Physics 151
Simple Circuits
Prelab Assignment
Name ____________________________________
In this week’s lab, Simple Circuits, you will perform one of the most common of electrical measurements.
You will generate what is called an i-V curve. An i-V curve is simply a graph of current vs. voltage (or
voltage vs. current) for some material or electronic device, like a resistor or diode. In lab you will generate
i-V curves for a light bulb and a resistor. If you are an electrical engineer you will undoubtedly study many
i-V curves throughout your academic and professional career.
The basic procedure to generate an i-V curve is straightforward. The device under test (DUT) is connected
to an adjustable power supply. In lab you will use an adjustable dc voltage supply, so we will use that as an
example. (You can also use adjustable current supplies.) The supply is set at some initial voltage. Then
the current through and the voltage drop across the DUT are measured simultaneously and recorded. Then
the supply voltage is increased and another simultaneous current and voltage measurement is recorded.
This procedure continues over some chosen range of supply voltages. In lab you will make measurements
over a supply range of 0 – 10 V in roughly 1 V increments. The resulting set of current and voltage
measurements is then graphed. The shape of the graph reveals much information about the electrical
characteristics of the DUT, some of which you will explore in lab.
In preparation for this measurement draw a circuit diagram for the test circuit you will be using in lab. The
test circuit forms a complete circuit, a single loop, consisting of a dc voltage supply connected to a resistor
in series with a light bulb. Consider the light bulb to be the DUT. Draw this circuit in the box below. If
you need to look at some examples for guidance you can refer to the beginning sections in Chapter 27 of
your textbook which discusses series circuits. The only symbols you are allowed to use in your diagram
are the following:
+
dc Voltage
Supply
Incandescent
Lamp
1
Wire
Resistor
If the dc supply is on, there will be current in this circuit and a potential drop across the bulb. However,
our goal is to measure the current through the bulb and measure the potential drop across the light bulb.
We can’t do that with this circuit because there is no current meter or voltage meter connected to the
circuit. Draw the test circuit a second time in the box below, but this time add a current meter and a
voltage meter to the diagram. Both meters have two probes that must be connected properly to the circuit if
the measurement is to be successful. Remember, the current meter is used to measure the rate of charge
flow through the light bulb. The voltage meter is used to measure the potential difference across the light
bulb. What each measures suggests how each should be connected to the circuit. Draw the circuit using
the following two symbols for these meters.
A
V
Current Meter
(or ammeter)
Voltage Meter
(or voltmeter)
Two examples of i-V curves, using fabricated data points, are shown on the next page. The first has voltage
on the vertical axis and current on the horizontal axis. This is how your graphs will appear in lab. The
second has the axes switched. This is also very common.
One piece of information you can extract from an i-V curve is the DUT’s resistance as a function of current
(or of voltage). The resistance of a device or material is defined as the ratio of the voltage dropped across
it, to the current through it.
V
R
i
If the potential drop is in volts (V) and the current is in amps (A), then the resistance will be in ohms (Ω).
2
Determine the resistance of Device A for currents of 0.30 A, 0.60 A and 0.85 A. As an aid, draw a
freehand smooth curve through the data points. Record your results in the table and comment on whether
the resistance of Device A increases, decreases or remains the same as the current increases.
Voltage (V)
Device A: i-V Curve
10
9
8
7
6
5
4
3
2
1
0
i (A)
V (V)
R (Ω)
0.30
0.60
0.85
As i increases R ______________
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Current (A)
Determine the resistance of Device B when it has 2 mV, 10 mV and 18 mV dropped across it. As an aid,
draw a freehand smooth curve through the data points. Record your results in the table and comment on
whether the resistance of Device B increases, decreases or remains the same as the current increases.
Device B: i-V Curve
4.0
3.5
i (nA)
Current (nA)
3.0
V (mV)
2.5
2.0
2.0
10.0
1.5
R (Ω)
18.0
1.0
As i increases R ______________
0.5
0.0
0
2
4
6
8
10
12
14
Voltage (mV)
3
16
18
20