Practicals
Class XII Physics
Experiment No 1
AIM: TO FIND RESISTANCE OF A GIVEN WIRE USING METER BRIDGE
APPARATUS: A meter bridge, galvanometer, one way key, a resistance box, a battery
jockey, unknown resistance wire about 1 meter long, and connecting wires.
Theory: A meter bridge is the practical application of Wheatstone bridge arrangement as
shown in figure below. The four resistances are connected to each other as shown and if the
bridge is in balanced state, i.e., there is no deflection in the galvanometer (G),
P/Q=R/S
Circuit Diagram ( To be drawn on first blank page)
Observations:
Least Count of Meter Scale : 0.1cm
S. No R(Ω) L(cm) (100-l) cm X= R(100-l)/ l
Ω
1
2
3
Mean X
RESULT:
1. The unknown resistance of the given wire is found to be X = … … Ω
Precautions and Sources of Error
1. The circuit should be neat and tight
2. The connecting wires should be rubbed with sand paper
3. The jockey should be tapped on the wire and not slide.
4. The wire may not be of uniform cross section
5. Parallax error should be avoided while taking reading of galvanometer.
Experiment 2
AIM: To verify the laws of combination of resistances (series) using a metre bridge.
APPARATUS AND MATERIAL REQUIRED : A metre bridge, a sensitive galvanometer,
two different resistances (carbon or wire-wound resistors), a resistance box, a jockey, a
rheostat, a plug key, a cell or battery eliminator, thick connecting wires and a piece of sand
paper.
Theory: When two resistances R1 and R2 are connected in series, the resistance of the
combination RS is given by
RS = R1 + R2
Observations:
Observations:
Least Count of Meter Scale : 0.1cm
Resistamce r1
S. No R(Ω) L(cm) (100-l) cm r1= R(100-l)/ l
Ω
1
2
3
Mean r1
Resistance r2
S. No R(Ω) L(cm) (100-l) cm R2= R(100-l)/ l
Ω
1
2
3
Mean r2
Resistamce Rs
S. No R(Ω) L(cm) (100-l) cm Rs= R(100-l)/ l
Ω
1
2
3
Mean Rs
CALCULATIONS 1. The theoretically expected value of the series combination of resistances is RS =
R1 + R2 = ___________Ω
Result: The theoretical and experimental values of series combination of resistances are
_________ Ω and ________________ Ω
PRECAUTIONS
1. All the connections and plugs should be tight.
2. Jockey should be moved gently over the metre bridge wire.
3. Plug keys of the resistance box should be made tight by rotating it in clockwise direction.
4. Null points should be in the central region of the wire (30 cm to 70 cm).
SOURCES OF ERROR
1. The jockey should not be pressed too hard on the metre bridge wire. Otherwise, the wire
may become non-uniform during the course of time
2. The length measurement l and l′ may have error if the metre bridge wire is not taut and
along the scale in the metre bridge.
3. If large current is passed for a sufficiently long time, the wire AC may get heated and its
resistance may change considerably during the time of experiment.
4. Galvanometer pointer is expected to be at zero when no current flows through it. However,
many a time it is observed that it is not so. In such cases, pointer has to be adjusted to zero by
gently moving the screw below the scale with the help of a screw driver. Otherwise, null
point must be obtained by sliding the jockey on wire AC and observing the point, where
tapping the galvanometer does not produce any deflection in it.
5. Many a time, it is found that the resistance offered by resistance box is not the same as is
indicated on it. Therefore, the error in R will cause an additional error in the result.
Experiment 3
Aim: To Determine Resistance of a Galvanometer By Half-deflection Method And to Find
its Figure of Merit
Apparatus: A weston type galvanometer, a voltmeter, a battery or battery eliminator, two
(10,000 Ω and 200 Ω) resistance boxes, two one-way keys, a rheostat, a screw gauge, a metre
scale, an ammeter of given range, connecting wires and a piece of sand paper
Theory:
Circuit Diagram
Observations:
Resistance
S.No R(Ω) Θ θ/2 S(Ω) G= RS/ R-S
(Ω)
1. 30
2. 28
3. 26
4. 24
5. 22
Mean G:
Figure of Merit
EMF of cell :
S.No R(Ω) Θ K= E/(R+G)θ
(A/div)
1 30
2 28
3. 26
4. 24
5. 22
Result : The galvanometer resistance is -------------- Ω and figure of merit is _______
A/div
Precautions and Sources of Error
1. ll the connections should be neat, clean and tight.
