To test the ohmic and non-ohmic behavior in a resistor and a bulb
Ohm’s Law: When the temperature of a metallic conductor is kept constant, the current
through the conductor is proportional to the potential difference across it; I a V. This statement
is known as Ohm’s law. Materials obeying Ohm’s law thus have a constant resistance at constant
temperature. A graph of I (current) versus V (voltage) gives a straight line through the origin if
the material obeys Ohm’s law. Most materials obey Ohm’s law at low temperatures, but as
temperature increases deviations from this law are seen.
Experiment 1: Aim: To test a bulb for ohmic or non-ohmic behavior.
Hypothesis: It is presumed that the light bulb will obey Ohm’s law as long as the current
through it is small. As the current is increased, the temperature of the bulb increases
and so does the resistance.
Material:
- Bulb (6V, 0.05A)
- Power supply
- Voltmeter
- Ammeter
- Wires
- Clamps
Variables:
Dependant variables: current
Independent variables: voltage
Controlled variables: Bulb, ammeter, voltmeter, power supply, wires.
Method:
1. A wire was connected from power supply to the (mA) outlet of ammeter.
2. A second wire was connected from the (com) outlet of ammeter to the bulb.
3. A third wire was connected from power supply to bulb.
4. Two wires connected from bulb to voltmeter in parallel. One wire to the (v)
outlet of the voltmeter and one to the (com) outlet.
5. The power supply was then turned on and the current in the ammeter was read
at different voltage values which were set by adjusting the knob on the power
supply equipment.
6. Values were noted down right until the bulb was destroyed.
Result:
mV (±3%)
2.04
4.01
6.03
8.09
10.03
12.07
14.06
16.02
18.02
20.93
m A (±3%)
27.7
40.5
50.08
59.9
67.5
74.6
81.3
87.2
90.5
Lamp
broken
Calculation 1: R = V/I
l. R = 0.0736
2. R = 0.0990
3. R = 0.120
4. R = 0.135
5. R = 0.148
6. R = 0.161
7. R = 0.172
8. R = 0.183
9. R = 0.199
Experiment 2: Aim: To test a resistor for ohmic or non-ohmic behavior.
Hypothesis: It is presumed that the resistor will show an ohmic behavior i.e. the
voltage across the resistor is presumed to be proportional to the current through it.
Material:
- Resistor (Brown, black, black, brown, gold): lOOO fi ± 5%
- Power supply
- Voltmeter
- Ammeter
- Wires
- Clamps
Variables:
Dependent variables: current
Independent variables: voltage
Controlled variables: Resistor, ammeter, voltmeter, power supply, wires
Method:
1) A wire was connected from power supply to (mA) outlet of the ammeter.
2. A second wire was connected from (com) outlet of ammeter to the
3.
4.
5.
6.
resistor via a clamp.
A third wire was connected from power supply to the resistor with the help of
another clamp.
Two wires were connected from resistor to voltmeter in parallel. One to
(V) outlet and another to (com) outlet.
The power supply was then turned on and the current in the ammeter was
read at different voltage values which were set by adjusting the knob on
the power supply equipment.
Six different values were noted down.
Result:
mV (±3%)
2.60
4.60
6.05
m A (±3%)
2.60
4.06
6.05
8.02
10.05
12.07
8.01
10.04
12.07
Calculation 2: R = V/I
1. R = 1
2. R = 1
3. R = 1
4. R = 1.00
5. R = 1.00
6. R = 1
Discussion: From calculation 1, it is evident that the resistance increases as the
current in the circuit increases leading to increase in temperature. On the other
hand, fig 1.1 shows a non-linear relationship between voltage and current,
which is evidently a non-ohmic behavior.
As for the second experiment, calculation 2 shows that the resistance has
remained constant throughout the experiment. Similarly, fig 1.2 shows a linear
relationship between voltage and current, which is an ohmic behaviour.
Limitation: Temperature change in both bulb and the resistor could not be
measured during the experiment, as temperature plays an important role for
ohmic or non ohmic behaviors as stated above under “Ohm’s law”.
Improvement: In this experiment, the ohmic or non-ohmic behavior was tested
using only one bulb and one resistor. For a better conclusion it might be
reasonable to test ohm’s law with more than one bulb and resistor.
Conclusion: Experiment one: From the discussion and introduction, it could be
concluded that the bulb shows a non-ohmic behavior since with increasing
current, the temperature inside it increases and so does the resistance. The
hypothesis is in agreement with the result, except for the fact that in this case, the
bulb does not show any ohmic behavior initially either.
Experiment two: As mentioned earlier in the discussion, the resistor shows an
ohmic behavior since increasing current does not lead to a significant increase in
temperature in this case, and hence the resistance remains constant throughout
the experiment. The hypothesis holds true with respect to the result gained
from the experiment.