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Surface Tension Experiment (Capillary Rise Method)

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Surface Tension Experiment (Class 11)

 

Aim:

 

To find the surface tension of water by capillary rise.

 

Apparatus:

 

A narrow capillary tube of uniform bore of known radius, beaker containing water, travelling microscope, etc.

 

Principle:

 

Surface Tension, T = (1/2) x rhρg

where, h - capillary rise

r – radius of the bore

ρ – density of water

g – acceleration due to gravity

 

Procedure (Method 1):

A capillary tube is cleaned first with an acid, then with an alkali and finally with water. It is then passed through a hole in a cork and is arranged vertically with its lower end dipping in water contained in a beaker. A long pointer is passed through the same cork and fixed vertically so that its tip just touches the surface of water in the beaker. The water rises in the capillary tube. Least count of the vernier of the travelling microscope is noted. The microscope is kept horizontally and focussed on water level in the tube. Adjust the microscope till the horizontal crosswire just touches the lower meniscus. The total reading (R1) of the vernier is found out. The beaker containing water is carefully removed and the microscope alone is lowered till the horizontal cross-wire just touches the image of the tip of the pointer. The reading (R2) of the vernier is taken. The capillary rise, h = R1 — R2, of water in the tube is calculated, Surface tension (T) of water is calculated by the equation,

T = (1/2) x rhρg;

where r is the radius of the capillary tube and ρ is the density of water.

 

Procedure (Method 2):


A clean capillary tube is passed through a cork and is arranged vertically on a stand. Its lower end is arranged to be dipped in water taken in a beaker. A pointer is also passed through the same cork and its tip is made to touch the liquid surface. Due to capillarity water rises in the tube. The least count of the given travelling microscope is noted. The microscope is then focussed at the meniscus of the liquid in the capillary tube and the horizontal cross wire is made tangential to the meniscus. The microscope reading (MSR and VSR) is taken on the vertical scale. Total reading = M.S.R. + (V.S.R. x L.C.) is calculated. The beaker containing the liquid is carefully removed. The microscope is then focussed at the tip of the pointer and the microscope reading on the main scale and vernier scale are also noted. The difference between the two readings gives the capillary rise (h). The experiment is repeated by lowering the capillary tube to different depths in the beaker and the mean rise ‘h’ is determined. Knowing the values of r, ρ and g; surface tension (T) can be calculated.

 

Observations and Readings

 

To find capillary rise h


Least count of the vernier = ____ cm


Trial

Reading of meniscus

Reading of tip of pointer

Height, h=R1–R2

MSR

VSR

Total R1

MSR

VSR

Total R2

1

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

Mean height, h = _____m

Radius of the capillary tube = r = _____m

Density of water = ρ = 1000 kgm-3

Surface Tension of water, T = (1/2)rhρg = _____Nm-1


Result:


Surface tension of water =  ____

Potentiometer Experiment

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Potentiometer Experiment with Readings (Class 12)


POTENTIOMETER EXPERIMENT - 1


Aim:


To compare the e.m.fs of two primary cells (Daniel and Leclanche cells).


Apparatus:


Potentiometer, Accumulator (E), The given primary cells (E1 and E2), Rheostat (Rh), Key (K), Two way key (K1), Connecting wires etc.


Principle:


When a steady current flows through the potentiometer wire, the e m.f of the cell in the secondary circuit is proportional to its balancing length.


Let E1 and E2 be the e.m.fs of a Daniel cell and a Leclanche cell and l1 and l2 be their balancing lengths respectively, then


E1 l1 and

E2 l2

Therefore, E1/E2 = l1/l2


Procedure


The terminals A and B of the potentiometer are connected to an accumulator (E) through a rheostat (Rh) and a key (K). This forms the primary circuit. The positive terminal of the accumulator is connected to A. The Leclanche cell (E1) and the Daniel cell (E2) are included in the circuit using a two way key (K1) along with a galvanometer (G) and a high resistance (HR) through a jockey (J). This forms the secondary circuit.


