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Resitor principle

  1. Oct 11, 2014 #1
    Dear all,
    I would like to ask you one basic thing. I know exactly how resitor works on electrical base. But I want to know if I know good how it works on physical base.

    This is my knowledge from primary school (for basic circuit).
    Input current and output current in circuit is still equal. This current depends on resistor parameters.

    But I am confusing by this problem:
    How can resistor influence current size in way, that this current is same on input and aslo output. Because if current is decreasing by conversion of energy to another energy, than this current should be different on input to resistor and output of it.

    Thank you very much.
     
  2. jcsd
  3. Oct 11, 2014 #2

    phinds

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    current flowing in a circuit is like a bicycle chain ... it flows equally at all points around the circuit. Anything that causes the current to change anywhere in a circuit causes the change to occur at all points in the circuit.
     
  4. Oct 11, 2014 #3

    Drakkith

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    Staff: Mentor

    Current doesn't decrease when you convert energy to another form. In fact, the more energy you convert, the more current you generally have. This is easy to see:

    Let's say we have a resistor of 100 ohms in a circuit with 10 volts. Current through the circuit is I=V/R, so I=10/100 or 0.1 amps. Power is P=IV, or P=0.1 x 10, giving us 1 watt of power.

    Now let's change the resistor to 10 ohms. Current is now I=10/10 or 1 amp. Power is P=1 x 10, or 10 watts. So by decreasing the resistance, we've increased the current flow and increased the power usage of the circuit.

    Also, think of your resistor this way. If current flow into the resistor was different than current flow out, you would build up an imbalance of charges whose electric field would eventually oppose the current and soon all current would stop flowing.
     
  5. Oct 11, 2014 #4
    Thank you for replies. But I still do not understand how resistor influence current on physical base.
     
  6. Oct 11, 2014 #5

    Drakkith

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    Staff: Mentor

    I think it may help to think about it in terms of voltage across the resistor instead of current. Current doesn't flow through the resistor simply because it is flowing throughout the circuit, it flows because there is a voltage across the resistor that makes the electrons within the resistor itself flow. In other words, the electric field is causing electrons to flow within the resistor, regardless of what is happening to the rest of the circuit. Adding or subtracting other components from the circuit simply makes the voltage across the resistor change, which affects current flow through it.

    For example:

    Let's say we have a simple circuit with 10 volts applied to a single resistor of 10 ohms. For the purposes of this example we are considering the voltage source and the connecting wires to have 0 resistance. In this case, ALL of the voltage is "dropped" across the resistor. This just means that their is a difference of 10 volts between one side of the resistor and the other. 10 volts across 10 ohms is 1 amp of current.

    Let's add a resistor:

    Now we have a second resistor, also 10 ohms, placed in series with the first resistor. Now if we measure the voltage drop across resistor 1 we will find that it has decreased from 10 volts to 5 volts. Resistor 2 also has 5 volts dropped across it. So total resistance of the circuit has gone from 10 ohms to 20, which by Ohm's law means that the current falls from 1 amp to 0.5 amps. But what about each resistor? Let's look:

    Resistor 1 has 5 volts dropped across it. 5 volts divided by 10 ohms is 0.5 amps.
    Resistor 2 also has 5 volts dropped across it, so the current through the resistor is also 0.5 amps.

    So total current through the circuit is 0.5 amps, which makes sense seeing how components in series with each other always have equal current flow through them.

    But, what about a parallel circuit?

    Let's say we put resistor 2 in parallel with resistor 1 instead of in series. The voltage is shared across parallel components, so each resistor has 10 volts dropped across it instead of 5 like they did when they were in series.

    Resistor 1 has 10 volts dropped across 10 ohms, giving us 1 amp of current.
    Resistor 2 also has 10 volts dropped across 10 ohms for a current of 1 amp.
    Current through a set of parallel components adds, so we have 1 amp + 1 amp, giving us a total of 2 amps through the circuit as a whole.

