Ohm's Law Experiment
Ohm's Law
Experiment for Class 12 with Readings
Aim
To
establish a relationship between voltage and current (Ohm's law) for a given
resistance using an ammeter and a voltmeter and hence to find
(i) the
resistance of a wire
(ii)
resistivity of the material of the wire.
Apparatus
Battery
(E), Ammeter (A), Voltmeter (V), A resistance wire (R), Rheostat (Rh). Key (K),
Screw gauge, Connecting wires etc.
Principle
Ohm's law
states that at constant temperature the current [I] flowing through a conductor
is directly proportional to the potential difference [V] between its ends.
ie, I ∝ V or V/I = R, the resistance of the conductor.
The
resistivity (ρ) of the material of the resistance
wire is calculated using the equation
ρ = Rπr^{2}/L
where, r - Radius
of the wire
L - Length
of the wire
Procedure
The battery
(E), resistance wire (R), rheostat (Rh), ammeter (A) and key (K) are connected
as shown in the figure. The voltmeter (V) is connected in parallel to the
resistance wire (R). The key (K) is closed and the rheostat is adjusted to read
a current I (say 0.2 A). The corresponding voltmeter reading (V) is noted. The
ratio V/I = R is calculated. The experiment is repeated for different values of
currents (0.4 A, 0.6 A, 1 A etc) by adjusting the rheostat. In each case the
ratio V/I is calculated and the mean value of R is found.
A Graph is
plotted taking current (I) along the X - axis and voltage (V) along the Y -
axis. It is straight line. The diameter of the wire and hence radius of the
wire (r) is measured using a screw gauge. Its length (L) is also measured using
a meter scale. The resistivity (specific resistance) of the material of the
wire is calculated using the formula, ρ = Rπr^{2}/L
Observations
and Readings
(i) To
verify the Ohm’s law
Trial |
Ammeter reading I(A) |
Voltmeter reading V(Volt) |
V/R = R(Ω) |
1 2 3 4 5 6 |
.2 .24 .28 .32 .36 .4 |
.2 .24 .28 .32 .36 .4 |
1 1 1 1 1 1 |
Mean
Resistance, R = 1Ω
Volt –
Current graph
OB = 0.16
A
AB = 0.16
V
R = AB/OB =
1Ω
Mean value
of R = 1Ω
2. To find the
resistivity of the resistor
Radius of the
conductor using screw gauge
Pitch = 1
mm
LC = .01
mm
Zero
correction = +10 div
Trial |
PSR |
HSR |
Corrected HSR |
Fraction HSR x LC (mm) |
Correct diameter |
1 2 3 4 |
0 0 0 0 |
42 43 44 41 |
52 53 54 51 |
.52 .53 .54 .51 |
0.52 0.53 0.54 0.51 |
Mean
diameter = 0.53 mm
Radius of
the resistor = r = 0.265 mm = 0.265 x 10^{-3} m
Length of
the resistor = l = 32.5 cm = 32.5 x 10^{-2} m
R = 1Ω
Resistivity
of the resistor = ρ = Rπr^{2}/L = 6.78 x 10^{-7}
Ωm
Results
(i) V/I is
found to be a constant. Hence Ohm's law is verified.
(ii). The
resistivity of the material of the given wire = 6.78 x 10^{-7} ohm.m
MODEL VIVA
VOCE QUESTIONS AND ANSWERS
1. State
Ohm's Law.
Ohm's law
states that current flowing through a conductor is directly proportional to the
pd across it at constant temperature.
2. What is
an ohmic conductor?
A conductor
that obeys the rule of Ohm's law is known as an ohmic conductor.
3. Define
ampere, volt and ohm.
ampere:
ampere is the unit of current. The current is said to be one ampere if one
coulomb of charge flows per second across any cross section of the conductor.
volt : volt
is the unit of potential. It is the potential difference across a conductor of
resistance one ohm, when a current of one ampere flows through it.
ohm : ohm
is the unit of resistance. The resistance of a conductor is said to be one ohm
if a potential difference of one volt is established across the conductor when
unit current flows through it.
4. We have
two wires of copper, one thicker than the other.
a) Which of
these will have more resistance?
b) Which of
these will have more resistivity?
a) The
thinner will have more resistance.
b) Both the
wires will have same resistivity.
5. Why is
an ammeter always connected in series in a circuit?
An ammeter
has a very low resistance. It will not alter the current in the circuit only if
it is connected in series.
6. Why is a
voltmeter always connected in parallel in a circuit?
A voltmeter
is a high resistance instrument. When it is connected in parallel it draws
negligible current from the main circuit and hence the current in the main
circuit practically remains unaltered.
7. Why do
we use thick connecting wires?
Thick
connecting wires offer negligible resistance.
8. What is
the effect of temperature on the resistance of a conductor?
The
resistance of the conductor increases with increase in temperature.
9. Define
resistance.
The
resistance of a conductor is the obstruction shown by the conductor to the flow
of electric current through it.
10. Name
some substances whose resistance decreases with increase in temperature.
Si, Ge, C
11. On what
all factors resistance of a wire depends?
(i)
Resistance of the conductor is directly proportional to the length.
(ii) The
resistance of the conductor is inversely proportional to the area of cross
section or the conductor.
(iii) It
depends on the nature of material and the temperature across the conductor.
12. Define
resistivity or specific resistance of a conductor.
The
resistance of a conductor of unit length and unit area of cross section is
known as resistivity.
13. Define
electrical conductivity.
The reciprocal of resistivity is called electrical conductivity.
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