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EMF, Voltage, Current, Potential Difference etc.

  1. Nov 4, 2006 #1

    danago

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    Hey. At school, we are currently studying electricity, and i am having some trouble understanding some of the concepts.

    From what i understand, EMF is the energy given to each unit of charge, which allows it to flow around a circuit. Why do the charges need energy? Is the energy they recieve converted to kinetic energy?

    Now, voltage really confuses me. Is voltage measured in volts? Or is voltage and the volt completely unrelated? According to my book, 1 joule per coulomb is equal to 1 volt, but my teacher is saying that voltage is the pushing force of charges in a circuit.

    And then theres potential difference...my book explains it in a way that i just dont understand.

    Pretty much everyone in my class is having the same problems. I think the way the book explains things is a bit hard to grasp. So if anyone could maybe explain these terms, that would be great.

    Thanks,
    Dan.
     
  2. jcsd
  3. Nov 4, 2006 #2
    Ok emf is pretty much the same as voltage for most purposes. Voltage and EMF (electromotive force) are potential differences. That is to say if you have two plates seperated by a space (a capacitor?) each with a certain amount of charge (in coulombs) on it there will be a potential difference across it of a certain amount of volts, which depends on the difference in charge.

    Thus you measure voltage across something unlike amperage which is just at a spot. EMF is basically the force a charge feels due to this potential difference. An analogy for current and voltage (albiet a slightly flawed one) is that if you have a an object moving at a certain speed, the voltage is the size of the object and the amperage is the speed. I use this to explain to people why amperage kills you not voltage but it doesnt work in all situations.

    Current is the amount of charge flowing per unit time as per I = q/t (i think) or dq/dt for a function of changing t.

    Hope that helps, if not pm me or keep writing here.
    -G
     
  4. Nov 4, 2006 #3

    danago

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    Thanks for that. Im beginning to understand things now. In a circuit, a cell creates a potential difference right? How does it do so?

    If i was told a light globe has a potential difference of 3V, would that mean that for every coulomb of charge that goes through that tungsten wire, 3J of energy is transformed into light energy?
     
  5. Nov 5, 2006 #4

    andrevdh

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    Energy is a pretty abstract concept, but it makes it easier to understand some processes on a simpler level, that is without considering the real details of the problem.

    The analogy is being made between a mass in a gravitational field and a charge in an electric field.

    A mass will move on "its own" from a region of high gravitational potential energy to a region of low gravitational potential energy. By moving between the two points the gravitational potential energy is converted to some other form of energy. The same concept is applicable in an electric circuit - positive electric charge moves through a circuit from a point where region of high potential excists to a point of low potential - if a conductive path excists between the two points for the current to flow along. The equivalent of voltage or potential difference in an electric circuit would be the amount of gravitational potential energy difference per unit mass between the two points that the mass is dropping through. In the electric circuit the potential energy difference gets converted to other forms of energy (heat, mechanical energy, light, stored electric energy a.o. by the component as the charge flows through it) in the component over which we find the potential difference.
     
  6. Nov 5, 2006 #5

    danago

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    Hmm ok. How is the potential difference created?
     
  7. Nov 5, 2006 #6

    verty

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    Danago, water is a good analogy. A battery is like a pump that pumps water around the circuit. If a switch is open, the pressure will be higher behind the switch because the pump is on. Voltage is difference in electric pressure, if you want to think of it that way. A volt meter measures the difference in pressure between two wires. An ammeter measures how fast the water is flowing.

    Of course, it isn't water that flows but charge. And charge is the sum of the charges of the electrons that are moving. So you can say a battery/cell is a charge-pump.

    I would think of EMF as the battery's torque, although this might be confusing.
     
  8. Nov 5, 2006 #7

    andrevdh

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    Inside of the cell chemical reactions releases the units of charge. Each unit of charge is therefore the result of some chemical process occuring. This means that each unit of charge gets a certain definite amount of energy as it flows out of the high potential terminal of the battery. This energy is expended in the components in the circuit. The electric charge arrives at the low potential terminal with all the energy released in the circuit, unless the terminals are shorted by a wire - which is a dangerous process since the energy will then be deposited back into the battery.

    It should not be thought that the same unit of charge that leaves the high potential terminal of the cell arrives at the low potential terminal with all its energy expended. The energy is passed on to the other units of charge sharing the motion in the circuit. This is achieved via the electric interactions between the units of charge.
     
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