Physics 142
Laboratory 2, Part 1
Testing Ohm's Law
In this lab, we will explore the current and voltage for several circuits involving resistors and capacitors.
Remember:
Current ( I ):
The amount of charge per second running past a point.
Potential (V):
The electrostatic potential. Differences in potential can be used to push charge through a
resistor, or to charge a capacitor.
Resistance (R):
Opposition to current. A potential difference is required to pump current across a resistor.
An ideal resistor satisfies the equation ?Z œ MV (Ohm's Law).
Measuring Instruments
A voltmeter is used to measure the potential difference between two points in a circuit. To use a voltmeter, you
must connect it between the two points in question:
An ammeter measures the current flowing through it. To use an ammeter, you must inert it into the circuit at
the point where you want to measure the current:
SAFETY WARNING: Always remember that current can flow through an ammeter. Whenever you
connect an ammeter, make sure you are not creating a short circuit!
Testing a Lightbulb
A lightbulb functions roughly as a resistor. Connect a lightbulb to your power supply, together with an ammeter
to measure current and a voltmeter to measure the potential difference:
SAFETY WARNING: To avoid burning out the lightbulb, please do not go above 10 volts.
Data Collection: Measure the current through the lightbulb for several different values of ?Z (at least five or
six data points), and record the results.
Writeup Instructions:
1. Does the lightbulb obey Ohm's law? Explain.
2. Based on your data, what is the resistance of the lightbulb?
3. Include a plot of M vs. ?Z for the lightbulb in your report.
Testing a Resistor
Connect a single resistor to your power supply, with an ammeter to measure current and a voltmeter to measure
the potential difference:
SAFETY WARNING: To avoid burning out the resistor, please do not go above 10 volts.
Data Collection: Measure the current through the resistor for several different values of ?Z (at least five or
six data points), and record the results.
Writeup Instructions:
1. Does the resistor obey Ohm's law? Explain.
2. Based on your data, what is the resistance of the resistor?
3. Include a plot of M vs. ?Z for the resistor in your report.
Physics 142
Laboratory 2, Part 2
Measuring Capacitance
A capacitor is a device that stores charge:
U œ G ?Z
Here U is the stored charge, ?Z is the potential difference across the capacitor, and G is the capacitance.
Your goal in this lab is to measure the capacitance G of your capacitor:
G œ
U
?Z
It is easy to measure ?Z with a voltmeter, but measuring U is harder. The plan is to slowly discharge the
capacitor, and measure the current M that comes out. The total charge U is given by the formula
U œ ( M .>
Charging a Capacitor
Charge the capacitor by connecting it to a power supply. Make sure to include a small resistor in the circuit so
that there isn't too much current:
small V
The capacitor should only take a few seconds to charge.
We are using compact electrolytic capacitors, which have a very high capacitance for their size. It is
possible to ruin these capacitors if they are hooked up improperly:
SAFETY WARNINGS:
1. It matters which end of the power supply is connected to which end of the capacitor. Hooking up
a capacitor in the wrong direction may cause it to explode.
2. Do not let the power supply go above the maximum voltage for the capacitor.
Discharging a Capacitor
You can discharge the capacitor by connecting its two ends. Include the big resistor in the circuit to make it
discharge slowly:
A
big V
You can measure the current in the circuit with an ammeter. As the capacitor discharges, the potential
difference decreases, which decreases the current being forced through the resistor.
SAFETY WARNING: If you hook up the ammeter directly to the capacitor, it will make a short circuit!
Two-Loop Circuit
You can set up the following circuit to charge and discharge the capacitor easily:
small V
A
big V
This circuit has a free terminal near the positive end of the power supply. Plugging this terminal in charges the
capacitor, and unplugging it makes the capacitor discharge through the big resistor.
Procedure:
1. Set the power supply to approximately "! V.
2. Connect the free terminal to the power supply and wait a few seconds to charge the capacitor.
3. Measure the potential difference across the capacitor with the voltmeter. Make sure to disconnect the
voltmeter after you finish your measurement.
4. Record the current through the ammeter, and then disconnect the terminal from the power supply. The
capacitor will begin to discharge.
5. Using a stopwatch, record the current every "! s while the capacitor is discharging. Continue taking data
until the current is less than "% of its original value.
Repeat this procedure for three or four different values of ?Z .