Simple Circuits
INTRODUCTION
All circuits share some features: an energy/power source, like a battery;
wires to connect different components/devices together; and
components/devices to put into the circuit. There are as many different ways
to create circuits as there are applications. These applications depend on the
components/devices in a circuit.
In today’s lab, we will examine a simple DC circuit. The term DC means
direct current. A DC circuit has current flowing in only one direction and is
usually powered by a battery.
Below is a table of the quantities and units that are used to describe
electricity in a circuit.
Quantity
voltage (V)
Unit
volt (V)
current (I)
ampere (A)
resistance (R)
ohm (Ω)
power (P)
watt (W)
Table 1. Electrical quantities and units
Voltage (V) can be thought of as the pressure that causes electrons to flow
in a circuit. It is measured in volts (V). Electrons will flow when there is a
difference in voltage between two points in a circuit. The greater the voltage
difference, the greater the electron flow. A voltage difference is caused by a
difference in the charge distribution from one point to another in a circuit.
This creates and electric field. The electric field lines are in the same
direction as the voltage drop.
Current (I) is the name for the flow of electrons. It is measured in amps (A).
When there is a difference in voltage between two points in a circuit, looselyheld electrons in the atoms of conductors will begin to move. Because they
are negative charges, they will move in the opposite direction of the electric
field lines (negative to positive). The direction arrow for current, however, is
conventionally drawn from positive to negative.
Resistance (R) is the tendency of a material to “resist” the flow of current. It
is measured in ohms (Ω). An example of a good resistor material is an
insulator like ceramic or rubber. An example of a poor resistor material is
any type of conductor like metal. There are other factors that can affect
resistance, including the number of resistors and how they are connected.
For a single resistor, the material, its geometry, and how it is constructed
determine its resistance.
In this lab, there are two different ways to connect electrical components:
series and parallel. When two components are in series, it means that they
are connected sequentially, with no opportunity for current to branch.
Figure 2. Lamps in Series
When two components are in Parallel, it means that both ends are connected
together, and an electric current must branch in order to go through each of
them. After passing through the components, the current must then
recombine.
Figure 3. Lamps in Parallel
Materials
Two batteries
Three lamps
Lamp holder
Wires
Multimeter
OBJECTIVES:
• Design and construct simple circuits;
• Draw circuit diagrams using symbols;
• Measure voltage and current using a multimeter;
• Learn the differences between series and parallel connections.
Activity 1: Make it work!
1. Explain, in your own words, how the lamp works. Use your textbook or an
online source if needed.
2. Include a sketch of the inner workings of the light lamp. Show where
electrical connections are made that will send a current through the lamp.
3. Using only one battery, one wire, and one lamp (without the lamb base),
construct a circuit arrangement that lights the lamp. Generate a sketch of
the circuit the results in a lit lamp. Don’t use circuit symbols -- your sketch
should be understood by anyone interested in lighting a lamp.
NOTE: Please disconnect the battery after each experiment. This will
make the batteries last much longer. Thank you!
Activity 2: Measuring current
The circuit you constructed in Activity 1 should resemble a closed loop. In
this activity, you will measure the current in this closed loop.
Set a multimeter to read DC Amps (indicated by a straight line over a
dashed line). When using this setting, the multimeter is called an ammeter.
Not all multimeters look exactly like the Fluke meter pictured below, but the
basic symbols and connections are still the same. The red probe should be in
the mA current terminal of the ammeter, and the black probe should be in
the common ground terminal of the multimeter.
Figure 4: Example of multimeter terminals for reading current or voltage.
The multimeter must be inserted into the circuit in such a way that the
current you are measuring must flow through the meter. The current
terminal of the ammeter should be connected to the positive terminal of the
battery (see Figure 5). The positive terminal of the battery is indicated by
the "+" sign on the battery.
* A note on wire color. In a conventional circuit, a red wire indicates that
a positive DV current or voltage is expected, and a black wire indicates a
negative DC current or voltage is expected. However, don't always trust to
wire color! An experienced electrician will always test to make sure.
