A diode valve is connected to a battery and a load resistance. The filament is heated so that a constant current is obtained in the circuit. As the cathode continuously emits electrons, does it get more and more positively charged?

Figure Shows a diode connected to an external resistance and an e.m.f. Assuming that the barrier potential developed in diode is 0.5 V , obtain the value of current in the circuit in milliampere.

(i) A poor emitter has a large reflectivity . Explain why. (ii) A copper tumbler feels much colder than a wooden block on a cold day. Explain why. (iii) The earth would become so cold that life is not possible on it in the absence of the atmosphere. Explain why? (iv) Why clear nights are cooler than cloudy nights? Why does a piece of red glass when heated and taken out glow with green light? (vi) Why does the earth not become as hot as the sun although it has been receiving heat from the sun for ages? (vii) Animals curl into a ball when they are very cool. Why? (viii) Heat is generated continuously in an electric heater but its temperature becomes constant after some time. Explain why? (ix) A piece of paper wrapped tightly on a wooden rod is observed to get charred quickly when held over a flame as compared to a similar piece of paper when wrapped on a brass rod. Explain why? (x) Liquid in a metallic pot boils quickly whose base is made black and rough than in a pot whose base is highly polished . Why?

Figure shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod, and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod =15 cm, B =0.50 T, resistance of the closed loop containing the rod 9.0 m Omega . Assume the field to be uniform. (a) Suppose K is open and the rod is moved with a speed of 12 "cm s"^(-1) in the direction shown. Give the polarity and magnitude of the induced emf. (b) Is there an excess charge built up at the ends of the rods when K is open ? What if K is closed ? (c) With K open and the rod moving uniformly, there is no net force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain. (d) What is the retarding force on the rod when K is closed ? How much power is required (by an external agent) to keep the rod moving at the same speed (= 12 cm s^(-1) ) when K is closed ? How much power is required when K is open? (f)How much power is dissipated as heat in the closed circuit ? What is the source of this power ? (g) What is the induced emf in the moving rod if the magnetic field is parallel to the rails instead of being perpendicular ?

In order to get a current of 0.5A in a circuit by connecting a bulb of resistance 20Omega with 12V battery, what should be the resistance to be connected in series? What will be the voltage drop across the bulb?

Assertion: Current is passed through a metallic wire, heating it red. When cold water is poured on half of its portion, then rest of the half portion become more hot. Reason: Resistance decreases due to decrease in tem perature and so current through wire increases.

An experimental setup of verification of photoelectric effect is shown in the diagram. The voltage across the electrode is measured with the help of an ideal voltmetar, and which can be varied by moving jockey 'J' on the potentiometer wire. The battery used in potentiometer circuit is of 20 V and its internal resistance is 2omega . The resistance of 100 cm long potentiometer wire is 8 omega . The photo current is measured with the help of an ideal ammeter. Two plates of potassium oxide of area 50 cm^(2) at separation 0.5 mm are used in the vacuum tube. Photo current in the circuit is very small so we can treat potentiometer circuit an indepdent circuit. The wavelength of various colours is as follows : |{:("Light",underset("Violet")(1),underset("Blue")(2),underset("Green")(3),underset("Yellow")(4),underset("Orange")(5),underset("Red")(6)),(lambda "in" Årarr,4000-4500,4500-5000,5000-5500,5500-6000,6000-6500,6500-7000):}| The number of electrons appeared on the surface of the cathode plate, when the jockey is connected at the ened 'P' of the potentiometer wire. Assume that no radiation is falling on the plates.

An experimental setup of verification of photoelectric effect is shown in the diagram. The voltage across the electrode is measured with the help of an ideal voltmetar, and which can be varied by moving jockey 'J' on the potentiometer wire. The battery used in potentiometer circuit is of 20 V and its internal resistance is 2omega . The resistance of 100 cm long potentiometer wire is 8 omega . The photo current is measured with the help of an ideal ammeter. Two plates of potassium oxide of area 50 cm^(2) at separation 0.5 mm are used in the vacuum tube. Photo current in the circuit is very small so we can treat potentiometer circuit an indepdent circuit. The wavelength of various colours is as follows : |{:("Light",underset("Violet")(1),underset("Blue")(2),underset("Green")(3),underset("Yellow")(4),underset("Orange")(5),underset("Red")(6)),(lambda "in" Årarr,4000-4500,4500-5000,5000-5500,5500-6000,6000-6500,6500-7000):}| When radiation falls on the cathode plate a current of 2muA is recorded in the ammeter. Assuming that the vecuum tube setup follows ohm's law, the equivalent resistance of vacuum tube operating in this case when jockey is at end P.

An experimental setup of verification of photoelectric effect is shown in the diagram. The voltage across the electrode is measured with the help of an ideal voltmetar, and which can be varied by moving jockey 'J' on the potentiometer wire. The battery used in potentiometer circuit is of 20 V and its internal resistance is 2omega . The resistance of 100 cm long potentiometer wire is 8 omega . The photo current is measured with the help of an ideal ammeter. Two plates of potassium oxide of area 50 cm^(2) at separation 0.5 mm are used in the vacuum tube. Photo current in the circuit is very small so we can treat potentiometer circuit an indepdent circuit. The wavelength of various colours is as follows : |{:("Light",underset("Violet")(1),underset("Blue")(2),underset("Green")(3),underset("Yellow")(4),underset("Orange")(5),underset("Red")(6)),(lambda "in" Årarr,4000-4500,4500-5000,5000-5500,5500-6000,6000-6500,6500-7000):}| When other light falls on the anode plate the ammeter reading remains zero till, jockey till, jockey is moved from the end P to the middle point on the wire PQ. Thereafter the deflection is recorded in the ammeter. The maximum kinetic energy of the emitted electron is :