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Why direct currents aren’t used in Home circuits?

How can an alternating current change its direction while moving in a wire or any conductor? Why direct currents aren’t used in Home circuits?
Alternating current is produced by rotating a coil in a strong magnetic field (or by rotating magnet around the coil)
So, during every half rotation the direction of emf (voltage) induced in the […]

Jaz posted this question from electrostatics

“I’ve tried so many times, but I’m unable to arrive at the correct equation; See attachment for question” Question   Jaz posted this question E=k(2λ/r) due to positive wire E’=k(2λ/(D-r)) due to negative wire The resultant will be the sum of these as both are in the same direction At the mid point both distances are r = […]

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.

If you want real time answers to your queries by a group of Physics Lovers, you can post them at the Physics Fans group with facebook at https://www.facebook.com/groups/askphysics/

  1. 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).
  2. 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?
  3. 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.
  4. 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. diagram
  5. 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.
  6. 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).
  7. 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.
  8. diagram1 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 ?
  9. The electron in the hydrogen atom circles around the proton with a speed of 2.18 x 106 m/s in an orbit of radius 5.13 x 10 -11 m. What magnetic field does it produce at the centre?
  10. A proton moves with a speed of 7.45 x 105 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)
  11. diagram2 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.

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  • numericals of physics of class 12

Electrostatic induction – Where does the charge go on earthing?

Rajkumar asks:”in electrostatic induction in step 3 when earthed wire is removed then where the free negative charge go.does they go to earth then what happen to them.”

Waiting for responses from visitors

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Why we specifically note resistance in a circuit while all part of a wire have some resistance..?

Why we specifically note resistance in a circuit while all part of a wire have some resistance..???


The resistance of the rest of the circuit compared to the resistance of the resistor is negligible.

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Current electricity – Numerical

A 10m length of aluminium wire has a diameter of 1.5mm. It carries a current of 12A. Find (i) the current density. (ii) the drift velocity (iii) the electric field in the wire. aluminium has approximately 10^29 free electrons per m^3

you are given two nichrome wire of same length and different thickness Which wire will get heated first Give reason.?

You are given two nichrome wire of same length and different thickness Which wire will get heated first Give reason.? Asked Merlin Mary Mathew


If the wires are connected one by one to a source, (or connected in parallel to the same source) the one having less resistance (Greater thickness) will get heated up first. This is because, for constant voltage, Heat dissipated is inversely proportional to resistance.

If both are connected in series to a power supply, the one with less thickness will get heated up as same current is flowing through both and then heat generated is directly proportional to resistance.

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