# Negative Voltage

Ok, I am starting to understand what voltage is. But i'm sort of confused when it comes to negative voltage. I know that voltage is defined as potential energy per unit charge. I understand it as analogous to "pressure". I don't understand what it means to have negative voltage though. How can "pressure" or even energy be negative?

rcgldr
Homework Helper
The sign convention for potential (voltage) is positive for a field from a positive charge, and negative for a field from a negative charge. In the case of a circuit, a point on the circuit with negative voltage means the potential is negative compared to a "neutral" ground.

CWatters
Homework Helper
Gold Member
Voltage is a relative quantity. Pressure isn't allways a good analogy for voltage because pressure is an absolute quantity.

Consider a 9V battery. The voltage on the +ve terminal is 9V with respect to the -ve terminal. However it's equally true to say that the -ve terminal is at -9V relative to the +ve terminal. You cannot just say that the +ve terminal is at 9V without citing what the reference voltage is.

Frequently we are lazy and don't allways bother to cite what the reference is on circuit diagrams. Sometimes we write a small foot note which says "All voltages are measured with respect to...".

Consider the circuit below.

What is the voltage on Node A relative to node B?
What is the voltage on Node A relative to the 0V node?
What is the voltage on Node B relative to 0V node?

What would you write on the circuit diagram?

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Andy Resnick
Voltage is a relative quantity. Pressure isn't allways a good analogy for voltage because pressure is an absolute quantity. <snip>

'Gauge pressure' is a relative quantity, so perhaps that quantity is more analogous.

rcgldr
Homework Helper
A better analogy for voltage using a constant gravity source such as the gravity near the earths surface is height. Gravitational potential reasonably near the surface of the earth equals g h, where g = 9.80665 m / s^2 and h = height in meters. Treating g as constant, the gravitational potential is a function of height. So if an object is above the surface it has positive gravitational potential, and if an object were below the surface such as inside a hole it would have negative gravitational potential. You could also define gravitational potential to be zero at the top of a 100 meter tower, so that gravitational potential equals g (h - 100). This would result in negative potential for h < 100 and positive potential for h > 100.

The units for gravitational potential is joules / kg, while the units for electrical potential (voltage) is joules / coulomb.

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As CWatters was pointing out, ground is not always the "least potential" in a dual power supply.

The other way to think about it is if you see something like "+5V, Ground and -5V", it's the same thing as "+10V, +5V, 0V" as far as potentials.

sophiecentaur