Figure 5. Schematic of an ammeter between the lamp and the positive
battery terminal
5. Measure the current in the circuit with the ammeter connected between
the lamp and the positive terminal of the battery. The mA current probe
should be connected to the positive battery terminal, and the common
ground probe should be connected to the lamp. Remember that the meter
displays current in units of mA (milliamps).
6: Prediction: How do you think the current will change if another lamp is
added in series with the first? Will the current increase? Decrease? Stay the
same?
7. Add a second lamp in series with the first lamp. You are creating an
electrical path so that the path of current is from the battery, through one
lamp, then the next lamp, and then back to the opposite end of the battery.
What is the new current reading? Did the brightness of the first lamp
change?
Figure 6. Schematic of an ammeter between the first lamp and the positive
battery terminal
8. Prediction: Predict what the value will be if you measure the current
between the two lamps (see figure 7). Will the current be smaller? Greater?
The same? Zero?
Figure 7. Schematic of an ammeter between the lamp and the negative
battery terminal
9. Move the ammeter so that it is between the lamp and the negative
terminal of the battery (see Figure 6). The common ground terminal of the
ammeter should be connected to the negative terminal of the battery. What
current do you now measure? Is it what you predicted?
10. Often students think that current gets "used up" on its way through the
light bulb, but obviously this is not the case here. Take the average of the
currents that you measured. This is the series current.
Activity 3: Role reversal
11. Draw the schematic of the simple circuit used in steps 7-8 above. If
conventional current flow is from the positive terminal of the battery toward
the negative terminal, draw in an arrow which shows the direction of the
current.
12. Prediction: What do you predict will be the effect of reversing the
direction of current through the circuit?
13. Do this by disconnecting the battery wires, rotating the battery, and
then reconnecting the wires on the opposite ends of the battery from where
they were initially. Look at the meter. What are your observations? Return
the battery to its original position when finished.
14. The ammeter shows a positive value when current is flowing from the
current terminal to the common ground terminal. If current flows the other
way, it will read a negative value. From this experiment, explain how you
can use the ammeter to determine what direction current is flowing in a
circuit.
Activity 4: This has potential
In this activity, you will measure voltage differences in the circuit. Remove
the ammeter from the simple circuit and reconnect so the lamps come on.
Set a multimeter to read DC Volts (indicated by a straight line over a dashed
line). The common ground is still used, but now the volt/ohm terminal will
be used instead of the current terminal. When using this setting, the
multimeter is called a voltmeter.
A voltmeter is used differently than an ammeter. An ammeter measures the
current flowing through the circuit, so the main body of the current must
actually pass through the ammeter. The ammeter is in series with the
component you are measuring.
The voltmeter, however, is reading the voltage difference between two
points in the circuit. It will not be part of the main circuit. Instead, it will
siphon off just enough current to make its own measurement. The voltmeter
is in parallel with the component you are measuring (see Figure 8).
Figure 8. Reading the voltage across a battery
15. Using the voltmeter, measure the voltage difference between the
negative terminal of the battery and the positive terminal. Do this by placing
the common ground probe at the negative terminal of the battery, and the
volt/ohm probe at the positive terminal of the battery (see Figure 7). Record
this measurement. This is called a voltage rise. The main purpose of the
battery is to supply a source of voltage for the rest of the circuit to use.
During the next steps, you will measure the voltage drops across the two
lamps and the two wires that make up the simple circuit. This is done by
moving the probes around the simple circuit, crossing components until you
return to the starting point.
16. Move the common ground probe to the positive terminal of the battery,
and the volt/ohm wire to the lamp terminal closest to the positive terminal
of the battery. Record your observations. This is the voltage drop across the
first wire.
17. Keep going; measure the voltage drop across the wire that connects the
two lamps, the voltage drop across the second lamp, and the voltage drop
across the wire that connects the second lamp to the negative terminal of
the battery.
18. Where does most of the voltage drop occur?
19. The laws of conservation tell us that total of the voltage drops must
equal the total of the voltage rises. Sum up all of the voltage drops you just
measured. Compare this to the voltage rise due to the battery.
POSTLAB QUESTIONS
1. Explain, in your own words, the differences between the way the meters
are set up and attached in order to measure current and voltage in a
circuit.
2. Why do you think that current is the same throughout a circuit, but
voltage gets "used up?" (HINT: Think of the water analogy.)