Experiment to Verify Ohm's Law


Ohm's Law is a mathematical formula that expresses the relationship
between the electromotive force, electric current, and resistance in a
circuit. The German Physicist, George Ohm discovered this relationship
in 1826. When applied to a direct-current circuit, Ohm's Law states,
that the electromotive force (E) measured in volts =the current (I) in
amperes multiplied by the resistance (R) in ohms; therefore E = IR.
When the law is used for an alternating-current circuit, resistance is
replaced by impedance (Z), also in ohms. The flow of alternating
current produces a counterelectromotive force, which resists the
current. The strength of such resistance depends on how rapidly the
current alternates. Impedance consists of resistance, called reactants
combined with the circuits regular resistance to direct current.

Electricians use Ohm's Law to determine the efficiency of circuits.
For example, they can calculate how the flow of current will be
affected by various arrangements of such circuit components as
connecting wires, capacitors and resistors.

Electric Current

Electric Current is the movement or flow of electric charges. A charge
can be either positive or negative. The protons that make up part of
the nucleus of every atom have a positive electric charge. The
electrons that surround the nucleus of every atom are negatively
charged. An electric current can consist of positive, negative or both
types of charge.

Direct and alternating current

An electric current is either direct or alternating, depending on its
source. Direct current (DC) flows in the same direction. Alternating
current (AC) regularly reverses its direction of flow. It is produced
by AC generators and is used in most homes. Each time AC completes two
changes of direction, it goes through one cycle. The number of cycles
per second is called the frequency of the AC. This alternating current
is talked about in the paragraphs above.

Apparatus

100cm of constantan wire

Ammeter

Voltmeter

Battery

Crocodile clips

Diagram

[IMAGE]




Method

1. The apparatus was set up as in the diagram above and the voltage
across the wire was set to be 3Volts.

2. The initial length of the Constantan wire was set to be 1metre. The
current was measured. The distance between the crocodile clips was
adjusted until it was 90 cm as measured with the metre rule. The
voltage adjusted and the current recorded.

The wire was laid out in a straight line on the desk so that any heat
generated would be lost to the surroundings.

[IMAGE]3. The value of the current through the wire was measured using
the ammeter and recorded.

This is repeated with 10cm intervals (each length 3 times.)

[IMAGE]

4. The length of wire between the crocodile clips was finally reduced
to 30 cm, the voltage adjusted and the current recorded.

Safety
I will make my experiment safe by maintaining minimal direct contact
with the wire, as it will heat up due to the current passing through
it.

I shall make sure all surfaces are free from water. This is because
water is a conductor of electricity and contact with electricity may
cause electric shack to present parties.


I shall wear goggles and conduct the experiment standing up, making
sure my chair is under the table out of the way, I am wearing no loose
clothing and my hair is tied back.


I shall make sure all equipment is placed so that it is near no edges
of the table.


Fair Test

I shall make it a fair test by conducting the experiment on the same
day, in the same room, with the same wire material (constantan), and
by repeating each measurement three times.


It is hard to maintain temperature that is why constantan is used, it
is not greatly affected by temperature and therefore fluctuations in
temperature will not affect the results of my experiment.


I will repeat the experiment three times in order to make sure my
results are accurate. I shall also make it a fair test by measuring
the distance with the same ruler each time and placing the crocodile
clips in place as accurately as possible.



Prediction
==========

I believe that as the length of wire increases the resistance will
increase and that that doubling the length of wire will equal to
double the resistance. Doubling the length of a prism such as a wire
doubles its volume, therefore doubling the number of atoms it
contains, doubling its resistance. An increase in resistance should
therefore be due to an increase in the length of wire. The longer wire
has a greater number of free electrons that will collide with other
free electrons, metal atoms and any metal impurities. Each electron
passing through the wire must lose a certain amount of energy every
time an atom obstructs it. Therefore if it hits twice as many atoms it
looses twice as much energy.

