Verifying laws of combination (parallel) of resistance using meter bridge

Aim: To verify laws of combination (parallel) of resistance using meter bridge .

Apparatus: a meter bridge , a leclanche cell , a galvanometer , a jockey , a jockey , a resistance box , two resistance wire , connecting wires , set square .

Theory

The resistance (r) of a resistance wire or coil is given by r=(100-l)/L *R where R is the resistance from resistance box in the left gap and L is the length of meter bridge wire from O end upto balance point.

When r₁ and r₂ are connected in parallel , then their combined resistance

Rp = r₁r₂ /r₁+r₂

Procedure

1. Mark the two resistance wires as r₁ and r₂
2. Connect the two coils r₁ and r₂ in parallel as shown in fig. in the right gap of meter bridge and find the resistance of this combination . Take at least three set of observation
3. Record your observation .

Result

Within limit of experimental error , experimental and theoretical values of R_p are same .

Hence law of resistance in parallel are verified .

Precaution

The key should be inserted only while taking observation .

Connection should be neat and tight .

Source of error

The galvanometer may not show opposite deflection

The key in the resistance box may be close .

OBSERVATION TABLE

Value of r₁

Resistance of R.B. (ohm)	Balance L (cm)	(100-l) (cm)	[(100-l)*R]/L	Mean r1
0.5	59	41	0.347	0.6393
1	37	63	0.587
2	33	67	0.985

Value of r₂

Resistance of R.B. (ohm)	Balance L (cm)	(100-l) (cm)	[(100-l)*R]/L	Mean r2
0.5	83.2	16.8	0.1009	1.729
1	80.8	19.2	4.208
2	69.4	30.6	0.881

Value of r_p

Resistance of R.B. (ohm)	Balance L (cm)	(100-l) (cm)	[(100-l)*R]/L	Mean rp
0.1	27	73	0.27	0.36
0.2	32	68	0.42
0.5	56	44	0.39

Calculations

Rp = (R₁R₂)/R₁+R₂ = (0.693×1.729)/(0.693+1.729) = 0.4667 ohm

Experimental Error = [(0.36-0.4667)/(0.36)]*100 = 29.6%

 

Viva question

1. How does resistance change in parallel combination ?

Resistance decrease in parallel combination

2. explain the decrease of resistance in parallel combination ?

Effective area of cross section increases as r x 1/A , resistance decreases .

3. Why should jockey not be pressed too hard on wire while sliding ?

Sliding the jockey with a hard press will scratch the wire and make its thickness non-uniform .Resistance per unit length of wire will be inconsistent as resistance depends on area of cross section .

4. What is the resistance of an open key ? Explain.

Open key has infinite resistance as it makes current zero (R=V/I)

5. Define unit of resistance .

SI unit of resistance is OHM or volt per ampere . 1 Ohm is the resistance of a conduction carrying 1 ampere current when potential difference maintained across its end is 1 volt .

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