268. Force is given byF= Q (VxB). Of these, Name the Pairs of vectors which are always at right angles to each other.
269. Mention one advantage and one disadvantage of AC over DC.
270. Name two electro magnetic waves that are emitted by an incandescent bulb.
271. Mention one advantage of reflecting type telescope.
272. With what purpose was famouse Davisson- Germer experiment with electrons performed?
273. Draw the symbol of the universal logic gate.
274. Draw energy band diagram of P-type semiconductor.
275. Capacitor P, Q and R each have a capacitance ‘C’. A battery can charge the capacitor P to a potential V. If after charging P, the battery is disconnected from it and the charged capacitor P is connected in following separate instances to Q and R
     1. In parallel to Q and
     2. In series to ‘R’
276. A metal wire is stretched to increase its length by 10% What is the percentage change in its resistance? Will the resistivity of the wire change?
277. When a battery of emf. ‘E’ and internal resistance ‘r’ is connected to a resistance ‘R’, a current I flows through it. Derive a relation between E, I, r and R.
278. Mention 2 properties of alloys from which permanent magnets are made.
279. An inductor of 2mH carries a current of 5A. The direction of current is reversed in it in half a second due to which a voltage is produced across the inductor. What is this phenomenon known as? Find the Voltage Produced.
280. Plot a graph to show how the following quantities vary with the frequency of the source.
     1. Capacitive reactance
     2. Inductive reactance.
281. Vehicles moving in foggy weather use yellow color headlights. Why?
282. Give reasons for :
     1. The sky appears blue
     2. Sun appears reddish at sunset and sunrise.
283. Draw a labeled block diagram for a communication system.
284. Define electric field intensity at a point. Derive an expression for the electric field intensity at a point on the axial line of a short dipole.
285. Show that the potential on the equatorial line of an electric dipole is zero
286. What is an electric dipole? Derive an expression for the torque acting on an electric dipole, when held in a uniform electric field. Hence, define the dipole moment.
287. State Gauss theorem in electrostatics. Using this theorem, derive an expression for the electric field
288. intensity due to an infinite plane sheet of charge of charge density σ
289. Show mathematically that for any point out side the shell, the field due to a uniformly charged thin
290. spherical shell is the same as if the entire charge of the shell is concentrated at the centre.
291. Apply gauss theorem to find electric field intensity at a point due to an infinitely long thin, uniformly
292. charged straight wire.
293. Define the capacitance of a capacitor. Give its S.I unit for a parallel plate capacitor, prove that the total
294. energy stored in a capacitor is ½ cv²and hence derive expression for the energy density of the capacitor.
295. Derive an expression for the capacitance of a parallel plate capacitor when the space between the plates is partially filled with a dielectric medium of dielectric constant k.
296. In a hydrogen atom, an electron revolves around a proton. Which of these two exerts a greater electrostatic force on the other?
297. What is the force experienced by a positively charges particle Q moving at right angles to a uniform electric field E.
298. What is the order of voltages that can be built up using a Van De Graff generator?
299. What is the angle b/w Electric field and Dipole moment at an axial point?
300. Define gyromagnetic ratio. What is it’s value?
301. State the condition in which terminal voltage across a secondary cell is equal to its emf.
302. The dielectric strength of air is 3 x