Electromotive Force and Current

[MCATMountain]

Hey guys,

Does electromotive force oppose current? The electromotive force is defined in the Berkeley book as moving charges from an area of low potential to high potential. (As a side note, does this mean it can do work?). Current on the other hand is a movement from high potential to low potential. Wouldn't this mean that electromotive force opposes current?

Secondly, the formula for electromotive force is = Vab = I x R. And I = electromtoive force/R+r. Could someone explain these two formulas concepts? They don't seem to make sense.

Thanks in advance!

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[Temperature101]

Hey guys,

Does electromotive force oppose current? The electromotive force is defined in the Berkeley book as moving charges from an area of low potential to high potential. (As a side note, does this mean it can do work?). Current on the other hand is a movement from high potential to low potential. Wouldn't this mean that electromotive force opposes current?

Secondly, the formula for electromotive force is = Vab = I x R. And I = electromtoive force/R+r. Could someone explain these two formulas concepts? They don't seem to make sense.

Thanks in advance!

For all practical purpose electromotive force is a battery with an internal resistance (that small r in the formula).. Therefore, it can be considered as voltage source... The formula is Ohms' law... V = RI since V in that case is the electromotive force (voltage), they use it to find the current and the small r is the resistance inside the battery (electromotive force). The big R is the resistance of the circuit...

 

[MCATMountain]

For all practical purpose electromotive force is a battery that has an internal resistor (that small r in the formula)... If it is a battery, therefore, it is a voltage supplier... The formula is Ohms' law... V = RI since V in that case is the electromotive force (voltage), they use it to find the current and the small r is the resistor inside the battery (electromotive force). The big R is the resistor of the circuit...

Thanks! Ok, so electromotive force is simply a battery with resistence.

Now, what about the idea of electromotive force vs. current? Does the force oppose current, or is this completely off? For example, which way are the elctrons/positive charge/current flowing with regard/vs the electromotive force?

 

[Temperature101]

Thanks! Ok, so electromotive force is simply a battery with resistence.

Now, what about the idea of electromotive force vs. current? Does the force oppose current, or is this completely off? For example, which way are the elctrons/positive charge/current flowing with regard/vs the electromotive force?

I think there is force in the direction of the current, but there has to be also another force in the opposite direction to keep the potential difference across the battery terminals... I think TBR explains that stuff very well if I recall...

 

[MCATMountain]

I think there is force in the direction of the current, but there has to be also another force in the opposite direction to keep the potential difference across the battery terminals... I think TBR explains that stuff very well if I recall...

Hmm. I guess we'll wait until someone else posts. Thanks for the help though!

 

[Temperature101]

Hmm. I guess we'll wait until someone else posts. Thanks for the help though!

Yeah... I read about that stuff about 2 months ago...I am not sure if I remember all the intricate details about it. It would be good if someone else chime in on it.

 

[QrtrLifeCrisis]

Thanks! Ok, so electromotive force is simply a battery with resistence.

Now, what about the idea of electromotive force vs. current? Does the force oppose current, or is this completely off? For example, which way are the elctrons/positive charge/current flowing with regard/vs the electromotive force?

EMF = voltage (provided by a battery or by a magnetic force).

Without voltage, you don't have current.

 

[Temperature101]

EMF = voltage (provided by a battery or by a magnetic force).

Without voltage, you don't have current.

Yeah.. I think that all OP should know; however, it seems like OP wants to know every intricate details regarding EMF...

 

[QrtrLifeCrisis]

Yeah.. I think that all OP should know; however, it seems like OP wants to know every intricate details regarding EMF...

There's not much more to it than that.

The EMF provides the voltage (or potential difference) that drives a positive charge (the current) from high potential to low potential. It is called "electromotive force" for historical reasons, but it is NOT a force. The unit for EMF is volts.

 

[Temperature101]

There's not much more to it than that.

The EMF provides the voltage (or potential difference) that drives a positive charge (the current) from high potential to low potential. It is called "electromotive force" for historical reasons, but it is NOT a force. The unit for EMF is volts.

I guess you are right because that is all I know about it.

 

[MCATMountain]

There's not much more to it than that.

The EMF provides the voltage (or potential difference) that drives a positive charge (the current) from high potential to low potential. It is called "electromotive force" for historical reasons, but it is NOT a force. The unit for EMF is volts.

See, thats what got me confused. The book says that EMF drives charge from low potential to high. Therefore, opposing current. I'll look again though. :|
Thanks for the help.


Edit: Looked it up. Here are the exact words." If there is a separation of charge between two points on a device like a battery, adn there is a force moving the charge from lower potential to higher potential, then that device is referred to as a source of electromotive force." - Page 177 of the second physics book.

 

[milski]

See, thats what got me confused. The book says that EMF drives charge from low potential to high. Therefore, opposing current. I'll look again though. :|

It can be driving negative charges (electrons) from low to high potential. Ignoring their masses (which you can do most of the time), the two are the same anyway. The convention is to talk about current as movement of positive charges but a lot of times the actual movement is of negative charges in the opposite direction.

 

[QrtrLifeCrisis]

Edit: Looked it up. Here are the exact words." If there is a separation of charge between two points on a device like a battery, adn there is a force moving the charge from lower potential to higher potential, then that device is referred to as a source of electromotive force." - Page 177 of the second physics book.

They might be referring to negative charges (electrons).

Conventional currents are positive charges that move from high potential to low potential.

Real currents are negative charges (electrons) that move from low potential to high potential.

Again, these are based on historical reasons. Also, it is not a force, per se. It is a potential difference that is driving the movement of these charges.

The only time that I can think of where current is being opposed is when a capacitor is involved. When a capacitor is completely charged, a counter voltage is set up at the capacitor end that stops the movement from current from the voltage source.

 

[MCATMountain]

Makes sense. Thanks for the replies guys.

 

[neurodoc]

I think the best way to understand all this is to understand the basic electromagnetic units used to describe electrical charge, electrical potential energy, electrical current, and resistance. The unit of electrical charge is the Coulomb. The charge of one electron (the so-called "elementary charge," is equal to about 1.602 x 10 to the -19 Coulombs. By definition, electrons have negative charge. The coulomb is thus a pretty big quantity of charge. An Ampere is the unit of electrical current (flow of charge), equal to one Coulomb of charge passing through a defined point in space (where the current flow is measured) in one second, again a very large number of "elementary" electrical charges.

Coulomb's Law (which resembles Newton's equation for the gravitational force between two masses separated by a distance), describes the force that exists between two charges separated by a distance.

Voltage is defined as energy/unit charge, or Joules/Coulomb, and represents potential energy of charges in an electrical field (the presence of charges creates the electrical field). In order to move a charge in an electrical field, the movement of the charges either requires energy or dissipates energy, depending on the direction the charge moves in the electrical field. Voltage is the same as "electrical potential (energy) difference between two points in an electrical field. This potential energy difference is the same as Electromotive Force. Again, the sign of the force depends on the direction of movement in the electrical field.

You note that: electromotive force is = Vab = I x R. And I = electromtoive force/R+r. This is a formulation of Ohm's Law for DC electrical circuits (V=IR), with the "R+r" describing the internal resistance of a battery along with the external resistance of the circuit (both in Ohms).

It's all pretty straightforward if you understand the very basic electromagnetic theory you're expected to know for the MCAT. It could be a lot more complicated, but the MCAT doesn't expect premeds to know more than you get in Physics 1A/1B.😛