Diferences b/w emf & voltage,Resistance & resistivity

In summary, emf is a voltage and potential that is generated by an energy sourcing device, like a battery or a generator. Resistivity is resistance times cross-section area per length.
  • #1
hafiz16
4
0
hi I am new member of this forum..Plz tell me how to use this forum & how to ask Question from other members...Plz tell me differences b/w emf & voltage,Resistance & resistivity..
 
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  • #2
Welcome to PF!

Hi hafiz16! Welcome to PF! :smile:

emf ("electromotive force") means different things in different books.

usually, it means the same as voltage

See http://en.wikipedia.org/wiki/Electromotive_force#Terminology"
It is common in some fields, such as circuit theory, to refer to the voltage created by the emf as the emf. Some authors do not distinguish between the emf and the voltage it creates. Some use emf to refer to the open-circuit voltage and voltage to the potential difference when current is drawn
… etc etc etc

Resistivity of a material is resistance times cross-section area per length … see http://en.wikipedia.org/wiki/Resistivity#Definitions" :wink:
 
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  • #3
I believe that "emf" usually refers to a potential due to an energy sourcing device, like a battery, or a generator. This emf is indeed a voltage, as well as being a potential. But, if an energy dissipating device, i.e. passive element, incurs a voltage drop when carrying current, this is a drop in potential, or voltage, but the term "emf" is not used here.

Thus, potential, and voltage are general terms. The term emf is included, as emf is a voltage and potential. But emf is used specifically to describe a voltage/potential generated by an energy sourcing device. The term "drop" denotes dissipation. The emf and the drop are both measured in volts. One indicates energy being sourced, the other indicates dissipation.

Clear?

Claude
 
  • #4
The emf of a device measures the energy gained by unit charge passing through a cell or dynamo etc. The PD between two points in a circuit measures the energy lost per unit charge as it passes through those points. Both are measured in Volts (joule per coulomb)
One is energy gained. The other is energy lost.
In a closed circuit, energy gained equals energy lost. This is often expressed as Kirchhoff's Rule.
 
  • #5
The way I like to remember it, emf is the negative line integral of the electric field between two points, whereas a potential difference is the negative difference between the potential function of the electric field. When the electric field actually has a potential function, these would be the same thing (but we don't call it emf when the field is the gradient of some function). However, some electric fields don't have potential functions, for example a circular electric field can't be written as the gradient of a potential. But it will still have an emf.

The important thing here is that the voltage across a closed loop is always zero, but you can have a nonzero emf by integrating around a closed look. As you know,

[tex]\oint_C\vec{E}\cdot d\vec{l} = -\dfrac{d}{dt}\iint\vec{B}\cdot d\vec{A}[/tex]

So Faraday's Law itself gives an example of how emf can be different from a voltage. If the electric field had a potential function, then the left side would always be zero.

You could also think of the emf as the "work" done by an electric field.
 
  • #6
thxk for all..but i still not clear & i am confuse..What is emf & voltage..
 

Related to Diferences b/w emf & voltage,Resistance & resistivity

1. What is the difference between EMF and voltage?

EMF (electromotive force) is the potential difference created by a source of energy, such as a battery, while voltage is the measure of the potential difference between two points in an electrical circuit. Essentially, EMF is the cause of voltage.

2. Can you explain the difference between resistance and resistivity?

Resistance is the measure of how much a material resists the flow of electricity, while resistivity is the inherent property of a material that determines its resistance. Resistivity takes into account the material's dimensions, while resistance does not.

3. How do EMF and voltage relate to each other?

EMF is the cause of voltage, meaning that EMF creates a potential difference between two points in an electrical circuit. Voltage, on the other hand, is the measure of this potential difference.

4. Is there a formula for calculating resistance and resistivity?

Yes, the formula for resistance is R = V/I, where R is resistance, V is voltage, and I is current. The formula for resistivity is ρ = RA/L, where ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.

5. How do resistance and resistivity affect the flow of electricity?

Resistance and resistivity both hinder the flow of electricity by creating obstacles in the flow of electrons. The higher the resistance or resistivity, the more difficult it is for electricity to flow through a material.

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