
2. Read briefly through the experimental steps below. Think carefully about how you will record and present your readings.
3. Set up circuit a and measure the voltage (pd) across the resistor and the current through it. Record your readings.
4. Set up circuit b with two cells in series. Repeat the measurements of the voltage (pd) across the resistor and the current through it. Record your readings.
5. Repeat the readings for three cells in series.
6. Set up circuit c with two cells in parallel. Take care to connect the leads correctly. Measure the voltage (pd) across the resistor and the current through it.
7. Repeat the readings for three cells in parallel.
Questions
1 Before carrying out this experiment, a student predicted that the current in the series circuits would be in proportion to the number of cells.
(c) Do your results support this prediction? Use your own measurements to give a reason for your answer.
(d) The student claimed the prediction would have been correct if the cells had zero internal resistance. Do you think this claim is reasonable? Give a reason for your answer.
2 What scientific terms or notation did you use when recording your data?
Results
Record your results as you go. You may also wish to summarise them in a separate table.
Discussion
1. What pattern (or patterns) can you identify for the current and the pd across the cells in a circuit where the number of cells in series is increasing?
2. What pattern (or patterns) can you identify for the current and the pd across the cells in a circuit where the number of cells in parallel is increasing?
Conservation of Energy in a Circuit
Aims
In this experiment you will investigate a circuit using two laws known as Kirchhoff’s laws. The first law states:
At any junction in a circuit, the total current entering the junction equals the total current leaving the junction.
In other words, charge is conserved.
The second law states:
Around any closed loop in a circuit, the sum of the emfs is equal to the sum of the products of current and resistance in that loop.
In other words, energy is conserved.
In this investigation you will set up a circuit using various resistors and cells in order to investigate a circuit using these laws. Recall that, according to Ohm’s law, the product of the current and resistance across a device corresponds to the potential difference.
Safety
Check electrical equipment for loose wires. The resistors may heat up during the experiment.
Equipment, materials and method
1. For this experiment you will need a multimeter, 5 labelled resistors of varying resistance from 10 to 22 in suitable holders, 2 × 1.5V cells and connecting leads.
2. Measure the exact resistance of each of the five resistors using the multimeter set on the ohmmeter setting.
3. Connect the circuit shown below. Check that the cells are arranged with the correct polarity.
4. Measure the pd across each cell and across each resistor using the multimeter as a voltmeter. Record your readings. As soon as you have recorded the readings, disconnect the cells.
5. Disconnect the wire between R1 and R2; connect the multimeter as an ammeter in the gap and measure the current through the branch containing R1 and R2. Remove the multimeter and replace the disconnected wire. Similarly, measure the current through the branch containing R3, R4 and R5. Record your readings.
Results
1. Use your measurements to calculate:
(a) the pd across the two cells
(b) the current through the cells.
2) (a) Calculate the pd across each of the two branches using the resistance and current measurements for each resistor.
(b) Compare the result of your calculations in 2(a) of the pd across each resistor branch.
Discussion
1) What effect might the multimeter have on the circuit when measuring the current in step 5?
2) What are the sources of experimental error in this experiment?
3) Estimate the uncertainty in your result for 1(a) to discuss whether your pd values for 1(a) and 2(a) can be considered the same?