Physics 144 – Chowdary
How Things Work
Spring 2006
Name:__________________________________________________________
Partners’ Name(s):________________________________________________
Lab #6: Circuits, Continued
Introduction
In today’s lab, we’ll continue to learn about simple electric circuits. As we discussed previously, all electrical
and electronic appliances use electric circuits. In this lab, you will re-visit many of the circuits you created last
week, this time measuring voltage drops and currents in these circuits. You’ll learn how to apply energy
conservation, charge conservation, and the resistance rule to understand and predict the brightness of different
configurations of light bulbs.
The behavior of any electric circuit can be understood in terms of electric charges moving through the circuit. It is
conventional, but not essential, to consider positive charges flowing from the positive to the negative terminal of
the battery. A bulb will only light up when charges flow through it. This is analogous to the flow of water around
a closed network of pipes with a pump taking the place of the battery; here each connecting wire corresponds to a
very wide pipe and the bulb a resistive paddle wheel. Additionally, the amount the bulb lights up is related to the
power delivered to the bulb; the power depends not only on the charge flowing through the bulb, but also the
energy that the charge has in the form of electric potential energy that can be transformed into heat and light.
We will characterize the behavior of electrical circuits and the charges flowing through them in terms of two of
physical quantities: current and potential difference.
The current at any point in the circuit or within any single circuit element, denoted I, is the rate at which electric
charge passes that point. This is analogous to the rate of flow of the water through a system of pipes. Current is
measured in Coulombs/second, or Amperes (A). It is not necessarily the same at two different points in the circuit
(although it may be in special cases).
The potential difference or voltage between two points in a circuit, denoted V or V, is the energy required to move one
unit of charge between those points. This is analogous to the water pressure difference between two points in a
pipe system. Potential difference is measured in Joules/Coulomb, or Volts (V). For example, it requires energy to
move charge through a light bulb; V=3V across the bulb means that 3J of energy is needed to move 1 C of charge
from one end of the bulb to the other. The energy lost by this charge may manifest itself as heat and light.
Equipment
You will use the same equipment as last week: batteries in battery holders, light bulbs in sockets, knife
switch, and banana plug leads. You will also find several digital multimeters (yellow or black boxes with a dial on
them), which we can use to measure the voltage drop (potential difference, related to energy) between various
points of a circuit and also to measure current (the amount of charge that passes through a point in a fixed amount
of time) through some point or junction in a circuit.
Part I: How do you measure current or potential difference in a circuit?
Current is measured by an ammeter, and potential difference by a voltmeter. The digital multimeter
functions as either an ammeter or a voltmeter, depending the meter setting selected and how it is connected to the
circuit. An ideal measuring device should not disturb the circuit that is being measured. In the next few steps, you
will use the brightness of the light bulb as a visual check to show whether the ammeter or a voltmeter affects the
circuit.
Basic Configuration:
Connect the circuit shown in the schematic diagram of Figure 1. Note the
brightness of the light bulb in this basic configuration.
+
Measuring Current:
You will now measure the current flowing through the bulb, using the black
multimeter as an ammeter (there’s nothing special about the black multimeter
compared to the yellow multimeter, but the following instructions are for the black
meter). Turn the dial to the 10A setting. Insert one banana plug lead into the hole
labeled COM (the bottom hole); insert a different banana plug lead into the hole
labeled 10ADC (the top hole).
+
Figure 1:
Basic Configuration
There are many ways to insert the ammeter into your basic circuit; the correct one will not affect the current. You
will use the brightness of the light bulb to determine whether or not you have affected the circuit. Each circuit in
Figure 2 includes an ammeter. In configuration A, we would say that the ammeter is in parallel with the light bulb.
Do you see why we would use the term “parallel” to describe these two? For configuration B, we would say that
the ammeter is in series with the light bulb (in fact, for configuration B, the batteries, the light bulb and the ammeter
are all in series).