The primary circuit is closed and the rheostat is adjusted for a suitable current. The Leclanche cell (E1) is included in the circuit. The jockey is pressed at the ends of the potentiometer wire (A and B). If the deflections obtained at the two ends are in opposite directions connections are correct. The jockey (J) is moved along the wire from A to B till the galvanometer shows zero deflection. The high resistance (HR) is then cut off and the exact balancing point is determined. The balancing length (l1) of the wire (distance from A to the jockey) is measured. The Daniel cell (E2) is then included in the circuit and the balancing length l2 is measured. The ratio E1/E2 = l1/l2 is calculated. The experiment is repeated by adjusting the rheostat (Rh) for different currents and the mean value of E1/E2 is determined.


Circuit Diagram

Observations and Readings


Trial

Balance length

E1/E2

Cell of emf E1, l1 cm

Cell of emf E2, l2 cm

1

150

374

0.40

2

141

364

0.38

3

139

365

0.38

4

190

392

0.48

5

177

399

0.44

 

Mean, E1/E2 = 0.416


Result:


The ratio of e.m.f’s = 0.416

 

POTENTIOMETER EXPERIMENT - 2


Aim:


To determine the resistance of the given cell using a potentiometer


Apparatus:


A potentiometer, accumulator (storage cell), the given cell (ex: Daniel or Leclanche cell), resistance box, keys, galvanometer, high resistance etc.


Theory:


When the key K is open, E l  ------------- (i)

When the key K is closed, ER/(R+r) 1’ ------------(ii)

Solving equation (i) and (ii), we get the internal resistance, r = R(l — l’)/l’

 

Procedure:


Connections are done as shown in the figure. The storage cell, the rheostat and a key are connected in series between the terminals A and B of the potentiometer wire. The positive terminal of the cell should be connected to A. The given cell, the galvanometer and a high resistance (HR) are connected in series between the terminal A and the jockey J of the potentiometer. See that the positive terminal of the battery is connected to the terminal A. A resistance box R is connected in parallel to the cell through a key K.


The primary circuit is closed. The key K is opened and the balancing length l is determined. (The rheostat is adjusted to get l as large as possible. Thereafter the rheostat should be adjusted).


The key K is closed. A suitable resistance R, say 5Ω, is taken from R. The balancing length I’ is determined. The internal resistance r of the cell is calculated using the relation,


r = R(l – l’)/l’ 


The experiment is repeated for different values of R in closed circuit. In each case, the internal resistance r of the cell is calculated. [The balancing  length l with R in open circuit need to be determined only at the beginning and at the end of the experiment. The average value l is taken].


Circuit Diagram


Observations and Readings


Balancing length with R in open circuit (l)

(1) 373 cm

(2) 369 cm

Mean l = 371 cm


Balancing length with R in closed circuit (l’)


Trial

R(Ω)

Balancing length, l’ cm

r = R(l – l’)/l’

1

1

81

3.58

2

2

134

3.53

3

3

152

4.32

4

4

163

5.10

5

5

207

3.96

 

Mean r = 4.098

 

Result:


Internal resistance of the cell is determined for different values of the external R.

 

Viva Questions and Answers


1. What is a cell?


A cell is a device by which electric energy is generated due to chemical action taking place inside it.


2. Define primary cell.


Primary cell is a cell in which gives electrical energy from chemical energy. This cell cannot be recharged.

Eg: Dry cell, Daniel cell, Leclanche cell


3. Define secondary cell.


Cell which stores electrical energy as chemical energy and returns it back as electrical energy, is called a secondary cell. This cell can be recharged.

Eg: Lead (acid) accumulator


4. Why is potentiometer called so?


Potentiometer works on the principle that for a constant current, fall of potential along a uniform wire is directly proportional to its length.


5. Define e.m.f. of a cell.


e.m.f. of a cell is defined as the p.d between the terminals of the cell when the cell is in an open circuit.


6. Why we prefer potentiometer rather than a voltmeter to measure e.m.f. of a cell?


Potentiometer method is a null deflection method. It does not draw any current from the source whose e.m.f. is to be measured. A voltmeter always draws some current.


