HSC Science (General) 12th Board ExamMaharashtra State Board
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# Show Graphically the Variation of E.M.F. with Time (T). - HSC Science (General) 12th Board Exam - Physics

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ConceptCoil Rotating in Uniform Magnetic Induction

#### Question

Obtain an expression for e.m.f. induced in a coil rotating with uniform angular velocity in a uniform magnetic field. Show graphically the variation of e.m.f. with time (t).

Resistance of a potentiometer wire is 0.1Ω/cm. A cell of e.m.f 1.5V is balanced at 300 cm on this potentiometer wire. Calculate the current and balancing length for another cell of e.m.f. 1.4V on the same potentiometer wire.

#### Solution

Coil rotating in uniform magnetic field:

Consider a coil of ‘N’ turns with effective area NA placed in uniform magnetic induction B as shown in the figure below.

The coil is rotated continuously with constant angular velocity ω. The axis of rotation is in the plane of the coil and normal to the magnetic induction B.

At t = 0, the plane of the coil is perpendicular to the magnetic induction B.

The magnetic flux passing through the coil is NAB.

After t seconds, the plane of the coil is at an angle θ.

Thus, the magnetic flux Φ through the coil at time t is given by

Φ = NABcosθ = NABcosωt

As time changes, the magnetic flux goes on changing. Hence, the e.m.f. generated in the coil is given by

e=-(dphi)/(dt)=-d/dt(NAB cosomegat)

e = NABωsinωt

e = 2πfNABsinωt

This is the expression for induced e.m.f. generated in the coil at any instant t. It is known as instantaneous e.m.f.

Given that

R/L = σ = 0.1Ω/cm = 0.1 x 100 = 10Ω/m

l1 = 300 cm = 3 m, E1 = 1.5 V, E= 1.4 V

We know that

E1 = iR1 =iσl1

therefore i=E_1/(sigmal_1)=1.5/(10xx3)=0.05 A

Hence, the current for the other cell is 0.05 A.

For potentiometer, the balancing condition is

E_1/E_2=l_1/l_2

thereforel_2=l_1E_2/E_1=3xx1.4/1.5=2.8m

So, the balancing length for the other cell is 2.8 m.

Is there an error in this question or solution?

#### APPEARS IN

2014-2015 (March) (with solutions)
Question 8.1 | 7.00 marks
Solution Show Graphically the Variation of E.M.F. with Time (T). Concept: Coil Rotating in Uniform Magnetic Induction.
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