Connect each configuration of Figure 2.
Note that for configuration A you won’t
need to open the circuit, but for
configuration B, you will need to open the
circuit to insert the ammeter.
Compared to the basic configuration of
Figure 1, which way of inserting the
ammeter from Figure 2 disturbs the
circuit’s behavior noticeably? Which is
closest to an ideal current measurement?
In order to properly measure current
through the bulb, would you place the
ammeter in parallel or in series with the
bulb?
10 ADC
+
+
A
COM
10 ADC
+
A
+
COM
A
B
Figure 2: Inserting an ammeter into the basic
configuration of Figure 1.
Consider the configuration that measures current correctly. What happens if you reverse the order of the leads on
the ammeter? Probably the easiest way to do this is to switch the leads at the ammeter sockets. Describe the
difference that this causes in the reading and explain this in terms of the flow of charge.
Measuring Potential Difference:
You will now measure the potential difference across the bulb. Reconnect the basic configuration of Figure
1 and note the brightness of the bulb. Using the yellow multimeter (again there’s nothing special about the yellow
multimeter compared to the black multimeter, but the following instructions are for the yellow meter), set the dial
to the DCV 20, connect one lead to the V socket (all the way on the left) and the other lead to the COM socket (just
right of the V socket).
Connect each configuration of Figure 3. Note
that for configuration A you won’t need to
open the circuit, but for configuration B, you
will need to open the circuit to insert the
voltmeter.
Which configuration from Figure 3 creates
the most noticeable disturbance compared to
that of the basic configuration Figure 1?
Which is closest to an ideal potential
difference measurement?
In order to
properly measure potential difference across
the bulb, would you place the voltmeter in
parallel or in series with the light bulb?
V
+
+
V
COM
V
+
V
+
COM
A
B
Figure 3: Inserting a voltmeter into the basic
configuration of Figure 1.
Reconnect configuration A of Figure 3, reversing the connections to the the voltmeter. Describe the difference that
this causes in the reading and explain this in terms of the relative energy of the charges on either side of the bulb.
In your explanation, consider the motion of positive charges circulating from the positive to the negative battery
terminal. On which side of the bulb would you expect the energy of each positive charge to be higher? Given that
the energy of each charge changes as it passes through the bulb, explain what happens to the energy that each
charge loses.
Now consider the possible source of energy that propels the charges around the circuit: the battery. Predict the
sign of the voltmeter reading if the voltmeter is connected in parallel across one of the batteries with the V lead at
the negative end of the battery and the COM lead at the positive end of the battery. Verify your prediction.
Discuss your results from this section with your instructor before moving on.
Part II: Measuring Potential Difference and Current in a Circuit
Go back to the basic configuration of Figure 1. Wire in ammeters and voltmeters as shown in Figure 4; be
careful that you get the order of the leads as shown in the figure. Note that as our discovery in the previous
section, voltmeters are wired in parallel across a circuit element, and ammeters are wired up in series next to a
circuit element. Next to each voltmeter or ammeter, write down the reading on the meter. Include sign as well
as number, and make sure that you write down units.
10 ADC
A
COM
+
V
V
V
+
V
COM
COM
A
COM
10 ADC
Figure 4: Inserting voltmeters and ammeters
into the basic configuration of Figure 1.
A common idea is that current gets used up as it passes through a light bulb. To test this idea, consider the two
ammeters in Figure 4. If current gets used up as it passes through a light bulb, what would this imply about the
reading on the ammeter before the light bulb and the reading on the ammeter after the light bulb? Do your results
support or refute the notion that current is used up by the light bulb?
Does electric potential energy get used up in this circuit? If you were to follow a charge as it enters the negative
end of the battery chain, comes out the positive end, goes through the wires, goes through the light bulb, and
comes back to the negative end of the battery chain, you’d expect its increases and decreases in electric potential
energy to sum up to zero. Consider the reading of the potential difference across the battery chain and the
potential difference across the light bulb. Do your results support or refute the notion that the increases and
decreases in electric potential energy sum up to zero?