2. All the plugs in resistance boxes should be tight.
3. The e.m.f. of cell or battery should be constant.
4. Initially a high resistance from the resistance box (R) should be introduced in
the circuit (otherwise for small resistance an excessive current will flow
through the galvanometer or ammeter can be damaged).
Sources of error
1. The screws of the instruments may be loose.
2. The plugs of resistance boxes may not be clean.
3. The e.m.f. of battery may not be constant.
4. The galvanometer divisions may not be of equal size.
Experiment 4
Aim: To Convert The Given Galvanometer (of Known Resistance And Figure of Merit) into
a Voltmeter of Desired Range and To Verify the Same
Apparatus: A weston type galvanometer, a voltmeter of 0-3 volts range, range a battery of
two cells or battery eliminator, two (10,000 Ω and 200 Ω) resistance boxes, two one way
keys, a rheostat, connecting wires and a piece of sand paper.
Theory:
Calculations
Circuit Diagram
Observation Table
Result: The converted galvanometer has been verified as the results are within acceptable
range.
Precautions
1. All the connections should be neat, clean and tight.
2. The e.m.f. of the cell or battery should be constant.
3. The ammeter used for verification should preferably be of the same range, as
the range of conversion.
4. The diameter of the wire to be used for shunt resistance, should be measured
accurately.
5. Length of shunt wire used should be neither too small nor too large.
6. The resistance box should be a high resistance one.
7. The voltmeter used for verification should preferably be of the same range, as
the range of conversion.
8. Value of required series resistance should be calculated accurately.
Experiment No 5
Aim: To find, the focal length of a convex lens by plotting graphs between u and v.
Apparatus: An optical bench with three uprights , a convex lens with lens holder, two
optical needles.
Theory: The relation between u, v and f for a convex lens is
where,
f = focal length of convex lens
u = distance of object needle from optical centre of the lens
v = distance of image needle from optical centre of the lens.
According to sign-convention, u has negative value and v has positive value. Hence, f comes
positive.
When a graph is plotted between u and v, taking same scale on both axes, a rectangular
hyperbola is obtained. This graph when bisected at 45˚ gives the value of 2f on both x and y
axes.
Ray Diagram
Observations:
Rough focal length of convex lens: cm
Least count of optical bench: 0.1cm
Position of convex lens: 50cm
S. No Position of Position of Object Image
Object Image Distance(cm) Distance(cm)
Needle(cm) Needle(cm)
u v
Calculation:
Calculations of focal length by graphical methods:
(i) u-v Graph. Select a suitable but the same scale to represent u along X’-axis and v along Y-
axis. According to sign conventions, in this case, u is negative and v is positive. Plot the
various points for different sets of values of u and v from observation table second quadrant.
Draw a line OA making an angle of 45° with either axis (i.e., bisecting ∠YOX’) and meeting
The graph comes out to be a rectangular hyperbola as shown in graph between u and v.
the curve at point A. Draw AB and AC perpendicular on X’- and Y-axes, respectively.
The values of u and v will be same for point A. So the coordinates of point A must
Result: The focal length of the convex lens is ---------------cm.
Precautions
1. Tips of the object and image needles should lie at the same height as the centre
of the lens.
2. Parallax should be removed from tip to tip by keeping eye at a distance at least
30 cm away from the needle.
3. The object needle should be placed at such a distance that only real, inverted
image of it is formed.
4. Index correction for u and v should be applied.
Sources of error
1. The uprights may not be the vertical.
2. Parallax removal may not be perfect.
Experiment No 6
Aim: To find the value of v for different values of u in case of a concave mirror and to find
the focal length by using the mirror formula..