    This is the same as taking the "equivalent resistance" of the components and using that to find the current:

    Two parallel resistors at 10 ohms each gives us an equivalent resistance of 5 ohms. So, 10 volts divided by 5 ohms is 2 amps through the circuit, which agrees with the above.

    As you can see, it is the voltage dropped across each resistor that determines the current flow through that resistor. The reason that current flow through the components of a series circuit is always the same is that the voltage ends up being divided across each component in just the right amount. A high resistance component will have a proportionally higher amount of voltage drop across it when compared to a low resistance component so that the current flow through each one is equal.
     
  7. Oct 12, 2014 #6
    Dear Drakkith,
    thank you for comprehensive answer.

    I tried to use the same way of thinking before, but I had one problem which I could not explain.

    When we have simple circuit with one resistor, which have 10 Ohms resitance and 10 volts voltage source, than the current flows throught circuit is 1 Amps. But when we have the same circuit with 20 Ohms resistance the current flows throught it is 0,5 Amps. Why????

    The voltage drop is the same in both cases, but the current is different.
     
  8. Oct 12, 2014 #7
    First model was proposed by Drude: http://en.wikipedia.org/wiki/Drude_model
    But for some features this model isn't adequate. For more correct model of material conductivity you need to know a lot of quantuum physics.
     
  9. Oct 12, 2014 #8
    Dear all,
    I think I found out the answer.

    We can imagine these two cases (10 Ohms resistor and 20 Ohms resistor) as two bottles (the same capacity) with different diameter of throat. So when we have smaller diameter of throat (20 Ohms) water will flow in smaller quantity as bottle with bigger diameter of throat (10 Ohms). So we can imagine resistor as throat of bottle.
    So whole current is influenced by resistance of resistor with biggest resistance in circuit. Because this resistor does not allow by its molecular structure (it has smaller diamer - so electrons cannot flow in bigger quantity, here is nice animation http://www.regentsprep.org/Regents/physics/phys03/bresist/default.htm - first animation) to flow current in more quantity as resistor with bigger diameter..

    So for example:
    We have two resitors one has 10 Ohms resistance and one has 20 Ohms resistance. Current flows throught circuit with these two resistors cannot be higher than current flows throught circuit with just one resistor with bigger resistance (20 Ohms).

    Voltage (10 Volts), resistor (20 Ohms), current (0,5 Amps).

    Voltage (10 Volts), resistors (20 Ohms + 5 Ohms), current (0,4 Amps). We can add resitor with any low resistance, but current will be still influenced by biggest resitor in circuit.

    Do you agree with this explanation?

    Thanks
     
  10. Oct 12, 2014 #9

    sophiecentaur

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    That is more of an attempted analogy than an 'explanation'. The best way is to stick within 'Electricity' for an explanation.
    It is much better, imo, to try to avoid the idea of a force 'against' the flow. Just consider that it 'removes' energy from charges, as they pass through a conductor. Wide and narrow tubes with water flowing through them do not help to explain this energy thing so steer clear of that if you want to get an understanding.

    A small amount of Maths will describe the relationships far better than arm waving and words: The quantity that we call Resistance is simply the Potential Difference (Voltage) across a device ('resistor') that needs to be applied to the resistor for each Amp of current that passes. (R = V/I). Also, Potential difference is the amount of energy **lost / used / transferred when a charge flows. One Volt of PD corresponds to one joule of energy per coulomb that flows through a device ( E=QV). Those two equations can be jiggled around to give you the Power dissipated in a resistor and a lot more. Getting familiar with the Maths will help you accept what happens but an 'explanation' is only really achievable by keeping 'electrical'.
    **This energy transfer can be described at all sorts of levels. On a trivial basis, you can talk in terms of the electrons bumping into atoms, on the way through, and losing some of their energy. The Drude model (referred to earlier) is a Classical approach to describe the electrons losing energy as they progress through the lattice of ions. There is a much better QM model that takes you further. It all depends on how far you want to go.
     
  11. Oct 12, 2014 #10

    davenn

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    no,
    the current is influenced by ALL as in the total resistance of the circuit

    A side note ... there are 2 s's in resistor ;)

    Dave
     
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