Post office box Experiment for std 8 to 12 Physics

 POST OFFICE BOX

Aim:-

A post office box and other necessary instruments are given to you. Take three appropriate ratios of the resistances in the two arms of Post Office Box and determine the values of two unknown resistances.

Now connect the unknown resistances in series and parallel respectively and verify the laws of series and parallel combinations of resistances.

PRINCIPLE :- 

It works on the principle of balanced Wheatstone bridge.

APPARATUS :-

A Battery,

unknown resistance (Rx & Ry),

Post Office Box, 

a galvanometer, 

rheostat, 

connecting wires.

Post Office Box Apparatus

Condition 1 Resistance Rx connect between C and D

Post Box Wiring Diagram find Unknown Resistance Rx 

Condition 2 Resistance Ry connect between C and D

Post Box Wiring Diagram find Unknown Resistance Ry

Post OfficeBox line diagram Series connection Rx and Ry

Series connection of Rx and Ry wiring diagram

Parallel Connection line Diagram of Rx and Ry

Wiring Diagram Parallel connection of Rx and Ry

CALCULATIONS :-

Series connection, R’s = Rx + Ry + …..Ohm

Parallel connection, R’p = Rx Ry  /  Rx + Ry = ….Ohm.

RESULT :-

(1) Unknown resistance, Rx = ….Ohm. , Ry = ….Ohm

Observation for Post Office Box Experiment

Calculation table for Post Office Box

CONCLUSION :

The experimental and theoretical values of equivalent resistance for series and parallel connections are equal,within the experimental error. This implies the validity of the rules for the different combinations of resistances.

Precautions

 All plug keys of Post Office Box must be tight.

 Contact points of K1 and K2 must be cleaned  with sand paper.

 Galvanometer should be very sensitive.

 Throughout the experiment  press key K2(battery key) first and then key K1(galvanometer key).

More Video : Post Office Box Video presentation.

http://youtu.be/vG1Bkh1V7RY

Post Office Box Experiment.

Measurement of Resistance (Wheatstone's Bridge)

Practicle – 2
MEASUREMENT OF RESISTANCE ( WHEATSTONE’S BRIDGE)

AIM :-

You are given a Wheatstone bridge and other necessary instruments. Find the values of the unknown resistance Rx. Exchange the positions of unknown resistance and resistance box in the gaps of Wheatstone bridge and repeat the experiment.

PRINCIPLE :-

It works on the principle of balanced Wheatstone bridge.

APPARATUS:-    

Wheatstone-bridge (meter-bridge),

2 Volt Battery,

Rheostat,

Galvanometer,          

Jockey,

Key,

Resistance box(1 – 100  W),

Resistance coil.

Apparatus for Wheatstone's Bridge Practicle

 Precautions :-

1. Make proper and tight connections.
2. Keep the keys tightly closed in the resistance box.
3. Adjust the value of resistance box so that null point is obtained between 40 cm and 60 cm.
4. Do not slide the jokey continuously over the wire.

Note :-  Keep high resistance of rheostat in the beginning of each observation so that current is less and the galvanometer is not damaged. After getting approximate position of null point only decrease the resistance of rheostat.

(1) Condition :- Connect unknown resistance Rx between M – N.

Line Diagram For Wheatstone's Bridge

Wiring Diagram for Wheatstone’s Bridge.
Use this setup for carry out practical so it may be easy to understand.

Wiring Diagram for Wheatstone's Bridge

Measure L1 and L2 and note down in your Table.

(2) Condition :- Interchange unknown resistance Rx between P – Q.

In which interchange

Unknown  Resistance RX and Known Resistance Box RB.

Line  Diagram for Interchange RX And RB

Wiring Diagram for Wheatstone’s Bridge. (Condition-2)

In which interchange Unknown Resistance RX and Known Resistance Box RB.
Use this setup for carry out practical so it may be easy to understand.

Wiring Diagram for Interchange RX And RB

 

Conclusion :-

(1)   Unknown Resistance, Rx = …….. W

(2)    Unknown Resistance, Ry = ……...W