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Explain the connection between Potential Difference, Current & Ohm's Law

  1. Apr 11, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-4-11_2-59-49.png

    2. Relevant equations
    Ohm's Law
    R = V/I
    V = IR
    3. The attempt at a solution
    here is my answer, i have a feeling im missing something, any help would be appreciated.

    The graph represents a linear relationship between the potential difference (V) and the current (A). By calculating the slope of the graph, you will get the value of the resistance in Ohms. The slope of the graph is constant, demonstrating that the resistance (R) is constant as well.

    I just started learning about electricity and magnetism, and im not really understanding what potential difference means, its making questions like these seem really confusing. Correct me if im wrong, but from what i read, potential difference is either the amount of energy required to move a coulomb of charge from one point to another in a circuit, but then my lesson also says that potential difference is the decrease in electrical potential energy for each coulomb in a circuit. Im a bit confused, can someone explain to me (in english) what potential difference is??
     

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  3. Apr 11, 2016 #2

    andrevdh

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    Gravitational potential energy and electrical potential energy is very much alike.
    In the electrical case the voltage source or power supply sets up the (potential) electical field in the circuit.
    + Electrical charge flows from a high potential to a lower potential if a continuous path excists.
    The charge then ends up at a point of lower electrical potential energy - that is less energy is available to the circuit.
    The source of this energy is the voltage source.
    It does work on the charge as it pushes the charge internally, that is inside of the power supply, up against the electric field.
    This energy is made avialable to the circuit as charge flows through the components in the circuit.

    Hope this helps :smile:
     
  4. Apr 11, 2016 #3
    You are correct in your definition: Electrical Potential difference between two points is the work required to move a unit charge from one point to the other.
    Now, work (seen from the charge) can be positive (external actor does the work on the charge, i.e. gives the energy) or negative (the charge does the work, i.e. gives the energy to the surroundings).

    In a circuit, the power supply is the source of energy and charge (a chemical reaction in a battery for example). It gives the charges energy by placing them all in one location (every charges feel the (repulsive) electric fields of their neighbours, that is why the battery needs to work on the charges to place them there). the difference of potential between one side of the battery (the +), where the positive charges having this energy are located, and the reference (the -) is a positive potential difference. energy is given to the charge: this positive potential difference is called emf.

    In a resistor made of a lattice of atoms, the charges give the energy to the lattice by interacting with it (the lattice gains KE, that translate in T increases vs the surrounding, so heat towards outside the circuit (transfer of energy). Here the potential energy of the charges decreases. The potential energy per unit charge (the electrical potential) will be lower after the resistor than at its entrance. The potential difference is negative: it is called a potential drop.

    Conclusion: both cases are potential differences, it is their sign that differs expressing if the circuit receives the energy (+) or loses it (-). You can imagine a circuit like a box: when the PD is positive it's an entrance to energy, when the PD is negative, it's an exit. The magnitude of the difference shows how much energy moves through the door per unit charge. If you take this quantity per second, you get power delivered (when the PD is +) or power dissipated (when the PD is -).
     
  5. Apr 11, 2016 #4

    CWatters

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    In plain English...

    Potential is another word for voltage so potential difference is the voltage difference between two nodes (eg either end of a resistor).

    Too plain?
     
  6. Apr 11, 2016 #5
    ok so potential difference is basically joules (energy)/coloumb
    it is the amount of energy needed to move each coloumb from one point to the other in a circuit, but it is also the decrease in electrical potential energy for each coloumb of charge. My lesson said that 'the amount of energy gained by each coloumb of charge (through an electrical potential source, such as a battery) is equal to the amount of energy lost by each coloumb as it travels through the circuit to each load.
    thanks for the explanation everyone, im starting to get it now.
    for the actual question, did i explain the connections between potential difference, currrent, and Ohm's Law properly??
     
  7. Apr 12, 2016 #6

    andrevdh

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    The voltage sources acts as an electrical pump.
    It lifts the electric charge up to a higher potential inside of the source.
    This energy is made available or used by the eletrical components in the circuit when the electric charge
    flows downhill from the high potential terminal to the low potential terminal of the voltage source.

    As for the original question, you are 90% there. All that needs to be mentioned that if the graph is directly proportional then we are dealing with an ohmic conductor or alternatively it obeys Ohm's law. If the graph is not directly proportional then the conductor is non-ohmic and it does not obey Ohm's law.
     
  8. Apr 12, 2016 #7
    what do you mean by 'directly proportional'?
     
  9. Apr 12, 2016 #8

    cnh1995

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    It means linear. Graph of I vs V for ohmic materials is a straight line passing through the origin.
     
  10. Apr 12, 2016 #9
    ok thanks for the help :smile:
     
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