Measuring the resistance of pencil lead

Teacher Notes

Four labs can be conducted using the materials in the Resistance Board and Ohm’s Law kit:

  • the effect of length on resistance (using the Pencil Lead Resistance Board)
  • the effect of different materials on resistance (using the Pencil Lead Resistance Board)
  • the effect of wire diameter on resistance (using the Wire Resistance Board)
  • the effect of voltage on the current in a resistor (using the Ohm’s Law kit)


Pencil Lead Activity

Use the Resistance Boards Kit for this activity.


Concepts

Understanding how the resistance of a resistor varies with its length
 

Materials

  • Pencil Lead resistance board
  • Four alligator clip wires
  • A digital multimeter


 

Directions

  1. First, brainstorm with your group about factors that can affect the resistance of an object. Discuss whether each factor has a direct or inverse relationship with resistance; for example, “resistance increases as ________ increases/decreases.” Summarize your group’s discussion below:
  2. In this activity you will use a pencil lead as a resistor. Fasten the HB pencil lead in place by sliding it under the clamps of the Fahnestock clips. Be cautious; the lead breaks rather easily.
  3. Connect the circuit. Draw a diagram. Use a colored pencil and arrows to trace the path of electrons in the circuit shown.
  4. Part A: Use the HB lead. Design and conduct an experiment to determine the effect of the length of the pencil lead on the resistance measured.
    1. Your experimental question is:
    2. State your hypothesis:
    3. Your independent variable is:
    4. Your dependent variable is:
    5. What factors are held constant in your experiment?
    6. Procedure:
    7. Make a data table. Conduct your experiment, and record your results:
    8. Graph your data.
  5. Part B: You have two other leads on your board. What do you think will happen if you measured the resistance of the other two resistors in the same manner as you did in Part A? Repeat the experiment, collect data, present the data in a table and graph the data on the same graph as above.
  6. Create a data table for other leads.

 

Conclusions and Analysis

  1. From your experiment above, how does resistance vary with length?
  2. What type of mathematical relationship do you have between resistance and length?
  3. What differences did you notice between the slopes for the different pencil leads?  What do you think caused these differences?
  4. Suppose a 12 cm long piece of 2H pencil lead has a resistance of 24 W.  How much is the resistance of the lead/cm? If you broke off a 3-cm long piece of the lead, how much resistance would the 3-cm piece have?
  5. Does this experiment tell you how the resistance would change if you took a fatter piece of lead that was also 12 cm long? Explain your reasoning.
  6. Does this experiment tell you how the resistance would change if you took a piece of 3H lead that was 12 cm long? Explain your reasoning.

 
 
Download this activity in PDF format

More activities from the Resistors and Capacitors Module are available on the Exploring Physics CD.

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