verify the law of combination for parallal connection of resistances using metre bridge.
Answers
Aim:
To verify the laws of combination (parallel) of resistances using a metre bridge
Materials Required:
1. 2 different resistances (carbon or wire-wound resistors),
2. metre bridge,
3. galvanometer,
4. a cell or battery eliminator,
5. a jockey,
6. a rheostat,
7. a resistance box,
8. a plug key,
9. sandpaper and
10.thick connecting wires.
Theory:
Consider two resistances r1 and r2 are connected in series.
The series combination resistance RS is given by RS = r1 + r2
When connected in parallel, the resistance of the combination is given by Rp
resistance of the combination Rp
Diagram
Resistance connected in Parallel
resistance connected in parallel
Resistances r1 and r2 connected in parallel to one arm of a metre bridge
Resistances r1 and r2 connected in parallel to one arm of a metre bridge
Procedure
Set up the circuit as shown in the figure above.
Connect R1 and R2 as shown in the figure.
Tighten all plugs in the resistance box by pressing and rotating each plug to assure that all plugs make good electrical connections. Using sandpaper clean the ends of connecting wires before making the connections.
Remove some plug(s) from the resistance box to get the suitable value of resistance R
Get a null point D on the metre bridge wire by sliding the jockey between ends A and C.
Note the value of the resistance R and lengths AD and DC.
Calculate the experimental value of the equivalent parallel resistance
Repeat the experiment for four more values of resistances R. Obtain the mean value of unknown resistance.
Observations And Calculations
sl.no
Resistance R (ohm)
Resistance from the resistance box R (ohm)
Length AD=l
Length DC=100-1
Rp=100-1/l
Mean resistance (ohm)
R1 only
1
2
3
4
5
r1 =
r2 only
1
2
3
4
5
r2 =
r1 and r2 in parallel
1
2
3
4
5
Rp =
Precautions
Ensure the connection is neat, clean and tight
Insert key only while taking an observation
Move jockey gently over the metre bridge wire.
Sources Of Error
The instrument screw may be loose
Unavailability of thick connecting wires
Result
The experimental value of Xp = ohm
The theoretical value of Xp = ohm
The experimental and theoretical value of Xp is found to be the same. Hence, the law of resistance in parallel is verified.
Explanation: