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## Explain a method to create Uniform Electric Field.

The simplest method of creating a uniform electric field in a region is to keep two  equally charged thin plain metal sheets of opposite charge parallel to each other. As you can see from the derivation for electric field due to a plain sheet of charge, the electric field due to a plain sheet of charge is independent of distance (provided the distance is not large compared to the dimensions of the plain sheet of charge).

So, if you keep two plain metal sheets of same charge densities but opposite kind of charge, we get a fairly good uniform electric field in between the plates.

At regions close to a uniformly charged thin sheet also we get a uniform electric field, but as distance increases it may tend to become non uniform. ## Electricity field intensity within a conductor is always zero.Why?

Electricity field intensity within a conductor is always zero.Why?

The main difference between a conductor and an insulator is that the conductors contain plenty of free electrons which are free to move throughout the conductor; whereas the insulators do not have free electrons at room temperature.

When a conductor is held in an electric field, the free electrons inside them instantly drift opposite to the direction of electric field until the electric field due to the drift balances the external electric field and the net electric field inside the conductor becomes zero. ## Current Electricity – Drift Velocity based numerical

Two different wires X and Y of same diameter but different materials are joined together in series     across a battery . If the number density of electrons in X is  twice that in Y . Find ratio of drift velocity of electrons in two wires .

In series combination, the current is the same.

I = nAev

=> n1v1 = n2 v2

=> v1:v2 =n2:n1 ## Are the gases coming out when a refrigerator is opened vapours?

Are the gases coming out when a refrigerator is opened vapours? Then why is it called?

Asked by a 7th std student.

Air contains humidity (due to the presence of water vapour). Evaporation can take place at any temperature. However the rate of evaporation increases with increase in temperature..

The conversion of a liquid to gas can take place due to evaporation or boiling. but boiling is the phenomenon taking place at a higher but constant temperature.

Due to evaporation (and boiling) the air contains water vapour at room temperature.

When we open the fridge the water vapour condenses to tiny droplets of water and become visible (due to scattering of light).

(I feel that all the concepts might not have been cleared, please ask the questions which arise as comment to this post.) ## Hot air rises up; but how?

We used to say that hot air is lighter and so it rises up. But around the hot air, there is cold air. So, how did the cold air allow the hot air to rise up.

Asked by Rashmi and Srushti of class VIII B, KV AFS Bidar. You might have seen that helium of hydrogen filled balloon rising up. Also remember the case of hot air balloons. It is the same process here too.

First of all, the air is a fluid and the molecules are free to move as there is plenty of space between the molecules. So, when the air below becomes hot and lighter, they can easily go up through the empty space in between the cold and heavier molecules. the space vacated by the hot molecules are immediately occupied by the comparatively heavier cold molecules. ## A spring is held vertically downward and a current is passed through it . Then the length of the spring :

A spring is held vertically downward and a current is passed through it . Then the length of the spring :

A . increases     B. decreases    C . does not change    D the spring oscillates Since the adjacent turns are carrying currents in the same direction (just like two parallel wires) there will be attraction between the turns and therefore the length of the spring

tends to decrease. ## Questions from Alternating current

1.      Derive an expression for the average power in an ac circuit.
2.      Distinguish between resistance, reactance and impedance.
3.      Draw the phasor diagram showing voltage and current in LCR series circuit and derive an expression for the impedance 4.      Deduce the phase relationship between current and voltage in an ac circuit containing inductor only
5.      A series LCR circuit with L = 0.12 H, C = 4.8 x 10 7 f and R = 23 ohms is connected to a variable frequency supply.  At What frequency is the current maximum?
9.      A sinusoidal voltage V=200 sin 314t is applied to a resistor of 10 ohms resistance. Calculate (i) rms value of the voltage, (ii) rms value of the current and (iii) power dissipated as heat in watts.

(Posted by Sayantika Nath) ### Visitors So Far @ AskPhysics

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