Studying Ohm’s Law

Equipment needed / Quantity
Light bulb / 1
Resistance box (variable resistor) / 1
Wire leads / 2

Background

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 as


where 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.

Ohmic material

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,


where the slope remains constant.

Non-ohmic material

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.

Experiment

Investigation the resistance of a resistor

1. Connect the Science Workshop interface to a notebook, turn on the interface, and turn on the notebook.

2. Double-click the icon "Science Workshop English" on the Windows Desktop.

3. Connect the circuit as shown in Figure 1. Turn the dial on the resistance box to set the resistance to 10 .


- Figure 1 -

4. Click to set Sample V as shown in Figure 2.


- Figure 2 -

5. 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 output.


- Figure 3 -

6. Click to set Sample I as shown in Figure 2.

7. Drag the icon Scope to OUTPUT.


- Figure 4 -

8. Set 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 second).


- Figure 5 -

9. Click the icon MON (Monitor) as shown in Figure 2. You should obtain the result as shown in Figure 5.

10. Click the icon REC (Record) as shown in Figure 2, to obtain the result.

11. In order to analyse the data, drag the icon Graph to OUTPUT.

12. As shown in Figure 6, set the y-axis of the graph to V and set the x-axis to I.


- Figure 6 -


- Figure 7 -

13. Click the Autoscale button to resize the graph.

14. Click the icon to run the statistics tool.

15. Click the icon to analyse the graph.

16. Click as follows to use the linear fit function:

->Curve Fit ->Linear Fit


- Figure 8 -

Analysis

The straight lines as shown in Figure 9 can be expressed as y = a1 + a2x. a1 and a2 are constant.

1. What is the slope of the straight line?

2. Is the slope constant along the line?

3. Re-do the experiment using a frequency of 0.1 Hz as shown in Figure 4. Does the slope still remain constant along the line?

4. What is the physical meaning of the slope?

5. Is the slope equal to the resistance (i.e. 10 ) setting on the resistance box?

Extensions

Replace the resistance box by a light bulb and repeat the experiment. Remain other inputs the same but change the following conditions: Frequency = 0.1 Hz, recording rate = 50 samp/s (sample per second). Click MON icon to test whether you get the positive result. May need to wait for about 30 second.

Describe what happens to the light bulb after you click the MON icon.

Is the slope of the graph straight? If not, describe the change of the slope.

What is the relationship between the slope of the graph and the brightness of the light bulb?

Hence, how is the resistance of the light bulb affected by the temperature?

Increase the Frequency from 0.1 Hz to 10 Hz. What is the change in the slope?