Materials
Needed per group:
- ½ can of play dough
- Knife switch
- 4 alligator clip wires
- Voltmeter
- Ammeter
- 2 large paper clips, or long metal pins
- Ruler
- Power supply, voltage does not need to be variable
Procedure
Caution: Do not connect the wires in your play dough together because you will create a short circuit and blow the fuse in the power supply.
- Have the students write a hypothesis for their lab. Make sure that the students give reasons for thinking the way they do.
- Roll dough out into a flat pancake shaped sheet about one to two centimeters in thickness evenly across the pancake.
- Cut the pancake into four even sections.
- Make four cylinder shaped lumps of clay that are uniform and all the same length.
- Take one of the four cylinders of clay and place both paper clips in the dough about 0.3 centimeters from each end of the cylinder as shown below.
- This is your resistor. Measure the length of the resistor and record in the data table.
- Set up the resistor in a simple circuit with the ammeter and voltmeter connected in the proper places to take voltage and current readings for the resistor.
- Connect the knife switch in series with the circuit so the current can easily be turned on and off.
- Turn on the power supply and turn the voltage dial up until the current reads about 100 mA.
- Take a voltage and current reading from the meters and then immediately use the knife switch to stop the current. The longer the play dough sits with the current through it, the more the readings will change, it is important to consistently read the current and voltage quickly and as soon as current is set up in the circuit to maintain accurate results.
- Now connect the next piece of play dough as shown in the diagram below, making a longer resistor.
- Take data for the voltage, current, and length and record in the data table.
- Continue adding length and taking readings until all four strips have been added.
- Take all four strips apart.
- Set up your circuit how you originally had it with one strip of play dough.
- Decrease the voltage in the circuit until the ammeter reads around 30 mA.
- Take a voltage, current and height (thickness, cross sectional area) reading.
- Now add the next strip on top of the first strip as shown in the diagram below.
- Continue adding strips on top and taking readings until all four strips have been added.
Data Analysis
- Find the resistance of each resistor by using Ohm’s Law
- Graph resistance vs. length
- Graph resistance vs. cross sectional area
- Write a conclusion based on the hypothesis
Reinforcement Activities
- The above lab is a lot of fun, but can be messy and also does not allow students to investigate resistivity of metal conductors. Also it is tricky to take measurement if the readings are changing on the play dough. For a more conventional lab, or to extend this activity it might be helpful to have the students test the resistivity of different wires. The effects of temperature can also be studied using pre-made coils of wire as the wire heats up by passing current through the wire the resistance of the wire changes.
- Have students measure resistance accurately using a Wheatstone Bridge. An excellent activity for talking about error analysis.
