Laboratory 4 – Ohm’s Law
PHYS 204 Online Laboratory
Name: __________________________________
Ohm’s Law
Electric voltage, Current and Resistance
The purpose of this activity is to determine the mathematical relationship between battery
voltage (ΔV), current (I), and resistance (R) for a simple circuit.
•
Ohm’s Law
There are certain formulas in Physics that are so powerful and so pervasive that they reach
the state of popular knowledge. A student of Physics has written such formulas down so
many times that they have memorized it without trying to. Certainly, to the professionals
in the field, such formulas are so central that they become engraved in their minds. In the
field of Modern Physics, there is E = m • c2. In the field of Newtonian Mechanics, there is
Fnet = m • a. In the field of Wave Mechanics, there is v = f • λ. And in the field of current
electricity, there is ΔV = I • R.
The predominant equation which pervades the study of electric circuits is the equation
ΔV = I • R
In words, the electric potential difference between two points on a circuit (ΔV) is equivalent
to the product of the current between those two points (I) and the total resistance of all
electrical devices present between those two points (R). Through the rest of this unit of
The Physics Classroom, this equation will become the most common equation which we
see. Often referred to as the Ohm’s law equation, this equation is a powerful predictor of
the relationship between potential difference, current and resistance.
Getting Ready:
Navigate to
https://phet.colorado.edu/sims/html/circuit-construction-kit-dc/latest/circuit-construction-kit-dc_en.html
Click on Introduction or double click on Lab to start to build your circuit as shown in this lab.
Build, Measure, Analyze
1. If necessary clear your Workspace by clicking on all components; you only need a battery,
resistor, wires and a switch to build your circuit. Be sure you adjust the values of resistors.
Choose any value of the potential at the power supply, and keep it constant for the entire lab.
1
Your circuit should look something like this. Play around by changing the polarity of the battery
and observes what happens to the current. Explain why you see the result that you see.
Note also the relationship between the battery potential and the potential across the resistor. What
does this relationship imply about the resistance of the wire?
2. Use the voltmeter and ammeter to measure the voltage and current in each resistor. Record
your readings in Table 1.
3. Increase the resistance to the values shown in the table. Continue to measure the potential
across the resistor and the current in the circuit until Table 1 is complete.
2
Table 1:
Resistance (Ω)
Voltage (V)
At battery
Current (A)
In circuit
Potential
(Voltage)
measured
across
Resistor
1/R
(Ω-1)
10
40
50
80
100
4. Using Excel, make a graph of the data in the table with Current on the y-axis and Resistance on
the x-axis.
5. Using Excel, make a graph of the table with Current on the y-axis and 1/R on the x-axis.
6. Determine the equation of the line which goes through the points in the graphs. Have Excel place
this equation on the graph. Print both graphs so that both are on the same single sheet of paper
and attach the graphs to this report.
Table 2:
Resistance (Ω)
of each resistor
Voltage (V)
At battery
Current (A)
In circuit
before the 1st
resistor
Voltage
measured
across
Resistor 1
Voltage
measured
across
Resistor 2
10
40
50
80
100
7.
Return to the circuit that you made earlier. Add a second resistor (with a resistance equal to ½ of
the original resistance) in series with the first one. Now perform the measurements that you made
earlier and record your data in Table 2.
a. How does the current compare to the current measured in Table 1. Does this make sense
to you? Explain why it should.
3
b. How does the potential (voltage) measured across each of the resistors compare to each
other.
c.
Does this make sense to you? Explain why it should.
8. Modify the circuit again by moving the second resistor (from step 4) to be wired in parallel with
the original resistor. Again, perform the measurements that you made earlier and record your
data in Table 3.
a. How does the current compare to the current measured in Table 1. Does this make sense
to you? Explain why it should.
Table 3:
Resistance (Ω)
of each resistor
Voltage (V)
At battery
Current (A)
In circuit
immediately
before the 1st
resistor
Current (A)
In circuit
immediately
before the 2nd
resistor
Voltage
measured
across
Resistor 1
Voltage
measured
across
Resistor 2
10
40
50
80
100
a. How does the current that is measured before each resistor compare to each other, and to the
current recorded in Table 1. Does this make sense to you? Explain why it should.
b. How does the potential (voltage) measured across each of the resistors compare to each other,
and to the battery potential. Does this make sense to you? Explain why it should.
4
Questions
1. Explain how the shape of the graphs made in steps 1& 2 corresponds to Ohm’s Law. Why
does one graph curve rather than being a straight line? What can we learn from the plot that
has a straight line of data?
2. Compare Ohm’s Law and the general equation for a straight line to explain why
the graph you made in step 2 will in fact be a straight line. Use this comparison
to show what the value of the slope should be in terms of “V”. What should the
Y-Intercept of your graph be?
3. When the current through a resistor is increased by a factor of 4, what happens to
the power dissipated by the resistor? Justify your answer.
5
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