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HYSTERISIS

Namrata Asked:

What is hysteresis? I only know its related to the frequency.

 

Ans:

Hysterisis mens “Lagging Behind”

 

Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect

The lack of retraceability of the magnetization curve is the property called hysteresis

 

Magnetic hysteresis

When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.

The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.

If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence (RETENTIVITY).

If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The ‘thickness’ of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material.

Electric Flux

From: Nancy James

Subject: Electric flux
Message Body:
Could some one please explain what is written in the ncert book of PHYSICs class XII about ‘electric flux’?

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Gauss`s law

Nagendra chowdary asked : –

“A circular ring of radius r made of a non conducting material is placed with its axis is parallel to a uniform electric field if the ring rotated by 180 degrees Does the flux changes? A Q charge is uniformly distributed on a thin spherical shell If a point charge is brought near it what is the field at the center?Is u r answer depends on whether it is conducting or non conducting?”

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

 

Eddy currents and applications

What ere Eddy currents and what is their application?

Rahul asked

Answer: 

Eddy currents (also called Foucault currents) are currents induced in conductors, when the magnetic flux linked with a solid conductor changes. The change in magnetic flux can cause a circulating flow of electrons, or a current, within the body of the conductor.

The term eddy current comes from analogous currents seen in water.

 

Applications

Electromagnetic braking

Metal detectors

Eddy current adjustable-speed drives

Eddy-current testing

Electric meters (Electromechanical Induction Meters)

Eddy current brakes

Induction heating

Proximity sensor (Displacement sensors)

Traffic Detection Systems

Vending machines (detection of coins)

Coating Thickness Measurements

Sheet Resistance Measurement

Eddy current separator for metal separation

Mechanical speedometers

Safety Hazard and defect detection applications

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)

Answer:

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.

 

How does dielectric constant depend on nature of medium?

How does dielectric constant depend on nature of medium?
Does this have any relation with free electrons and force between molecules?

-Vaanya Asked

Answer:

Dielectric Constant, also called relative permitivity of a medium depends on the electric susceptibility of it, the ease with electric flux lines can pass through it.

The susceptibility depend on molecular polarization, the electric dipole moment acquired per unit volume in an external electric field.

So, the thing to note is that, dielectric constant is a physical quantity measured as a comparison of the strength of electric field in vacuum to that inside the material.

Dielectric ConstantDielectric constant can be defined in terms of electrostatic force

Dielectric Constant can also be defined in terms of capacitance.

Please visit the following links for more information.

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