Apparatus: An optical bench with three uprights , concave mirror, a mirror holder, two
optical needles .
Theory:
Ray Diagram:
Observations
Rough focal length of concave mirror: cm
Least count of optical bench: 0.1cm
Position of concave mirror: 50cm
S. No Position of Position of Object Image
Object Image Distance(cm) Distance(cm
Needle(cm) Needle(cm) )
u
v
Calculations
Calculate the focal length from the formula and find mean f.
Result : The focal length of the concave mirror is - ----------------cm
Precautions
1. Principal axis of the mirror should be horizontal and parallel to the central line
of the optical bench.
2. The uprights should be vertical.
3. Tip to tip parallax should be removed between the needle I and image of the
needle O.
4. To locate the position of the image the eye should be at least 30 cm away from
the needle.
5. Tips of the object and image needles should lie at the same height as that of
pole of the concave mirror.
6. Index correction for u and v should be applied.
Sources of error
1. The uprights may not be the vertical.
2. Parallax removal may not be perfect.
Experiment No 7
Aim: To draw the I-V characteristic curve of a p-n junction in forward bias and reverse
bias.
Apparatus: A p-n junction (semi-conductor) diode, a 3 volt battery, a 50 volt battery, a high
resistance rheostat, one 0-3 volt voltmeter, one 0-50 volt voltmeter, one 0-100 mA ammeter,
one 0-100 μA ammeter, one way key, connecting wires and pieces of sand paper.
Theory:
Forward bias characteristics. When the p -section of the diode is connected to positive
terminal of a battery and n-section is connected to negative terminal of the battery then
junction is said to be forward biased. With increase in bias voltage, the forward current
increases slowly in the beginning and then rapidly. At about 0.7 V for Si diode (0.2 V for
Ge), the current increases suddenly. The value of forward bias voltage, at which the forward
current increases rapidly, is called cut in voltage or threshold voltage.
Reverse bias characteristics. When the p -section of the diode is connected to negative
terminal of high voltage battery and n-section of the diode is connected to positive terminal
of the same battery, then junction is said to be reverse biased.
When reverse bias voltage increases, initially there is a very small reverse current flow,
which remains almost constant with bias. But when reverse bias voltage increases to
sufficiently high value, the reverse current suddenly increases to a large value. This voltage at
which breakdown of junction diode occurs (suddenly large current flow) is called zener
breakdown voltage or inverse voltage. The breakdown voltage may^tarts from one volt to
several hundred volts, depending upon dopant density and the depletion layer.
Diagram
For forward-bias
Range of voltmeter = …..V
Least count of voltmeter = …..V
Zero error of voltmeter = …..V
Range of milli-ammeter = …..mA
Least count of milli-ammeter = …..mA
Zero error of milli-ammeter = …..mA
Observation Tablre
S. No V F(V) IF(mA)
10
For reverse-bias
Range of voltmeter = …..V
Least count of voltmeter = …..V
Zero error of voltmeter = …..V
Range of micro-ammeter = …..μA
Least count of micro-ammeter = …..μA
Zero error of micro-ammeter = …..
Observation Table
S. No V r(V) Ir(μA)
10
Calculations
For forward-bias
Plot a graph between forward-bias voltage VF and forward current IF taking VF along X-axis
and IF along Y-axis.
This graph is called forward-bias characteristic curve a junction diode.
Rd
Rs
For reverse-bias
Plot a graph between reverse-bias voltage VR and reverse current IR taking VR along X-axis
and IR along Y-axis.
This graph is called reverse-bias characteristic curve of a junction diode.
Result: Junction resistance for forward-bias -------------Ω
Junction resistance for reverse-bias ____________Ω
Precautions
1. All connections should be neat, clean and tight.
2. Key should be used in circuit and opened when the circuit is not being used.
3. Forward-bias voltage beyond breakdown should not be applied.
4. Reverse-bias voltage beyond breakdown should not be applied.
Sources of error
The junction diode supplied may be faulty.
