DC Circuits
Objective: To learn how to build and run an electrical circuit. To measure current and voltage
anywhere in the circuit. To determine the relationships between current, voltage and resistance. To
learn and apply Kirchoff's Rules
Apparatus: SnapCircuits board, circuit elements, cables, DC power supply, multimeters (combination
voltmeter/ammeter/ohmmeter), light bulb (6V limit – do not exceed for a single bulb in series).
Introduction
So far our labs have focused on electric fields that have macroscopic effects – a charged rod that
attracts bits of paper or foil, a charged copper sphere, and scotch tape strips that attract or repel each
other. Now we will examine the microscopic effects of electrical fields in wires that conduct
electricity. In today's lab you will build simple circuits and learn to measure current, voltage and
resistance.
Theory
The following summarizes what you have have already learned in lecture about circuits:
Electric Potential
You should recall that the electric field E is related to the electric potential, V. If the electric potential
changes by the amount  V across a distance  s , the electric field is:
E=− V
s
Current
Current I is defined as the flow of charge Q per unit time:
I=
Q
t
Ohm's Law
For certain materials, the current, I, that flows is directly proportional to the applied potential
difference, V, where the constant of proportionality is the material's resistance, R.
This relationship is known as Ohm's law.
V = IR
Ohm's law essentially says that there's a linear relationship between the voltage applied to a material
and the amount of current that flows. Materials for which the voltage-current relationship is described
by Ohm's law are known as ohmic materials; those materials for which the voltage-current relationship
is not described by Ohm's law are known as, you guessed it, non-ohmic.
Kirchoff's Junction Rule
In general, the current entering any point in a circuit must equal the current leaving that point. This rule
is particularly useful when we apply it to a circuit junction. A junction is a location in a circuit where
three or more wires meet. This junction rule indicates that charge does not accumulate nor disappear at
a junction. The junction rule is a consequence of the law of conservation of charge.
In the junction on the right, the current i1 splits up
into i2 and i3. Because charge cannot accumulate
nor disappear at a junction, the following equation
must be true:
i1 = i2 + i3
or equivalently
i1 – i2 – i3 = 0
Kirchoff's Loop Rule
The algebraic sum of all potential differences around any closed loop in a circuit is zero. This is a
consequence of the law of conservation of energy.
Starting from the negative terminal of the battery
and proceeding clockwise, the sum of the
potential differences in a complete loop is given
by the following equation:
+Vb – iR1 – iR2 = 0
Procedure
You will build a series of circuits using SnapCircuit circuit elements and connectors, which will be
connected to a DC power supply. A DC (Direct Current) power supply supplies a constant potential
difference (voltage); a typical flashlight battery is another example of a DC power source
Circuit elements, including light bulbs and power supplies/batteries, have two terminals: plus (+) and
minus (-). Current enters through one and and leaves through the other. So a closed circuit (as in the
circuit containing two resistor and one battery, above) should be one continuous, closed loop (or series
of loops).
On your power supply, there is a green Power button. Before you push it, make sure that the “Coarse”
and “Fine” current knobs are set to the middle position, and that the “Coarse” voltage knob is set to the
minimum position. After you press the Power button, you can slowly adjust the voltage with the two
voltage knobs. Do not exceed 6V on the power supply if you are using a single light bulb in series
with it. The voltage (or current) is displayed on the red LED display on the face of the power supply,
with a lever that lets you select voltage or current. Remember, this is the voltage (or current) reading
through the power supply and not necessarily at every point in the circuit.
A voltmeter must be connected in parallel with the circuit element across which you are measuring the
voltage. In contrast, an ammeter must be connected in series with the circuit element through which
you are measuring the current. In the labs, we will be using measuring devices called multimeters,
which can serve as a voltmeter, ammeter or ohmmeter depending on the setting. Make sure that if you
are using the multimeter as a voltmeter (ammeter) that you have it connected in parallel (series) or your
results will not only be incorrect but you may damage the multimeter. Also, when you're using the
multimeter as an ohmmeter, you should not have a power supply or battery in the circuit – the only
thing between the ohmmeter's terminals should be the circuit element (resistor or bulb).
Record all drawings, observations and calculations in the hand-in sheet and submit.
