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**Guess Paper – 2008**

**Class – XII**

**Subject – Physics **

( Based on the new pattern of CBSE board examination 2008 )

EMI and AC

( NCERT units 6 and 7 )

*Time Allowed : 3 hours Maximum Marks : 70*

* *

*(a) All questions are compulsory *

*(b) Question numbers 1 to 8 are very short answer type questions, carrying one mark each.*

*(c) Question numbers 9 to 18 are short answer type questions, carrying two marks each.*

*(d) Question numbers 19 to 27 carry three marks each.*

*(e) Question numbers 28 to 30 are long answer type questions, carrying five marks each.*

*(f) Use of calculators is not permitted. However, you may use log tables, if necessary.*

Q.1> Define Alternating current and mention its SI unit

Q.2> Name the element in which current leads voltage by 90 degrees.

Q.3> What is the power factor of an LCR circuit at resonance?

Q.4> An inductor of 10mH carries a current of 2A. The current is increased to 5A in 10 seconds. What is the voltage induced in the inductor?

Q.5> Name the phenomenon associated with the production of back emf in a coil due to change in the current in the coil itself. What is the SI unit for measuring this characteristic?

Q.6> What is electro magnetic induction?

Q.7> A 100 ohm resistor is connected to a 200 volt, 50 Hz power supply. What is the net power loss In the resistance.

Q.8> A bulb is connected in series with a capacitor to a DC source. Explain what will happen.

Q.9> Why is choke coil needed in the use of fluorescent tubes with ac mains? Why can we not use an ordinary resistor instead of the choke coil?

Q.10> Can we use an ordinary ammeter to measure alternating current? If not, explain why.

Q.11>If in an LCR circuit, the frequency of the supply is doubled, how should the values of C and L be changed so as to keep the current in the circuit constant.

Q.12> An ideal transformer is supplied with 10KW power at 200 volts. Find the output voltage and current if the transformation ratio is 5.

Q.13> State Faraday’s law of electromagnetic induction.

Q.14> A coil of area 2m^{2} is placed in a magnetic field B(t) = 4t + 3 Tesla. The resistance of the coil is 100 milliOhm. Find the induced current in the coil.

Q.15> Give one line definitions for the following

(a) Resistance (b) Reactance (c) Impedance

Q.16> Draw a graph showing the variation of Impedance of an LCR circuit as a function of source frequency. Also plot current on the same graph and hence define resonance frequency.

Q.17> An aeroplane with a wingspan of 50m is flying with a speed of 1Km/sec. The magnetic field of earth at that place is 300 G at a dip angle of 30 degrees, Find the induced emf across the plane.

Q.18> An inward magnetic field exists in a region, Find the direction of induced current if

(a) a square loop ( lying outside the field ) is being pushed into the magnetic field region.

(b) A circular coil ( lying inside the field ) is pulled out of the magnetic field.

(c) A triangular loop is moved inside the magnetic field.

Q.19> Show that the net power loss in an ideal inductor is zero.

(b) The power factor of an ac circuit is 0.5. What is the phase difference b/w voltage and current?

Q.20> State Lenz law and show that it obeys energy conservation

Q.21> What are eddy currents? Explain a method to reduce them. Also mention 2 applications.

Q.22> Derive an expression for the mutual inductance of a pair of solenoids, where the smaller one is placed completely inside the bigger one.

Q.23> An AC source given by E = E_{o}sinωt is connected across a capacitor. Derive an expression for the current flowing in the circuit and plot the phasor diagram.

Q.24> On a graph plot the variation of voltage and current versus ωt for the case of

(a) Pure resistor

(b) Pure inductor

(c) Pure capacitor

Q.25> What is the principle of working of an ac generator? The armature of an AC generator has 1000 turns of 1m^{2} area. Its strong horse shoe magnets produce a magnetic field of 0.25T. The armature is connected to a windmill due to which it rotates at 5 revolutions per minute. Find the maximum, average and the rms voltage produced by this generator.

Q.26> A 400mH inductor and a 3 ohm resistor are connected in series to a 500 volt , 5 / π Hz source. Find the peak current and the phase difference b/w the V and I. Also draw the impedance triangle.

Q.27> If the frequency of ac is increased then what will be the effect on current in case of following a.c. circuits. Give reasons

(1) Resistance circuit

(2) Inductor circuit

(3) Capacitor circuit

Q.28> Briefly explain the construction and working of a transformer. Mention 3 factors responsible for the decrease in efficiency.

Q.29> A series LCR circuit is connected to variable frequency 220 volt source. L = 5H, C = 80microFarad and R = 20 ohms.

(a) Determine the source frequency, which drives the circuit in resonance.

(b) Obtain the impedance of the circuit and amplitude of current at resonance

(c) Find the potential drop across each of the 3 elements. Is the algebraic sum greater than the source voltage? If yes, resolve the paradox.

Q.30> When an inductor L and a resistance R in series are connected across a 12 V , 50Hz ac source, a current of 0.5A flows in the circuit. The current differs in phase from applied voltage by π/3 radian. Calculate the value of R.

(b) A capacitor and a bulb are connected in series to an ac source. Explain, how the brightness of the bulb will change when

(a) Distance b/w the plates of the capacitor is increased

(b) A dielectric slab is introduced in the capacitor

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