Open and Short Circuit questions

  • I am confused on the terms Open, Short, and Closed when talking about circuits. As far as I know:

    • a) Open circuit means the wires are cut off so there will be no current flow, but there is voltage.
    • b) Closed circuit means the wires are connected so there will be flow of current, but there is no voltage
    • c) Short circuit also refers closed circuit.

    Is my knowledge (a,b,c) about the question correct?

    There is also another thing which confuses me:

    • d) Voltage is the force that makes the current flow. How can there be current but no voltage or voltage but no current? (from formula: \$V = I \cdot R\$)

    Please explain a, b, c and d so it won't bother me anymore if I am going to solve circuits..

  • For (a,b,c) that's more or less correct. In general, there doesn't have to be a voltage/current just because there is a short/open, there just can't be any voltage in a perfect short and there can't be any current in a perfect open.

    Another way to re-word these two terms is that a short circuit has 0 resistance (R=0), and an open circuit has infinite resistance (R=infinity).

    So in Ohm's law, \$V = IR\$.

    If \$R = 0\$, then \$V = 0\$.

    If \$R = \infty\$, then using some mathematical trickery:

    $$ I = \lim_{R\rightarrow \infty} \frac{V}{R} = 0 $$

    As far as the force analogy goes, if it's useful think about you pushing on a building. Just because you are applying a force doesn't mean the building is going anywhere. These type of analogies tend to break down when dealing with theoretical 0's and infinities, so I wouldn't rely too heavily on them but rather look at the mathematics.

    +1 for answering the later part of the question, and showing that Ohms law still applies. Questioner do the math. Draw a circuit `V10 — r=1 — R=1 — and back to start` then: repeat at least 6 times: Multiply the value of R by 10 and recalculate. Observe what happens as R gets big. Now do the same, but this time make R become smaller.

  • I may be wrong since I did not study this in English, but I see a major difference between b) and c). From a very practical point of view, a closed circuit is good, a short circuit is very bad !

    A short circuit is, for instance, connecting a wire directly between the poles of a battery or power supply. Whereas a closed circuit is just a "normal" load between the poles. Oops (let me be careful here, I don't want to get sued or anything), don't actually, physically, do it at home (or anywhere else), the wire might melt, you might burn yourself, start a fire, cause the world to stop revolving, etc...

    From \$V = RI\$ : in theory, for short circuit R=0 (this is never actually the case unless you got superconductors) so that I becomes "infinite". Actually again, your battery will deliver its max current, heat up and die quickly, your PSU will either do that also or shut its output down if sanely designed.

    Regarding d), once again, this is the theory: as mentioned above, a wire does have a resistance so there is some voltage (potential) difference when current runs through it. Similarly, there can be leak currents that contradict the "voltage but no current flow".

    I learned with hydraulics analogies, it was rather evocative, but that's a little too long to elaborate about here.

  • You are right about open circuit, the wires are disconnected.

    In case of short circuit as well as closed circuit, the wires ARE connected but the difference is that in case of short circuit, the resistance between the connection is extremely low so very high current flows as per ohm's law, whereas in case of close circuit, the connection offers considerable resistance, hence no high current issue.

  • There are a few misconceptions in your question, and it might help you understand things if we clear those up. For a) and b), replace "there is" with "there may be". The way you have defined an "open circuit" means that there is effectively an open circuit between any two points in a circuit that are not connected together by an ideal wire. When there is not an ideal wire connecting two points in a circuit then it's possible that those two points will have a different voltage. So, removing and ideal wire from a circuit means that it becomes possible for the voltages at those two points to differ.

    Your definition of a "closed circuit" is any two points connected by an ideal wire. By the definition of an ideal wire the voltage at those two points must be the same (the voltage difference must be zero). Current might flow through the wire, depending on where you connected it in the circuit.

    For circuit analysis, it can be helpful to use a zero-ampere current source to represent an open circuit and a zero-volt voltage source to replace a short circuit.

    The reason that Ohm's law doesn't apply here is that the closed circuit is created using an ideal wire, and Ohm's law only applies to resistors. The ideal wire, by definition, has no resistance.

    When considering these circuit analysis concepts it's important to remember that we're talking about ideal circuit elements rather than real circuit elements.

  • A short is a current path with no resistance. All that is determined by what you are measuring across. The reason why you can read current and voltage across a battery is because batteries have internal resistance. Battery by itself is open. When you put your leads on battery, you complete or close the circuit. Shorts bypass the load straight to ground but is not a complete circuit, like circuit was shorted, you owe him some load. You give him the load and make it even, so the deals is now Closed.

    Closed circuit is when everything is connected in the path to ground or reference.

    An open is when there is NO current flow. You can have a voltage reading across an open but there is no current because there is no physical path.

    Best advice is to take a PSPiCE class. There you learn to plot a circuit using coordinates.

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Content dated before 6/26/2020 9:53 AM