2
\$\begingroup\$

I might not have drawn the second one right, but imagine if it was a live wire in a bathtub going through the water and the plumbing to ground instead of returning to neutral, except way more volts.

Is there a difference between the voltages between the two sets of prongs, if the source voltage is the same and high?enter image description here

\$\endgroup\$
3
  • \$\begingroup\$ What’s the voltage between ground and + in the second one? What’s the voltage between ground and + in the first one? \$\endgroup\$ – winny Oct 25 '20 at 19:05
  • \$\begingroup\$ i don't know, im trying to learn, I just said that the source voltage is the same. Is there no difference in voltage between the prongs when the current returns to the source vs when it escapes the circuit to ground? \$\endgroup\$ – Kefas Oct 25 '20 at 19:08
  • \$\begingroup\$ Neutral is near 0V \$\endgroup\$ – Tony Stewart EE75 Oct 25 '20 at 21:29
7
\$\begingroup\$

Your first drawing is an isolated DC power supply such as a battery with no connection to ground. The voltage measured between the terminals will be the battery voltage.

Your second supply has two breaks in the circuit. The supply is still isolated so no current will flow. The voltage measured between the terminals will be zero as the battery negative is not connected to anything. You could test this with a multimeter and a battery: connect one meter lead to an earthed point (cooker / fridge chassis) and the other lead to the positive of a battery (with no connection on the negative).


Then how does my bathtub analogy work? Clearly there is current running there. So does that mean that that current comes back into the source through the ground?

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Your domestic supply is earthed at the supply transformer. Earthing the bath ensures that in a fault the voltage of the bath is kept close to earth potential.

With a high-current earth fault the fuse will blow. RCD / GFCI devices add an additional level of protection.

Also doesn't real current run from the negative,

No. The convention is that current flows from positive to negative. It's true that in metals that the charge carriers are electrons and that they drift in the opposite direction to conventional current. In other circumstances current flows in positive ions. Don't get hung up on this.

... so if I connect the positive of the battery to the fridge, there would be 2 reasons for why the multimeter reads 0.

No, we're reading voltages so it doesn't matter whether current is flowing or not.

Links:

\$\endgroup\$
4
  • \$\begingroup\$ Then how does my bathtub analogy work? Clearly there is current running there. So does that mean that that current comes back into the source through the ground? Also doesn't real current run from the negative, so if I connect the positive of the battery to the fridge, there would be 2 reasons for why the multimeter reads 0 \$\endgroup\$ – Kefas Oct 25 '20 at 19:26
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Oct 25 '20 at 19:42
  • \$\begingroup\$ "The voltage measured between the terminals will be zero" That's because a real voltmeter measures current and then "uses" Ohm's law to compute the voltage, but since the current is zero the voltmeter reads \$0 \text{ mA} \times R_\text{internal} = 0 \text{ V}\$, right? In an ideal voltmeter, the reading would be undefined, since there's no way to compute it, right? \$\endgroup\$ – Alejandro Nava Oct 25 '20 at 23:45
  • \$\begingroup\$ @Alejandro, I usually consider the multimeter's impedance to be high enough to ignore the current and consider it as a fairly ideal voltmeter. In this case of measurement between two isolated circuits the meter will now connect the two circuits but with, typically, 10M resistance. In practice stray pickup will be a problem with the meter reading in this situation. \$\endgroup\$ – Transistor Oct 26 '20 at 12:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.