CBSE Class 12 Physics Practical Note with readings | Section A Experiment – 3

Section A Experiment – 3

Objective

To verify the laws of combination (parallel) of resistances using a metre bridge.

Apparatus Required

A metre bridge, a Leclanche cell or battery eliminator, a galvanometer, a resistance box, a jockey, two resistance wires or two resistance coils known resistances, a set square, sand paper and connecting wires.

Formula Used

 \frac{1}{R_{p}}=\frac{1}{R_{1}}+\frac{1}{R_{2}}

 R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}

S=(\;\frac{100−l}{l}\;)\;R 

Diagram

Here R₁ = 1 Ω and R₂ = 2 Ω

Calculation

 R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}

 R_{P}=\frac{1×2}{1+2}

R_{P}=\frac{2}{3}

R_{P}=0.66\;Ω 

Result

Experiment value of parallel combination=RP= 0.60 Ω

The theoretical value of parallel combination,

 R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}

 R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}

 R_{P}=\;0.66\;Ω 

Hence, laws of combination (parallel) of resistance is verified using a metre bridge.

Precautions

• All connections should be neat and tight.
• All the plugs in the resistance box should be tight.
• Clean the connecting wires and the connecting points of metre bridge properly with sandpaper.
• The jockey should be held perpendicular to wire of metre bridge.
• Try to obtain the balance point between 40 cm and 60 cm.
• Move the jockey gently on the wire and do not keep the jockey and the wire in contact for a long time.

Sources of errors

• The keys of the resistance box may not be clean and tight.
• The wire may not be uniformly thick throughout.
• The screws of the instrument may be loose.