Where does the voltage in a circuit go from 120v down to 0v?

Question

I've always wondered where precisely the voltage on a circuit drops from 120v down to 0v. It's "somewhere" inside of the load (a lightbulb or whatever), but doesn't exactly make sense, because if it's just "It drops to 0 when there's resistance in the circuit", well, there's resistance in the wire the entire time, and it doesn't drop in uniform fashion throughout that circuit. So I figured I'd draw 2 things.

For the theorists: A picture eliminating the typical load to hide behind as an answer.

And the equivalent for the practitioner: A picture of witnessing someone being shocked and trying to know where it's safe to touch them (if at all) to pull them off the line.

Edit: The guy getting shocked shows voltages listed, but no meter / or where the voltage is tested from. But it's implied that if you saw a hot (120v) wire from an outlet, the voltage is the difference between that wire and ground. The idea of the pictures is to help pinpoint the idea that no one is talking about where exactly the point is that the voltage steps down from 120v to 0v.

Answer 1

Think of voltage as electrical pressure.

It doesn't "go" anywhere. It is the difference in potential between point A and point B. You can never measure voltage at a single point, it is always in comparison to another point. So, if you put both leads of your meter on the same electrical point, as you did with the neutral and the ground since they are terminated on the same electrical point, you read zero. Birds sitting on a high voltage wire are only touching the wire at one point and there is no comparison point for a measurable voltage. So they are not hurt.

We create electricity by creating a difference in potential (voltage) between two points and then allowing the difference to flow (current) through a load (resistance) to create work.

As long as you don't get between those two points, you are safe.

Learn more here for free. Go to the education tab for the open source textbook and lectures.

Good luck in you learning and stay safe!

Answer 2

The resistance in the wire is very small, unless something is wrong with the wire (usually a bad connection - wire itself is quite reliable.)

The abstraction that the wire is of zero resistance is close enough to true (in comparison to a load that should ever be connected to that wire) for many purposes, but not true. As such, your meter, set to a low enough range, will actually show some small voltage, not 0, which will depend on exactly where it's connected to the wire. Likewise the "120V" reading will vary slightly depending on where that is connected. But that voltage (in a properly-designed/built/meets code circuit) is small enough to ignore for practical purposes, so we normally do.

Example - a 100 watt light bulb at 120VAC - resistance about 144 ohms. connect it to a typical 15A circuit wired with 14 Ga wire, say it's 100 feet from the panel to the bulb - the wire (on each side) has a resistance of 0.25 Ohms. 120V / 144.5 ohms = 0.83A. The wire (on each side) amounts to roughly 0.208 volts (measured from the panel to the bulb), and the bulb thus sees 119.584 Volts rather than 120V.

Now, if you "get rid of the load" you have roughly 0.5 ohm at 120V, also known as a dead short, and 240 amps flows in the wire, with about 60V drop on each side to the point where they connect. In a properly built circuit, this condition does not last very long at all as the circuit breaker or fuse will quickly disconnect the circuit. In an improperly built circuit, a fire will soon result.

As for not becoming the second victim in an electrocution, you turn the power off. In a typical house that means you find the main breaker and throw it off.

For household voltages, if for some reason you cannot turn the power off, you use an insulating tool (such as a dry wooden broom handle or a long plastic item) to knock the wire away from the victim, or vice versa. Don't try that with power lines (which run at much higher voltages), should you be so unfortunate as to find one of those connected to a person.

In the "pictured scenario" you could also just pull the plug.

Answer 3

Voltage is really a relative measure. Note how you compared the voltage compared to ground. You can sit on a live high voltage wire as long as you don't touch anything that isn't on the same potential. It's not the voltage that kills you it's the voltage difference, (technically it's current but you need a significant voltage differential to conquer the resistance in your body and the killing current current).

For a uniform resistive wire the voltage along the length will drop linearly. If you take 2 resistors of equal value in series across a voltage source you will be able to measure half the voltage from the middle point to each leg of the voltage source.

However the human body is not a uniform resistor.

If you see someone getting electrocuted they are part of a live circuit.

Pulling him away from the live wire may cause him to break contact with the ground before breaking contact with the live wire which may result in current flowing through you to your connection to ground as well.

The safer method of saving someone touching a live wire is to cut of the power or shove with an isolating pole, don't let yourself become part of the circuit. Then call 911/112/999 (whatever the emergency service number is in your area).

Answer 4

In your left drawing the load is shown in yellow. If the load is primarily resistive, such as the heating element of an electric space heater, then the voltage drops progressively and continuously from 120 V on the hot side to 0 V at the neutral conductor. Mid-way it will be 60 V.