Physics Hots Class Xii

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UNIT-1 ELECTROSTATICS
Where the energy of capacitor does resides? Do electrons tend to go to region of low or high potential? What is the net charge on the charged capacitor? A Gaussian surface encloses an electric dipole within it. What is the total flux across sphere? 5. Find the dimension of 1/2εoE2. 6. In a certain l m3 of space, electric potential is found to be V Volt throughout. What is the electric field in this Region? 7. If Coulomb law involves 1/r3 instead of 1/r2 dependence, would Gauss law be still true? 8. An electrostatic field line can’t be discontinuous, why? 9. The given graph shows that the variation of charge versus potential difference V for the two capacitors C1 & C2. The two capacitors have same plate separation but the plate area of C2 is doubled than that of C1.Which of the line in the graph corresponds to C1 &C2 and why? 1. 2. 3. 4.

C1 V C2

Q 10. Three charges, each equal to +2C are placed at the corners of an equilateral triangle. If the force between any two charges be F, then what will be the net force on either Charge? 11. A point charge q is placed at O as shown in the figure. Is VP-VQ +ve or –ve when (i) q>0, (ii) qr) such that the surface densities are equal. Find the potential at the common centre. 35. An electric dipole is held in an uniform electric field. Using suitable diagram, show that it doesn’t undergo any translatory motion, and (ii)Derive an expression for torque acting on it and specify its direction. 36. The field potential inside a charged ball depends only on the distance from its centre as V=ar2+b,where a and b are constants. Find the space charge distribution ρ(r) inside the ball.

CHAPTER-2 CURRENT ELECTRICITY
1. 2. Magnesium is used for making standard resistors, why? The sequence of bands marked on a carbon resistor are: Red, Red, Red, Silver. Write the value of resistance with tolerance. 3. A wire of resistively ρ is stretched to three its initial length, what will be its new resistively. 4. If p.d.v applied across a conductor is increased to 2v, how will the draft velocity of the electrons change? 5. A 10Ω thick wire is stretched so that its length becomes three times. Assuming that there is no change in its density on stretching. Calculate the resistance of new wire. 6. You are given 8 Ω resistor. What length of wire of resistance 120 Ωm-1 should be joined in parallel with it to get a value of 6 Ω ? 7. Three resistance 3Ω,6Ω and 9Ω are connected to a battery. In which of them will the power

dissipation be maximum if a) They are all connected in parallel b) They are all connected in series Give reason. 8. A silver wire has a resistance of 2.1Ω at 27.5˚c and a resistance of 2.7Ω at 100˚c. determine the temperature coeff. of resistivity of silver. 9. Give any two applications super conductors. 10. Two wire of equal length one copper and manganin have same resistance , which wire is thicker?.

11. Why manganin is used for making standard resistor? 12. A copper wire of resistivity ρ is stretched to reduce its diameter to half of its previous value .What will be the new resistances? 13. The variation of potential difference with length incase of two potentiometres A and B is given below.Which of the two is more sensitive. A Potential difference B

Length 14. If the length of the wire conductor is doubled by stretching it , keeping potential difference constant by what factor the drift speed of the electron changed. 15. If the temperature of the conductor increases, how does the relaxation time of electron changes. 16. A heater joined in series with the 60W bulb .With change of bulb with 100 W in the circuit, the rate heat produce by the heater will more or less or remain same. 17. What will be the change in the resistance of the circular wire , when its radius is halved and length is reduced by ¼ th of original length. 18. Two 120V light bulbs , one of 25W and another of 200W are connected in series . One bulb burnt out almost instantaneously ?.Which one...
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