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HPP Activity 71v1
BIOELECTROMAGNETISM: OHM’S LAW
Objectives

to understand Ohm’s Law qualitatively and quantitatively

to understand that Ohm’s Law is only valid for linear resistors
Exploration
Equipment:
1 multimeter
8 wires with alligator clips
1 bulb
1 socket
1 30cm nichrome wire
4 batteries and holder
GE 1.1
You have explored currents through bulbs and wires. Are there any
applications to the human body? You have also compared the human heart and
other muscles to the electric field set up by an electric dipole. Is it possible to
make the human body part of a circuit for any useful purpose?
Set up the circuit in the diagram below.
1. Measure the current through the bulb and the potential difference across the
bulb. Measure the current through the nichrome wire and the potential
difference across the nichrome wire. Record your results.
2. Add a battery to the circuit, and again record the current through and the
potential difference across the bulb and the nichrome wire.
Activity Guide
 2002-2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 71v1
2
3. Continue to add batteries and record the current through and the potential
difference across both the bulb and the nichrome wire, up to four batteries.
4. Plot the potential difference vs. the current for both the nichrome wire and
the bulb. Paste your plots in the space below.
Circuit elements for which a plot of potential difference vs. current is a
straight line are called linear resistors.
5. Should the point V = 0, I = 0 be on your plot? Explain.
6. Are either of the elements (the bulb or the nichrome wire), linear resistors?
7. Write an equation for the potential difference vs. the current for the
nichrome wire. Hint: You can get this equation from Excel!
Invention
For linear resistors, the potential difference and the current are related by a
constant
V  IR
Remember that we grow tired of writing V (representing a difference of
potential between two points) and instead just write V. So the common form
of Ohm’s Law is:
V=IR
The constant is the resistance measured in ohms. The symbol for ohms is .
1  = 1V/A (NB: the units for the slope of your line are V/A!)
Exploration
Equipment:
1 multimeter
GE 2.1
Activity Guide
 2002-2010 The Humanized Physics Project
HPP Activity 71v1
3
Set your multimeter to measure resistance.
1. Measure your body’s resistance by holding one lead in each hand. Compare
your resistance to the resistance of your partners. Record your observations.
2. Measure the resistance of the same parts of your body that you tested with
the chirping chick. Do these resistances help explain why or why not the chick
chirped when it did?
Fortunately for us, the resistance of dry skin is a very important protection at
low voltages. Typically, over an ordinary contact area, the skin inserts 10 100 k between an outside conductor (like the end of a banana lead or
multimeter probe) and the internal body fluids. Therefore, at voltages below
about 50 V, the skin provides safe current limiting protection - if it is dry! Be
extremely careful not to have electrical contact with a voltage supply if you
have wet or sweaty skin.
The severity of shock depends on the amount of current flowing through the
body
SUMMARY
You should understand Ohm’s Law qualitatively and quantitatively and understand that
Ohm’s Law is only valid for linear resistors. You should be able to work problems using Ohm’s
Law, Kirchhoff’s First Rule and Kirchhoff’s Second Rule.
Activity Guide
 2002-2010 The Humanized Physics Project