Discuss your results from this section with your instructor before moving on.
Part III: Using Energy Conservation and Charge Conservation
Light Bulbs in Series
Set up the circuit shown in Figure 5B. Figure 5A shows the circuit without any voltmeters or ammeters; note that it
consists of two batteries in series with two light bulbs that are in series. Next to each voltmeter or ammeter, write
down the reading on the meter. Include sign as well as number, and make sure that you write down units.
10 ADC
A
COM
V
Bulb 1
Bulb 1
+
COM
+
V
V
+
V
10 ADC
A
+
COM
Bulb 2
Bulb 2
COM
V
V
COM
A
COM
A
10 ADC
B
Figure 5: Measuring potential difference and
current for two light bulbs in series.
“Follow” a charge as it makes a loop beginning at the negative end of the battery chain, comes out the positive end,
goes through bulb 1, goes through bulb 2, and returns to the negative end of the battery chain.
What do you notice about the sum of the potential increases and decreases around this loop? What do you notice
about the current in all parts of this series circuit?
Light Bulbs in Parallel
Set up the circuit shown in Figure 6B. Figure 6A shows the circuit without any voltmeters or ammeters; note that it
consists of two batteries in series; the batteries are both in series with light bulbs; but the light bulbs are in parallel
with each other. Next to each voltmeter or ammeter, write down the reading on the meter. Include sign as well
as number, and make sure that you write down units.
+
Bulb 1
Bulb 2
+
A
“Follow” a charge as it makes a loop beginning at the negative end of the battery chain and comes out the positive
end. Note that any single charge can only make one loop, so any single charge that passes through the batteries can
either go through bulb 1 or through bulb 2, but NOT both. Then the charge can return to the negative end of the
battery chain.
Consider the loop consisting of the battery chain and bulb 1. What do you notice about the sum of the potential
increases and decreases around this loop?
Consider the loop consisting of the battery chain and bulb 2. What do you notice about the sum of the potential
increases and decreases around this loop?
Now, look at the ammeter just before the parallel junction (labeled ammeter I). Also, look at the ammeters in each
parallel branch (labeled II and III). What do you notice about the current entering the junction and the currents
leaving the junction? What about comparing II, III, and IV (the ammeter after the second junction)?
Light Bulbs in Series and in Parallel
Set up the circuit shown in Figure 7B. Figure 7A shows the circuit without any voltmeters or ammeters. This
circuit has light bulbs in series and in parallel. Next to each voltmeter or ammeter, write down the reading on the
meter. Include sign as well as number, and make sure that you write down units.
+
Bulb 1
+
Bulb 2
Bulb 3
A
As before, consider the potential increases and decreases in loops (the loop consisting of the batteries, bulb 1 and
bulb 2 as well as the loop consisting of the batteries, bulb 1 and bulb 3). Also consider the current entering and
leaving any junctions.
+
+
Part IV: The Resistance Rule
The Resistance Rule is more commonly known as Ohm’s Law, and states that the potential drop across a
(resistive) circuit element can be related to the current in that element by the resistance of that element:
V  I  R .
Consider the circuit you measured in Figure 7. You know the potential drop across light bulbs 1, 2, and 3, as well
as the current through each of those bulbs. Use this information to calculate the resistance of each bulb. Are the
resistance of the bulbs similar?
Power to Bulbs
The important thing in lighting a bulb is the power that is delivered to each bulb. The power that is
delivered to each bulb is the product of the voltage drop (potential difference) across each bulb along with the
current through that bulb:
P  I  V .
Again, consider the circuit you measured in Figure 7. Use the Resistance Rule, Energy Conservation, and Charge
Conservation, along with the definition of power, to explain why each bulb lights up the way it does. Also, use this
to explain what happens when you unscrew bulb 2. Make any additional measurements that you need to.