The voltage is defined as the integral of the electic field along a line joining two points. Herein is derived for your pleasure (i)the voltage due to a time varying magnetic field and (ii)the voltage that develops across a resistor.

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In terms of an electromagnetic field, the voltage along a closed line is given by the rate of change of magnetic flux through that circuit. To show this, start with one of Maxwell's equations, Faraday's law

curl **E**=-δ**B**/δt

Here **E** is the electric field and **B** the magnetic field. Now integrate over the surface that the closed circuit describes.

∫curl **E**.d**s**=-δ(∫**B**.d**s**)/δt

Stoke's Theorem allows us to express the surface integral of the curl of a vector as a closed line integral ∫_{c} of that vector. Furthermore, the surface integral of the magnetic field is simply the magnetic flux Φ through that surface. So the previous equation becomes

∫_{c}**E**.dl=-δΦ/δt

To simplify matters, assume that the closed line is a circle of radius R. Then the integral of the electic field over the line is simply 2πRE_{//}, where E_{//} is the component of the electric field along the circle. This, in fact, is the voltage around the circle (since voltage is the integral of electric field around the circuit). Thus,

V=-δΦ/δt

It follows that the faster the increase in magnetic flux, the more negative the voltage generated. This equation relates to the voltage due to a

solenoid.

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If there is a steady current flowing through an ohmic resistor, the voltage between the two ends of the resistor is the product of the electric field and the length L of the component.(V=EL). From Ohm's law, we know that the current is proportional to the electric field. The constant of proportionality is the electrical conductivity σ The inverse of σ is the resistivity ρ. Thus we have E=ρL and

V_{resistor}=ρJL

where J, the current density, in this case is defined as the current per unit cross-sectional area in the resistor.It is beyond the scope of this w/u to explain the the total resistance R of the resistor is directly proportional to the resistivity and length and inversely proportional to the cross-sectional area (R=ρL/A).The previous equation may be rewritten

V_{resistor}=RAJ

Of course the cross-sectional area multiplied by the current density is simply the total current I flowing in the resistor (assuming that J is uniform across the area) One finally has an expression for the voltage in terms of the more easily measured quantities R and I.

V_{resistor}=RI

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Obviously, the greater the resistance encountered by the current the stronger the potential difference (voltage) developed across it.