Ohm discovered that the voltage (V) across a resistor
changes as the current (I) passes through the resistor.
The relationship between V and I can be expressed
R is the resistance of the resistor. In other words,
as the voltage increases, so does the current. The
proportionality constant is the value of the resistance.
A resistor is 'ohmic' if the voltage across it varies
with the current as a linear relationship. The slope
of a graph of V against I is the value of the resistance,
the slope remains constant.
For a light bulb, the resistance of the filament
will change as it heats up and cools down. At high
a.c. frequencies, the filament doesn't have time
to cool down, so it remains at a nearly constant
temperature and the resistance stays relatively
constant. At low frequencies, the filament has time
to change temperature. As a result, the resistance
of the filament changes dramatically.
Connect the Science Workshop interface to a notebook,
turn on the interface, and turn on the notebook
Double-click the icon "Science Workshop English Normal"
on the Windows Desktop.
Connect the circuit as shown in Figure 1. Turn the dial on
the resistance box to set the resistance to 10 .
Click to set Sample V as shown in Figure 2.
Click to set sine wave in the AC Waveform panel.
Input the Amplitude as 3V and the Frequency
as 10 Hz. Click the icon auto to set to automatic power
Click to set Sample I as shown in Figure 2.
Drag the icon Scope to OUTPUT.
the y-input of the scope to V and 1.000 V/div.
Set the x-input of the Scope to I and 2.000 V/div.
Set the recording rate as 200 samp/s (200 samples per
Click the icon MON (Monitor) as shown in Figure
2. You should obtain the result as shown in Figure 6.
Click the icon REC (Record) as shown in Figure
2, to obtain the result.
In order to analyse the data, drag the icon Graph
As shown in Figure 7, set the y-axis of the graph to V
and set the x-axis to I.
Click the Autoscale button to resize the graph.
Click the icon
to run the statistics tool.
Click the icon
to analyse the graph.
as follows to use the linear fit function:
->Curve Fit ->Linear Fit
lines as shown in Figure 9 can be expressed as y = a1
+ a2x. a1 and a2 are constant.
is the slope of the straight line?
the slope constant along the line?
the experiment using a frequency of 0.1 Hz as shown in Figure
4. Does the slope still remain constant along the line?
is the physical meaning of the slope?
the slope equal to the resistance (i.e. 10 )
setting on the resistance box?
the resistance box with a light bulb and repeat the experiment.
Keep other inputs the same but change the following conditions:
frequency = 0.1 Hz, recording rate = 50 samp/s (sample per second).
Click the MON icon to test whether you get a positive result.
You may need to wait for about 30 seconds.
what happens to the light bulb after you click the MON icon
the slope of the graph straight? If not, describe the change
of the slope.
is the relationship between the slope of the graph and the brightness
of the light bulb?
how is the resistance of the light bulb affected by the temperature?
the Frequency from 0.1 Hz to 10 Hz. What is the change
in the slope?