# Common ground and supply voltages

• Funky Oordvork
In summary, the bottom plates of the two capacitors are at different potentials, but the potentials across the capacitors themselves are unchanged.
Funky Oordvork
I have already asked this question at electronics.stackexchange but I still don't get it. I thought I understood it in terms of a water analogy but when I went back to electrons it still didn't make sense to me. My first reaction when encountering the idea of common grounding 2 separate power supplies was that it would alter the voltages of the supplies, but that doesn't seem to happen. So I thought of the simplest example I could (using capacitors instead of batteries etc.). Here are two capacitors acting as power supplies in circuits with very high resistance loads, one at twice the voltage of the other and with twice as much charge on its plates :

http://i.imgur.com/46rPk.png

if I common ground them :

http://i.imgur.com/RGWKK.png

I realize that this does not create an additional closed circuit. However the bottom plates were at different potentials yet they must now be at the same potential. That means the potential of one has increased and the potential of the other has decreased. Meanwhile the potentials of the top plates are unchanged so to my mind the voltages across both capacitors must have changed. Could someone please explain where I am going wrong in terms of potentials and / or electrons, before and after.

Funky Oordvork said:
I realize that this does not create an additional closed circuit. However the bottom plates were at different potentials yet they must now be at the same potential. That means the potential of one has increased and the potential of the other has decreased.
Potential with respect to what? Where are you connecting the leads of your voltmeter?

Funky Oordvork said:
Meanwhile the potentials of the top plates are unchanged so to my mind the voltages across both capacitors must have changed. Could someone please explain where I am going wrong in terms of potentials and / or electrons, before and after.
Again, the top plates are unchanged with respect to what point? Where are your voltmeter leads connected?

The bottom plates may or may not be at different voltages before connection, the voltage difference is undefined. However that does not matter. In this circuit connecting the bottom ends has no effect on each circuit's energy. The right hand still has the same voltage diff across the capacitor, the left is equally unaffected. There is no closed circuit to carry current or provide a voltage reference, so each cct can be analysed completely independently even after connecting the bottom ends.

Assume the original voltage across the right capacitor is 2v and the left is 1v. The potentials at the tops on each side are changed as the charge on the capacitor sets the right hand top as two volts above the bottom, and the left hand top as one volt above the bottom, so the difference of the tops is now defined as 1v.

Last edited:

## 1. What is common ground?

Common ground refers to a shared reference point or connection for different electrical circuits. It is typically connected to the negative terminal of a power supply and is used to establish a common voltage reference for all components in a circuit.

## 2. Why is common ground important?

Common ground is important because it allows for proper communication and functionality between different electrical components. It ensures that all components are operating at the same voltage level, which is necessary for accurate measurements and reliable operation.

## 3. What is a supply voltage?

A supply voltage is the electrical potential or energy provided to a circuit or device by a power source. It is typically measured in volts and is used to power and operate electronic components and devices.

## 4. How is supply voltage determined?

The supply voltage for a circuit or device is determined by the power source it is connected to. For example, if a device is connected to a 9V battery, the supply voltage will be 9V. In some cases, the supply voltage may also be regulated by a voltage regulator to ensure a stable and consistent voltage level.

## 5. What happens if the common ground or supply voltage is not properly connected?

If the common ground or supply voltage is not properly connected, it can result in incorrect readings, malfunctions, or even damage to electronic components. It is important to ensure proper connections to avoid any potential issues with the circuit or device.

• Electrical Engineering
Replies
7
Views
2K
• Electrical Engineering
Replies
21
Views
2K
• Electrical Engineering
Replies
19
Views
9K
• Electrical Engineering
Replies
19
Views
4K
• Electrical Engineering
Replies
6
Views
2K
• Electrical Engineering
Replies
9
Views
3K
• Electrical Engineering
Replies
4
Views
2K
• Electrical Engineering
Replies
15
Views
3K
• Electrical Engineering
Replies
78
Views
4K
• Electrical Engineering
Replies
7
Views
1K