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 .
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
- Mark the two resistance wires as r1 and r2
- 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
- Record your observation .
Within limit of experimental error , experimental and theoretical values of Rp are same .
Hence law of resistance in parallel are verified .
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 .
Value of r1
|Resistance of R.B. (ohm)||Balance L (cm)||(100-l) (cm)||[(100-l)*R]/L||Mean r1|
Value of r2
|Resistance of R.B. (ohm)||Balance L (cm)||(100-l) (cm)||[(100-l)*R]/L||Mean r2|
Value of rp
|Resistance of R.B. (ohm)||Balance L (cm)||(100-l) (cm)||[(100-l)*R]/L||Mean rp|
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%
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 .