Optional Practical: Circuit simulations (Falstad)

Introduction

In this exercise Paul Falstad's circuit simulator will be used to simulate the simple circuits covered in the course. Note that the default symbol used for resistance is not the same as the one used by the IB, if you want the IB symbol choose "European resistors" from the options.

The simplest circuit

The simplest circuit consists of a cell and a resistor connected by wires.

  • Open the circuit simulator and choose "blank circuit" from the "circuits" menu.
  • Right click anywhere on the window and you will get a list of options. Select "voltage source 2 terminal" from the "inputs/outputs". The place it by click and drag.
  • To change the properties of the component hover over it with the cursor (it will turn white), right click then choose edit from the list.

  • Add a resistor and connect it to the battery with wires (all options in the right click list).
  • To measure the properties of a component hover over it with the cursor, all details of that component will then appear

Ohm's law

Ohms law states that the current passing through a resistor is proportional to the pd across it. The constant of proportionality is the resistance, R so:

V = IR

  • Set the voltage to 10 V and the resistance to 100 Ω. Check that the current flowing is consistent with your expectations.
  • Vary the voltage and notice how the current changes (note you have to apply the changes before they take affect).

A sawtooth input will change the voltage between two values at a constant rate. Select the cell and change it to "sawtooth" in the edit options. A frequency of 1 Hz will mean that the change from low to high takes 1s. The simulation will take a lot longer than 1s since it is not in real time, you can speed things up using the slider.

  • Observe the changing pd and current
  • Select the power supply and choose "view in scope". You will see a graph of how the voltage changes with time.
  • Right click the graph and choose display voltage but not current.
  • select power supply again and choose view scope but this time display current.
  • Observe how V and I change.
  • Now click one of the graphs and choose "Show V vs I"
  • Replace the resistor with a bulb (Lamp from inputs/outputs).
  • Plot the V/I characteristics for the lamp, you will have to slow down the sawtooth to maybe 0.1 Hz to see the change in resistance, why is this?

The not so simple circuit

Real cells have internal resistance, this can easily be represented by adding a resistor in series with the cell. Set the internal resistance to 1 Ω.

  • Vary the load resistor and observe how the PD across it changes. You should see that when the load is big the PD is about the same as the EMF of the cell.
  • Observe how the power dissipated in the internal resistance changes as the load is varied.

Resistors in series and parallel

Set up the series circuit below (switches can be found in passive components)

Closing each switch short circuits the resistor so that current dos not flow through it. By short circuiting the resistors you can change the number of resistors in the series combination.

  • To make the numbers easy set the cell EMF to 24 V
  • Display the current from the cell on the scope.
  • Use the switches to vary the number of resistors in the series combination and observe the effect on the current.
  • By measuring the voltage across the combination and the current though it show that the total resistance RT is given by

RT = R1 + R2 + R3 +......

Connect the parallel circuit below (you can make one resistor - switch combination then make copies by selecting all > copy > paste)

  • Set the cell EMF to 12 V.
  • Observe the effect on current of changing the number of resistors in the parallel combination.
  • By measuring the current through and voltage across the combination show that the total resistance RT is given by the formula

1/RT = 1/R1 + 1/R2 + 1/R3 + ....

Non ideal meters

There aren't any meters in this simulation but you can represent a meter by a resistor. A voltmeter should not draw current from the circuit so should have a high resistance.

  • Connect a 100 kΩ in parallel with a 100 Ω resistor as shown.

  • Use the 100 kΩ resistor to measure the PD.
  • How much current passes through the "voltmeter".
  • Reduce the resistance of the voltmeter and note the change in PD across it.

An ammeter is placed in series with a component, it should have a low resistance so it does not reduce the current in the circuit.

  • Connect a 0.001 Ω resistor in series with a 100 Ω resistor as shown.

  • Use the 1 mΩ resistor to measure the current in the circuit
  • What is the PD across the "ammeter".
  • Increase the resistance of the ammeter and note the change in current through the circuit.

Potential divider

  • Connect a potential divider like the one below.

If the load resistance (RL) is large then the PD across it will be given by

V subscript L space end subscript equals fraction numerator R subscript 2 over denominator R subscript 1 plus R subscript 2 end fraction V subscript s

  • Show that this equation holds for this circuit.
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