General Physics 2
Lab 6: Circuits
Siena College
Voltages in Series Circuits
1) Connect the circuit shown in the figure at the right, which consists of
two batteries connected to two bulbs in series.
2) Predict how the potential difference across the battery will compare
with the potential difference across the series combination of bulbs.
3) Using your multimeter to measure potential differences, make the following measurements:
Potential difference across batteries: __________
Potential difference across both bulbs: __________
4) How do the two potential differences compare? Did your observations agree with your prediction?
5) Assuming two bulbs are identical, predict how the potential difference across each bulb will
compare to the potential difference across the battery.
6) Measure the potential difference across each bulb:
Potential difference across bulb 1: __________
Potential difference across bulb 2: __________
7) Did your measurements agree with your predictions? Discuss any discrepancies.
8) Formulate a rule for how potential differences across individual bulbs in a series connection
combine to give the total potential difference across the series combination of the bulbs. How is
this related to the potential difference across the battery?
Voltages in Parallel Circuits
1) Connect the circuit shown in the figure above, which consists of two batteries connected to two
bulbs in parallel.
2) Predict how the potential difference across the battery will compare with the potential difference
across the parallel combination of bulbs.
3) Using your multimeter to measure potential differences, make the following measurements:
Potential difference across batteries: __________
Potential difference across both bulbs: __________
4) How do the two potential differences compare? Did your observations agree with your prediction?
5) Predict how the potential difference across each bulb will compare to the potential difference
across the battery.
6) Measure the potential difference across each bulb:
Potential difference across bulb 1: __________
Potential difference across bulb 2: __________
7) Did your measurements agree with your predictions? Discuss any discrepancies.
8) Formulate a rule for how potential differences across the different branches of a parallel circuit.
How are these related to the potential difference across the battery?
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Resistances in series and in parallel
In this experiment, you will investigate the equivalent resistance of several combinations of resistors.
You have been supplied with three resistors. Set the multimeter as an ohmmeter, and measure the
resistance of each of the resistors. Record the resistance of each on your paper as follows:
R1 = _____________
R2 = _____________
R3 = _____________
Now, connect the resistors as shown in each diagram, determine the equivalent resistance by
calculation and by measurement, and record the results below.
Calculated
Measured
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
Req = ________________
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Ohm’s Law
The figure to the right shows the circuit that will be used in this part of
the experiment. E is a source of emf; it provides a potential difference, or
voltage, to the circuit and R is either a light bulb or a resistor.
E
R
The emf source we are using is called a DC power supply. Your instructor will explain how to use this
device to power the circuit in the figure.
1) Make sure your power supply is off. Then connect one of the brown, cylindrical resistors in the
circuit. The resistor should have four colored bands painted on it in this order: red, red, brown,
gold. Turn both current knobs all the way down and turn both voltage knobs all the way up. Set the
AMPS button for the LO setting (button depressed) and set the display switch to read AMPS.
2) Turn your power supply on. Using the fine current adjustment knob, increase the current, i, from 0
to 0.080 amps in ten (10) steps. Record the current, i, and voltage, V, at each step.
Voltage (V)
Current (A)
3) Now enter the readings in an Excel spreadsheet. On the spreadsheet calculate the resistance of the
bulb, R = V/i, for each data point. Produce a plot of the voltage versus current, V vs. i, and
resistance versus current, R vs. i.
4) Provide a sketch of these graphs below for the resistor. Be sure to label the axes. Indicate the
values of the highest and lowest values of resistance calculated.
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5) Turn the current knobs all the way down. Substitute a light bulb in place of the resistor in the
circuit. Using both the coarse and fine adjustment current knobs, increase the current through the
light bulb to a maximum of 0.30 amps in steps 10 steps of 0.03 amps each. Do not increase the
current beyond 0.30 amps or the light bulb will blow.
Voltage (V)
Current (A)
6) Sketch your graphs for the light bulb below:
7) If a graph of V vs. i is a straight line, what quantity does the slope of the line represent?
8) Did either of the graphs of V vs. i result in a straight line? If so, which one?
9) Did either the light bulb or the resistor obey Ohm’s law? If so, which one(s) and why?
10) What is the resistance of your resistor?
R = _________
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Internal resistance of a battery
A battery is not a source of constant current – the current varies according to the resistance in the
circuit. A battery is, however, a nearly constant voltage source, but not perfectly constant as you may
have already noticed. If you have measured the voltage across the batteries in other parts of this
experiment while a current is being drawn (by a bulb or a resistor), you may have noticed that the
potential difference measured across the batteries drops below the value that you measure with no
current being drawn. This voltage drop occurs because the chemical reactions in a battery cannot
supply charge fast enough to maintain the full emf and there is always some hindrance to completely
free flow of charge within the battery. Therefore, a battery itself has some resistance, called its
internal resistance, and we can never separate it from the battery.
When we attach the leads of a voltmeter to a battery, we are measuring
the terminal voltage. When no current is drawn from the battery, the
terminal voltage equals the emf. But when current flows from the battery
through a circuit, there is an internal drop in voltage due to the internal
resistance. We can model a real battery as a perfect emf in series with a
resistor, r, as shown in the figure.
r
R
E
We can determine the internal resistance of our batteries using the circuit above. For this circuit, use
one D cell battery and one resistor with a known resistance (there should be plenty of 10 resistors
available).
1. First, measure the potential difference across the battery without the resistor attached. This is the
emf of the battery.
E = __________
2. Now connect the 10
the resistor.
resistor in series with the battery and measure the potential difference across
VR = _________
3. Calculate the current in the circuit.
i = _________
4. In our model, the battery’s internal resistance is just another resistor that is placed in series with the
10
resistor. Most of the emf of the battery is dropped across the 10 resistor, while a small
amount is dropped across the internal resistance, r. Using the values you determined above,
calculate the internal resistance of the battery, r.
r = __________
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