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Confused what voltage would be depending on its location?

Discussion in 'General Electronics Discussion' started by AndreeU17, May 8, 2016.

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  1. AndreeU17


    Apr 23, 2014
    I want to know if a simple circuit will have its voltage in take even if it becomes and open circuit rather than remaining a closed circuit.
    I tried working a simple circuit using the components and tools circuitlabs have but i personally do not have any clue on how to use their software.

    I want to know what the voltage is between those points? From my knowledge, i understand that a circuit will always have its voltage but will not present its current if the circuit is open. Once the circuit becomes "close" their will be current. However, look at the first schematic. If the switch is open, does that mean that there will be no voltage since the the lead does not lead to the negative terminal? Im just trying to understand if voltage will always have a place whether the circuit is "closed" or "open"? I do understand that current will not play its effect tho!

    If you have any better insight, then please tell me!!! Its a basic question that even to this day i can't seem to remember !

    Thank You!

    Apologies from my lack of understanding.
  2. chopnhack


    Apr 28, 2014
    Potential (voltage) exists between the two terminals of the battery and thus the attached wires. So in your example there is 5v present between + terminal and Voltage Here - both tags(2) and (3). Regardless of the resistor, this is just the potential between the two points. Kind of like height - potential kinetic energy - if you are on the tenth floor of a building, regardless of whether you are on the south side or west side, you are still 10 floors up. Same here, we have 5 'floors' of energy between the + terminal and those points returning to the - terminal. There is no voltage drop across the resistor because the circuit is incomplete and there is no current passing through the resistor.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Voltage is always measured (if it exists) between two points in a "circuit" of any kind, "closed" or "open". Both of your circuit diagrams show "open" circuits because switch SW1 is open in the top diagram and switch SW2 is open in the bottom diagram. Or does your top drawing attempt to show SW1 in the closed position? Either way, read on.

    You measure voltage with a voltmeter, and a voltmeter always has two measuring leads. Often, one of the leads is connected to a point called circuit common, and all voltages are measured using the other voltmeter lead. It is then said that these voltage are all measured with respect to circuit common. In your top diagram, the triangle symbols are a standard representation of circuit common. That means if you measure Voltage Here (2) with respect to circuit common the value will always be zero. Any two points attached to circuit common are all at the same potential with respect to any other point and at zero potential with respect to each other.

    In the top diagram with SW1, R1 and DC Voltage 5V, none of the labels Voltage Here (1) and Voltage Here (2) have any meaning because these labels refer to single points. Voltages are never measured at a single point. Voltages are always measured between two points.

    In the top diagraom, if SW1 is closed, then DC Voltage 5 V will appear across R1 between the Voltage Here (1) point and the Voltage Here (2) point because the triangle "ground" symbols both represent the connection to the negative or "-" pole of the DC Voltage 5V source. You would measure zero potential between the two triangle symbols.

    In the bottom diagram, If SW2 is closed, the same thing happens, DC Voltage 5V appears across R2. You have two identical Voltage Here (2) labels attempting to identify two different points in the circuit. This is not allowed. Each different point must have its own unique label. In any case, it makes no sense to ask "what is the voltage" at either one of the two identically labeled points.

    Now let us suppose you connect one lead of your voltmeter to the negative or "-" terminal of the DC Voltage 5V source. You then connect the other voltmeter lead to any of the labeled points in either the top or the bottom diagram. If in the top diagram you measure Voltage Here (1) you are implicitly making this measurement with respect to the negative terminal of the DC Voltage 5V voltage source. If SW1 is closed, you will measure 5 V. If SW1 is open, you will measure 0 V. If in the top diagram you measure Voltage Here (2) you will always measure zero whether SW1 is open or closed.

    The bottom diagram behaves exactly the same way, even though it doesn't have a triangle symbol to designate circuit common, because you still connect one lead of the voltmeter to the negative terminal of the DC Voltage 5V voltage source.

    As a matter of general principal, you must have at least one two-terminal voltage source in a circuit to have any voltage between any two points of any circuit, no matter how simple or how complicated the circuit. You can also have more than one two-terminal voltage source in any circuit, but if connected in parallel, then both voltage sources must have the same voltage and therefore either one can be replaced by a single source of that voltage.

    Just because you have a voltage source in a circuit doesn't mean you will be able to measure any voltages between two points in the circuit. Your example illustrates this: if the switch connected to the voltage source is open, no voltage appears across the resistor. The DC Voltage 5V voltage source doesn't go away. It sits there, still providing a 5 V potential across its two terminals, no matter what else is going on in the circuit.

    Quit playing around with Circuit Lab. Learn about Ohm's Law and Kirchoff's Laws from text books or on-line tutorials first. Do some text-book problems. To test your knowledge and understanding, solve for the voltage drops across all the resistors and the currents in the resistors by analyzing the circuit below. Do this with pencil and paper and perhaps a desk calculator. Do not simulate this circuit in Circuit Lab. Write your calculations on a sheet of paper, scan or photograph it, and upload the images here.
    Last edited: May 8, 2016
    chopnhack and davenn like this.
  4. Ratch


    Mar 10, 2013
    Voltage is the energy density of the charge. It is measured in the MKS system as joules/coulomb, which is the definition of a volt. When the energy density of a charge (voltage) at one point is higher than the energy density at another point, a current will be present from the higher energy density to the lower energy density, provided a conduction path exists. Any questions about that?

    hevans1944 likes this.
  5. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Ha! You are asking this of someone who doesn't have a clue yet. Maybe we should direct him to the Hyperphysics web site for fundamental reasons. Lots of links there to internal references that will take weeks, maybe years, to explore and understand.
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