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I know from V=IR that zero current means zero voltage drop, but what if we don't have a resistance?
let's consider the following example (Logical and made with diodes) :
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I know that if Va=0 or Vb=0 all the current will flow to the load, and there will be no current that passes to the right (where Vs is) but why will Vs be zero? What is Vs? (Vs is output voltage, doesn't mean a lot to me), can any one help me please?
Thank you

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    \$\begingroup\$ Null = zero. Tension = Voltage. Mass = load. \$\endgroup\$
    – Eugene Sh.
    Commented Nov 21, 2016 at 21:22
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    \$\begingroup\$ Zero current does not mean zero voltage. Consider a circuit with a 9V battery, switch and resistor in series. When the switch is open (not connected) there is zero current through the switch and there is 9V voltage across the switch. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Nov 21, 2016 at 21:29
  • \$\begingroup\$ The classic example is any charged capacitor. Capacitors can have a voltage across their terminals, but this happens at zero current. (Avoid thinking that currents have a voltage. That's a classic newbie error.) Voltage is actually it's own thing. Voltage the "Electro" part of Electro-magnetism. Think like this: inductors "store current" like a flywheel, while capacitors "store voltage" like a compressed spring. Or this: EM energy-flows can only exist if both voltage and current are present. \$\endgroup\$
    – wbeaty
    Commented Dec 22, 2016 at 3:03

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The thing to remember with voltage is that it's not an absolute value. It's the difference between potentials at two different points in the circuit - that is why it is also sometimes called potential difference.

Your \$V_S\$ is the the potential difference between that point and ground. Sometimes that will be small, such as the voltage drop across a diode (when an input is LOW). Other times it will be large, governed by the current flow through the load and the pullup resistor.

\$V_S\$ can never be 0V in your example circuit simply because the lowest it can be is the voltage drop across a diode. For a normal silicon diode that is around 0.7V. The only way you could get \$V_S\$ to be 0V would be to connect it directly to ground bypassing the diode. It is then 0V WRT ground because it is itself ground.

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