7. The e.m.f. of the cell used in the primary circuit of the potentiometer should be more than the potential difference to be measured. Why?


If it is not so, the balance point will not attain on the potentiometer wire.


8 What is the principle of a potentiometer?


For a constant current, the potential drop across a wire is directly proportional to its length.


9. Which materials are suitable for potentiometer wire?


Manganin, constantan


10. What do you mean by sensitivity of a potentiometer?


Sensitivity of a potentiometer is the smallest potential difference that it can measure.

 

AC Sonometer Experiment with Readings (Class 12)

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FREQUENCY OF AC MAINS EXPERIMENT WITH READINGS (CLASS 12)

Aim:

 

To determine the frequency of ac main using a sonometer.

 

Apparatus:

 

Sonometer, a step down transformer (230 V – 6 V or 230 V – 9 V), horse-shoe magnet (or two powerful bar magnets), weight hanger, slotted weights, screw gauge, rheostat etc.

 

Principle:

 

If l is the length of the sonometer wire vibrating in resonance with the frequency n of the ac main, then,

Where M is the mass suspended at the end of the sonometer wire and mis the linear density of the sonometer wire.

 

Procedure:

 

The low voltage side (secondary) of the transformer is connected, through a rheostat R, across the sonometer wire. The bridge C is now placed closer to the bridge B. A horse-shoe magnet is mounted mid-way between B and C with the poles of the magnet on either side of the sonometer wire so as to produce a magnetic field perpendicular to the wire. (Instead of a horse shoe magnet, two powerful bar magnets can be clamped on either side of the sonometer wire with unlike poles facing each other). The wire can vibrate freely without touching the poles. The rheostat may be adjusted to reduce the current through the wire in order to prevent overheating.

 

A suitable mass M (say 20 g) is suspended from the free end of the sonometer wire. A light paper rider is placed on the wire between the bridges C and B. The primary of the transformer is connected to the ac main and the main supply is switched on. Since the current flows through the wire perpendicular to the magnetic field, the wire experiences a force. Since the current is alternating, the wire vibrates. The movable bridge C is adjusted until the vibrations of the wire BC show a maximum amplitude, indicating that resonance has been obtained. Now the paper rider will be thrown off. (The magnet is moved as the bridge is moved so that it is at the mid-way between the bridges B and C). The resonating length l of the wire between the bridges B and C is measured. Then (M/l2) is calculated. The experiment is repeated for different values of the load M and the average value of (M/ l2) is determined.

 

The radius r of the wire is measured by a screw gauge. The linear density in of the wire is calculated by the equation, m = πr2d; where d is the density of the material of the wire. The frequency n of the ac main is calculated using the equation,

Observations and Readings


Trial

Tension

Resonating length

Frequency (n)

1 (cm)

2 (cm)

Mean (l)

1

14.7

52.9

47.2

50.05

100

2

19.6

50.5

48

49.25

105

3

24.5

55.5

51.3

55.5

97

 

n = 100.66

Frequency = n/2 = 50.33 Hz


Calculations


Weight Suspended, T = 1.5 kg, 2 kg, 2.5 kg

Length of Sonometer wire = 1 m

Mass of wire, m = 1.5 x 10-4 kg

M = m/l = 1.5 x 10-4 kg/m

 

Result:

 

The frequency of the ac main = 50.33 Hz.

Metre Bridge Experiment with Readings (Class 12)

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Metre Bridge Experiment with Readings (Class 12) 


METRE BRIDGE EXPERIMENT - I


Aim: 


To find the resistance and hence determine the resistivity of the material of the wire.


Apparatus: 


Metre bridge, Battery (E), Key (K), Resistance box (R), Given resistance wire (X), High resistance (HR), Galvanometer (G), Jockey (J), Screw gauge etc.


Principle: 


The resistance of the given wire X = R[l/(100-l)]

where, R - Known resistance (Resistance put in the resistance box).

l - Balancing length from the side of X.

The resistivity of the material of the wire, ρ=  Xπr2/L

where, r - Radius of the given wire

X - Resistance of the given wire

L - Length of the given wire