Electricity: An Investigation of Ohm’s Law
voltage
circuit
Important terms to remember
current
resistance
amperes
ohms
power
watts
Prior to lab: in your lab notebooks, describe a basic scenario of your experience with electricity.
It could be an observation you have made of the use of electricity in your own home, car,
museum or business or something you have heard or read regarding electricity. This could also
include questions or thoughts you have regarding this scenario, such as “ I wonder why this
happens?” Or “how does electricity work in my day to day experiences?”
Tak a look at the following websites for an introduction to electric circuits:
http://phet.colorado.edu/en/simulations/category/physics/electricity-magnets-and-circuits
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
Introduction
The purpose of this experiment is to provide an introduction to the study of electricity. We will
do this by studying a light bulb in an electrical circuit. Circuit components may be characterized
by their resistance R, measured in ohms (). In some cases the resistance of a circuit will remain
constant, but in others it may depend on temperature, current, and other factors. When connected
in an electrical circuit a current I, measured in amperes (A), will flow through the component and
a voltage V, measured in volts (V), will exist across the component. Ohm’s law relates the
voltage V, resistance R, and current I:
V = IR
(1)
The component in a circuit will convert electrical energy to other forms of energy, and the rate of
energy conversion or power, measured in watts (W), is given by
P = IV = I2R
(2)
For a bulb electrical energy is converted to light and heat, and for a carbon resistor electrical
energy is converted to heat. When the bulb is hooked up to a power supply, an electric current
flows from one contact to the other, through the wires and the filament. As electrons move
through the filament, they are constantly bumping into the atoms that make up the filament. The
energy of each impact vibrates an atom producing heat and visible light.
Electrical Circuit Symbols
The following are typical symbols used in drawing diagrams of the “flow” of electricity through
materials connected together in something called a CIRCUIT. We will be using these symbols in
the laboratory activities.
Voltage source V
Resistance R
Bulb
Objectives



To develop an understanding of simple circuits by building these simple circuits in the lab.
To use a digital multimeter to measure current, voltage and resistance in particular circuits.
To explore and apply the mathematical relationship between current, voltage and resistance
through a circuit element that students build in the lab.
Procedures and activities
A. Building a Simple Circuit
You will investigate the fundamental properties of working circuits and discover how a circuit
must be built so that “electricity” (charge) will flow by creating your own circuits. Build on your
knowledge of the circuits you investigated on the PhET website. Remember to consider your
questions and observations about electricity that you described prior to today’s lab.
Arrange a loose battery, a single wire (2 wires?), and a bulb in as many way as you can to
make the bulb emit light. Using the symbols for circuits, sketch each of your arrangements,
including failures as well as successes. Also describe the similarities between your successful
trials..
Question 1. What do the successful arrangements have in common? What do you propose as an
explanation for these successes? Support your response with specific references to your
observations and drawings.
B. Measuring Resistance
The electrical resistance of an object measures its opposition to the passage of an electric
current. A digital multi-meter (DMM) is a tool that measures resistance, voltage, and sometimes
current. Here you will use your DMM to measure the resistance of various single components,
and the resistance of two resistors in series and parallel.
1. Place one wire in the hole marked “com” (common, or ground) on your DMM, and another
in the socket labeled “V, ” (voltage or resistance). Turn the knob until it points to  – this
will allow you to measure resistance.
2. Measure the resistance of a “10 ” resistor. Measure the resistance in the opposite direction.
Question 2. Did the value you measured depend on direction? Was the value you measured
within 10% of 10 ?
3. Measure the resistance of a small light bulb.
4. Measure the resistance of a wire. Should we worry about accounting for the resistance of
wires in our circuits?
5. Hold the leads a few inches away from each other and measure the resistance of air. When
the DMM reads “OF,” it means the resistance is too high to measure.
6. Grasp the end of one lead from the (DMM) ohmmeter in each hand. What is the resistance
measured in your body? What does this say about electricity and your body?
Question 3. How does the resistance of your body compare to that of air? Current will always
take the path of least resistance. Given your results above, why is it unwise to go outside during a
lightning storm?
7. Arrange a 10  and a 15  resistor in series. Sketch this circuit diagram in your notebook
and measure the net resistance of both combined.
Question 4. Based on equation (2), what value do you expect to obtain? Does your measured
value agree with this prediction?
8. Now arrange the 10  and 15  resistors in parallel. Sketch this circuit diagram in your
notebook and measure the net resistance of this configuration.
Question 5. Based on equation (3), what value do you expect to obtain? Does your measured
value agree with this prediction?
C. Measuring Voltage
You will now use the DMM to measure voltage.
1. Turn the knob on the DMM to “V”. Leave the wires in the same holes.
2. Measure the voltage across a single battery. Measure the voltage in the opposite direction.
Does anything on the DMM change?
3. Measure the voltage across two batteries arranged in series, head to tail.
4. Measure the voltage of two batteries in parallel, with the heads and tails facing in the same
direction.
Question 6. Why would you arrange batteries in series? What advantage, if any, does a parallel
arrangement offer?
D. Measuring Current
When measuring both voltage and resistance we have placed the DMM in parallel with the
component to be measured (review the parallel circuit in the introduction if this doesn’t make
sense.) However, when we measure current the DMM must be in series with the component to
be measured so that the same charge flows through the component and the DMM. In this section
you will measure the current flowing through a circuit and verify equation (1).
1. Construct the circuit shown at the right, where
V
I
A
represents a DMM measuring voltage, and
represents a
DMM measuring current (A for amps). Turn the knob on one
DMM to mA/A in order to read current. Your voltage source
will be a 1.5 V battery.
1.5 V
2. Measure both the current and voltage.
A
10 
V
I
Question 7. Is equation (1) satisfied? Multiply your measured value of I by R = 10 . Is this the
same as your measured value of V?
E. Resistance and Power Consumption in a Light Bulb
1. In this section you will explore resistance, current, voltage and power consumption in a light
bulb.
2. Connect the light bulb to the power supply as show in the circuit
on the right. The supply is to be switched off while you complete the
wiring.
Question 8. You have been given a 6 volt bulb; it requires a driving
voltage of 6 volts to bring the bulb to normal brightness. Switch on
the power supply and bring the voltage up to 6 volts.
What is the current in the bulb at normal brightness?
Question 9. What is the resistance of the bulb? (use equation 1 and
give your result in ohms)
Question 10. How much electrical power is needed to produce normal brightness? (use equation
2 and quote your value in Watts)
Question 11. Does is matter which way current flows in a light bulb? Switch of the supply and
reverse the connection to the bulb; do you find any change in voltage, current, or brightness
when the supply is switched on again?
Question 12. Is the resistance of a bright bulb the same as that of a dim one? Reduce the driving
voltage to 3 volts and record the new value of current. Calculate the resistance from equation 1.
Question 13. When the voltage was reduced, the brightness decreased because the temperature
of the filament was reduced. Less power was available to heat the filament. Does the resistance
of the filament wire increase or decrease as the temperature rises?
Question 14: How did today’s lab help you better understand the scenario you describe prior to
lab? Describe what questions were answered in today’s lab and what questions you still have.