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Electric Field of Axons!

A nerve signal is transmitted through a neuron when an excess Na+ inos suddenly enters the axon, along cylindrical part of the neuron. Axons are approximately 10.0 micrometer in diameter, and measurements show that about 5.6×10^11 Na+ ions per meter ( each of charge e+)enter during this process. Although the axon is a long cylinder, the charge doesn’t all enter everywhere at the same time. A plausible model would be a series if point charges moving along the axon. Let us look at a 0.10-mm length of the axon and model it as a point of charge.


(a) If the charge that enters each meter of the axon gets distributed uniformly along it, how many coulombs of charge enter a 0.10-mm length of the axon?
(b) What electric field ( magnitude and direction) does the sudden influx of charge produce at the surface of the body if the axon is 5.00 cm below the skin?
(c) Certain sharks can respond to electric fields as weak as 1.0 microN/C. how far from this segment of axon could a shark be and still detect its electric field?

Motion of a charge particle in an electric field – Numerical Problem

At t=0 a very small object with mass 0.400mg and charge +9.00μC is travelling at 125 m/s in the -x-direction. The charge is moving in a uniform electric field that is in the +y-direction and that has magnitude E= 895 N/C. The gravitational force on the particle can be neglected. How far is the particle from the origin at t= 7.99 ms.

Hint for solving the above problem:

displacement along X direction; x=-uxt=125 x 7.99/1000

Displacement along Y direction, y=0.5 ay t2



Just substitute the values and get the answer

What are the similarities and differences in magnetic and electric field?

Similarities and differences in magnetic and electric field (Dilpreet posted this question)

Similarities Differences
 Both electric and magnetic field are conservative forcesBoth obey inverse square law

Both are non contact forces (Forces can be exerted without contact)

Both are attractive as well as repulsive (Like poles repel, like charges repel; unlike poles attract, unlike charges attract)

 Electric field is produced by a charge whether at rest or in motionBut magnetic field is produced only by a moving charge


The total magnetic flux through any closed surface is always zero, but the total electric flux through any closed surface is equal to the net charge enclosed by the surface multiplied by the reciprocal of absolute permitivity

Electric field lines are discontinuous as they have a starting point (+ charge) and an ending point (- charge); But magnetic field lines are continuous, they always form closed loops


Electricfield, Magnetic Field and Photon

We know that photon has no charge. Then where from it gets electric and magnetic field in light?

Malik Sajad asks


Electric and magnetic fields are not produced by the photon; it is massless too.

But, photon is created by time varying electric and magnetic fields, usually produced by accelerated charges.

As you might have already learnt, a charge at rest produces an electric field around it. A moving charge (current) produces a magnetic field around it as well as magnetic field. A varying current produces a varying magnetic field and hence an induce emf or current. An accelerated charge produces a time varying electric and magnetic fields.

Thus under suitable situations, the electric and magnetic fields can reinforce each other and propagate out as pure electromagnetic form of energy and such a packet of energy is called photon.

Solution to HC Verma Problem

“Referring to HC Verma Part 2, chapter 29, Electric field and potential, Q 69
I think that Force is inversely proportional to square of the distance between the charged particles, so acceleration is not constant and hence
v*v  =  u*u  + 2*a*s   is not valid in this case.
How is proceed with this question ?” Satyam asked

The mechanism of operation in a capacitor

The DC doesnt flow through capacitor. I thought it was because the electrons jus get saturated on one side and are lost on other side, such that it can’t take or give up anymore or plausibly due to voltage developed across it whereas in AC as directions keep changing, saturation doesnt occur. But it if it were so then AC would flow through two plates seperated by large distances or switches which can be considered capacitors.So please guide me as to what’s actually happening during current flow through capacitor and how it happens? (The question was posted by Thushar Misra)


Your logic is almost correct. A capacitor is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field.

A current is not “flowing” through a capacitor, the “current” that exists between the plates of a capacitor is called “Displacement current” which is actually caused by the time varying electric flux. This causes the circuit to be continuous though there is no electrical contact or flow of charges between the plates of the capacitor.

When a capacitor is connected to DC, there exists a change in electric flux when the switch is just turned on but soon it reaches steady state and there is no “current”. A capacitor can “conduct” only when there is a “change” in electric flux, which is continuously taking place in an AC.


Electric Field due to Polarisation

Sreeram asks:

“do charges produced due to polarization in a dielectric produce electric field outside?”


When a dielectric is kept in an electric field, it is polarized. The polarized ends are just like the two plates of a parallel plate capacitor. The electric field is confined between the two surfaces (except for some edge effect). So, in a macroscopic point of view, the answer is no.

(However, I agree that there is scope for disagreement and debate when we think and look at the microscopic level. Users of the site are requested to post comments)

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