Results

Length Of Wire

Voltage (V)

Current Flowing (A)

R=V
I (W)
(3s.f)

1

2

3

Average

1

2

3

Average

30

2.18

2.18

2.19

2.18

1.64

1.64

1.66

1.65

1.32

40

2.20

2.33

2.36

2.30

1.25

1.33

1.34

1.31

1.76

50

2.42

2.63

2.63

2.56

1.13

1.20

1.20

1.18

2.17

60

2.65

2.78

2.78

2.74

1

1.07

1.07

1.05

2.61

70

2.58

2.90

2.84

2.77

0.83

0.95

0.93

0.90

3.08

80

2.78

3.01

2.97

2.92

0.79

0.86

0.84

0.83

3.52

90

2.87

3.10

3.03

3

0.73

0.79

0.77

0.76

3.95

100

2.93

3.19

3.08

3.07

0.67

0.73

0.71

0.70

4.39

Graph

Analysis

The results show that the longer the wire, the more the resistance.
The graph shows that as the length of the wire is increased, the
resistance increases steadily. The graph has a positive gradient,
which indicates that the length of the wire is increasing in direct
proportion to the resistance. I believe this occurred because, the
longer the wire, the greater the resistance. This is because a long
wire has a greater number of free electrons that will collide with
other free electrons, metal atoms and any metal impurities.

The results support my prediction, because I stated that the longer
the wire, the greater the resistance. As shown in the results, it is
possible to see that as the length of the wire increased from 30cm to
40cm, the resistance also increased from 1.32W to 1.76W. I also
discovered in my research that this increase (according to Ohm's Law)
would be proportional. The resistance depends on the number of
collisions there are between the electrons and the electrons, atoms
and impurities in the metal wire per second. The number of collisions
per second doubles, when the length of the wire does, then there is
heat energy lost from the collisions, which means double the
resistance. The results show this to be true.

I predicted that if the length of wire doubled, the resistance would
also double. This occurred as shown in my results. As I doubled the
length of the wire from 40cm to 80cm, the resistance also
approximately doubled, from 1.76W to 3.52 W. Therefore, my results
match my prediction precisely. This was because as the length of wire
was doubled, the number of metal atoms, which could collide, was
doubled. Also the distance that the free electrons and metal atoms
could collide through was increased. Therefore, there would be more
collisions between the metal atoms and the free electrons, and
therefore the resistance also doubled.

Conclusion

My investigation was a fair test. I drew a line of best fit using the
7 different lengths of wire resistance results, which helped me to
prove my theory. The temperature was kept constant by performing the
experiments in the same room on the same day.

The length of the wire was measured using a metre rule. The smallest
scale division on the rule was 1mm. This gives an uncertainty of + or
- 1mm. There is an uncertainty of + or - 0.5mm at each end of the
length of wire. It was difficult to ensure that no kinks occurred in
the wire. Kinks in the wire would have meant that the wire was
actually longer than the measured value. Faulty connection may have
lead to inconclusive data.

The contacts between the crocodile clips and the wire may have
introduced extra resistance into the circuit. The amount of extra
resistance cannot be estimated and will have changed during the course
of the investigation..

Constantan wire was selected for the investigation because its
resistance does not change very much as the temperature changes. The
wire was laid out on the desk so that any heating effect would be
minimised. The heat generated would have been lost to the
surroundings.

The resistance of a metal wire increases as the temperature goes up.
It should be possible to develop a technique for connecting the wires
into the circuit, which would eliminate any uncertainty due to the
contacts. This might involve soldering connections to the wire under
test. This would involve quite a lot of extra work, which would not be
justified by the increase in accuracy obtained.

My prediction was proven almost accurate, the length of a wire and
resistance are directly proportional. Thus Ohm's law has been proven
right.

To extend my investigation I could explore a different variable like
temperature or material. Different metals have different values of
resistance. I could experiment to test the resistance of different
metals. A series of experiments could be carried out to measure the
change in resistance of a fixed length of Constantan wire as the
temperature of the wire is changed. Placing the wire under test in a
water bath and changing the temperature of the water bath by heating
it with a Bunsen burner could do this.



Bibliography
============

World Book Encyclopaedia

Letts GCSE Physics


CGP Physics

WWW Search engines

Encyclopaedia Britannica

GCSE Bitesize Revision