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# Tag Archives: potential drop

## Collection of Problems received from Class 12 Physics

These problems were posted by **Geena**. Hope that we will be able to post the answers to these questions soon; each in a separate post. By the time visitors can attempt to post their answers as comments to this post. (Only selected posts will be published)

Please note that the answers are not published until the Physics teachers are not satisfied with the answers. So, please be patient to get the correct answers rather than approximations in a haste.

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- A series battery of 10 lead accumulators each of emf 2 V and internal resistance 0.25 ohm is charged by a 220 V D.C. mains to limit the charging current a resistance of 47.5 ohm is used in series in the charging circuit. What is a) the power supplied by the mains and b) power dissipated as heat? Account for the difference of powers is (a) and (b).
- A potentiometer wire has a length L and a resistance Ro. It is connected to a battery and

a resistance combination as shown. Obtain an expression for the potential drop per unit length of this potentiometer wire. What is the maximum emf of a ‘test cell’ for which one can get a ‘balance point’ on this potentiometer wire? What precaution should one take, while connecting this ‘test cell’ , in the circuit? - A cell, of emf 4 V and .internal resistance 0.5 Ω, is connected across a load of resistance (i) 7.5 Ω, (ii) 11.5 Ω. Calculate (i) the ratio of the differences in the emf of the cell and the potential drop across the load, and (ii) the ratio of the currents in the two cases.
- In the figure shown, calculate the total flux of the electrostatic field through the spheres S1 and S2 The wire, AB, shown here, has a linear charge density, λ given by λ = kx where x is the distance measured along the wire, from the end A.
- A straight wire, of length L, carrying a current I, stays suspended horizontally in mid air in a region where there is a uniform magnetic field B . The linear mass density of the wire is λ. Obtain the magnitude and direction of this magnetic field.
- Two cells of same emf E, but different internal resistance rl and r2 are connected to an external resistance R as shown. in the figure. The voltmeter V reads zero. Obtain an expression for R in terms of rl and r2. Calculate the voltage across the cell of internal resistance r2. (Assume that the voltmeter V is of infinite resistance).
- A galvanometer with a coil of resistance 120 ohm shows full scale deflection for a current of 2.5 mA How will you convert the galvanometer into an ammeter of range 0 to 7.5 A ? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer.
- Figure shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph

showing the variation of induced e.m.f (E) with time (t). What does the area enclosed by the

E – t curve depict ? - The electron in the hydrogen atom circles around the proton with a speed of 2.18 x 10
^{6}m/s in an orbit of radius 5.13 x 10 -11 m. What magnetic field does it produce at the centre? - A proton moves with a speed of 7.45 x 10
^{5}m/s directly towards a free proton originally at rest. Find the distance of the closest approach for the two protons. (Given: mass of proton = 1.67 x 10^{–27}kg and e = 1.6 x 10^{–19}C) - Figure (a), (b) and (c) show three a.c. circuits in which equal currents are flowing. If the frequency of emf be increased, how will the current be affected in these circuits? Give reason for your answer.

## Questions normally not answered by Students in Viva

Here are some questions which are not properly answered by students during Viva.

- Name the Physical quantities to be kept constant for Ohm’s law to be true.
- State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
- How can we increase the sensitivity of a potentiometer?
- Define figure of merit of a galvanometer.
- Which has more resistance – a galvanometer or a milliammeter?
- How does an LED emit light?
- What is the difference between an ordinary diode and an LED?
- Define principal axis of a convex lens?
- What happens to the focal length of a concave mirror if it is immersed in water?
- What are the factors affecting the intrernal resistance of a cell?
- What are the difference between primary and secondary cell?
- Why can’t we use a dry cell for starting a car?

**Hints/Answers**

- length, area of cross section,temperature
**The potential drop across any length of a conductor**of uniform**cross section**and**composition**carrying a constant**current is directly proportional to the length.**- increasing the length of potentiometer wire, decreasing the current, decreasing the potential gradient
- Current for unit deflection
- galvanometer
- The energy released during recombination of electrons and holes across the junction is responsible for the release of light by LED
- In ordinary diode the energy emitted during recombination of electrons and holes is in the invisible region of the em spectrum but in the case of LED, the energy is in the visible region.
- Straight line joining the centres of curvature of the lens..
- The focal length of mirror does not change by changing the medium. Their is a pure geometrical relationship between the radius of curvature and focal length and the relation does not include any term depending on refractive index.
- the nature of electrolyte, the concentration of electrolyte, temperature, distance between electrodes, area of electrodes
- Primary cell cannot be recharged, secondary cells can be recharged; secondary cells have less internal resistance than primary cells.
- Due to the high internal resistance of a dry cell, it won’t be able to provide the current sufficient to start the car