Activity 1
Connect the cables of an ohmmeter (select this function on your multimeter) to the terminals of a
resistor (100 Ω or the lowest you can find) and confirm that the resistor's value is accurate.
Activity 2
Build a simple circuit where a flashlight bulb is powered by a DC power supply. Slowly raise the
voltage to between 5 and 6 Volts. Remember, do not exceed 6V on the power supply if
you are using a single light bulb in series with it. Draw the circuit in pictorial form, and
draw its representation in schematic form (using symbols for resistor, battery, etc).
Activity 3
Measure and record the voltage across the bulb using the multimeter in the voltmeter setting. Before
you make the measurement, make sure the leads are connected to the multimeter correctly. The red
lead should be placed in the V socket and the black lead should be placed in COM socket. Also make
sure that the meter is connected in parallel with the bulb. Check with your TA if you're unsure
about any of these steps. Also measure and record the voltage across the light bulb using the power
supply's voltage display. How do the two measurements compare?
Activity 4
Turn the power supply down and then off. In a moment you'll measure the current through the light
bulb using the multimeter in the ammeter setting. Make sure the leads are connected to the multimeter
correctly. The red lead should be placed into the 10A socket and the black lead should be in the COM
socket. Also remember to connect the ammeter in series with the bulb. Check with your TA if you're
unsure about any of these steps. Once you have the correct connections, turn on the power supply
and turn the voltage to the value you used in Activity 3. What is the current as measured by the
multimeter in the ammeter setting? What is the current as measured using the power supply's current
display? How do the two measurements compare?
Activity 5
Now vary the voltage across the light bulb from zero to the maximum rating of the light bulb at 11
evenly-spaced values of voltage. At each of these values, measure and record the voltage and the
corresponding current. (Measure the voltage using one multimeter in the voltage setting—make sure it
is connected in parallel with the light bulb. Measure the current using the other multimeter in the
ammeter setting—make sure it is connected in series with the light bulb.)
Activities 6-9
Repeat Activities 2-5 above, but this time for a resistor (pick a low resistance value, around 100 Ω).
Plot V vs. I for both bulb and resistor; you can use Graphical Analysis if you wish (recommended). Is
there a difference between the shapes of their graphs? Is the resistor an ohmic device? Is the bulb an
ohmic device? (Hint: What is the relationship between voltage and current for an Ohmic device?)
For the resistor, find a value for the slope of the V vs. I graph. How does this value compare to the
resistance value as labeled on the resistor?
Activity 10
Connect two resistors of known value in series and then connect them to the power supply. In a
moment, you'll turn the power supply on and set the power supply's voltage to 5V. Before doing so,
predict (calculate) the voltage across each resistor and the current through each resisitor. Then actually
measure these values. How close (give a percentage error) were your predicted values to your
measured ones? Draw the complete circuit and indicate the current through and voltage drop across
each circuit element (including the power supply). Is Kirchoff's Loop Rule obeyed? That is, do the
potential differences add to zero as you loop through the circuit?
Activity 11
Connect two resistors of known value in parallel and then connect them to the power supply. In a
moment, you'll turn the power supply on and set the power supply's voltage to 5V. Before doing so,
predict (calculate) the voltage across each resistor and the current through each resistor. Then actually
measure these values. How close (give a percentage error) were your predicted values to your
measured ones? Draw the complete circuit and indicate the current through and voltage drop across
each circuit element (including the power supply). Is Kirchoff's Junction Rule obeyed? That is, is the
current entering the junction formed by the parallel resistors the same as the sum of the currents
through each individual resistor? Is Kirchoff's Loop Rule obeyed? That is, do the potential differences
add to zero as you loop through the circuit?
Assessment and Presentation (Hand-in Sheet/Lab Notebook)
After entering your results from Activities 1-10 above in the hand-in sheet, answer the following
questions:
1. Resistance is often a function of temperature. How does the temperature of the light bulb influence
its resistance?
2. Christmas Tree light bulbs are connected in parallel, not series. Can you think of a practical reason
for this?
3. Connect a light bulb in series with a 100 Ω (or as close as possible) resistor and connect to a power
supply. Are you able to make the bulb light up? If not, explain why.