# Voltage drop greater than voltage supply

#### JackBidds

Joined Jan 10, 2021
9
Ok so I am currently watching a video on Kirchhoff’s Voltage Law and something has been bugging me. In the attached picture we are working out voltage drop/gain along components. Along the top resistor the voltage is 12v before it and somehow it has been calculated that the voltage drop over the resistor 12.5( larger than the voltage before it). How is this possible

#### Attachments

• 1.4 MB Views: 17

#### ericgibbs

Joined Jan 29, 2010
11,551
hi Jack.
Welcome to AAC
Consider the 12V and 8v sources are in series.
E

#### JackBidds

Joined Jan 10, 2021
9
hi Jack.
Welcome to AAC
Consider the 12V and 8v sources are in series.
E
Hi eric. Yes I understand this but the bottom left corner has been labelled 0 volts

#### ericgibbs

Joined Jan 29, 2010
11,551
hi J,
Consider that there is no 0V label, what's the current around the loop. ?
Post the current value.

E

#### JackBidds

Joined Jan 10, 2021
9
Current calculated is 0.25 amps

#### ericgibbs

Joined Jan 29, 2010
11,551
hi J,
OK.
Look at these 4 same circuits with the 0V point in different locations, the 0.25A is always the same.

The 0V label is just a reference point not a applied voltage.

E

#### Attachments

• 8.1 KB Views: 5

#### bertus

Joined Apr 5, 2008
21,024
Hello,

The sum of the voltages accross the resistors should be the same as the sum of the voltage sources.

Bertus

#### JackBidds

Joined Jan 10, 2021
9
hi J,
OK.
Look at these 4 same circuits with the 0V point in different locations, the 0.25A is always the same.

The 0V label is just a reference point not a applied voltage.

E
Ok thanks but why is it called 0v if it’s not actually 0. How can this be possible

#### ericgibbs

Joined Jan 29, 2010
11,551
hi J,
It is just a Common reference point for the circuit.

Say you were using a Voltmeter to measure the voltages around the circuit.
You would clip the -neg probe of the meter to 'A' and use the +positive probe of the meter to measure B , C or D.
So A is the reference point for the voltages.

The 0V means on your circuit, its a reference point, just a Label.

E

#### JackBidds

Joined Jan 10, 2021
9
hi J,
It is just a Common reference point for the circuit.

Say you were using a Voltmeter to measure the voltages around the circuit.
You would clip the -neg probe of the meter to 'A' and use the +positive probe of the meter to measure B , C or D.
So A is the reference point for the voltages.

The 0V means on your circuit, its a reference point, just a Label.

E
Ok I think I get it now, so it doesn’t have to actually be 0 volts

#### ericgibbs

Joined Jan 29, 2010
11,551
hi J,
That's correct.
Only a measuring point reference location label.

E

#### JackBidds

Joined Jan 10, 2021
9
But If the voltage is 0 before the 12v cell and 12.5 at the resistor after the cell how can this be

#### ericgibbs

Joined Jan 29, 2010
11,551
But If the voltage is 0
Hi J,
There is no voltage, it just a label like A,B,C,D

The 0V is not in the resistor loop, its just a reference point.

Consider you had a car, the chassis of the car is connected to the -neg of the battery, the chassis would be considered the 0V reference.

E

#### JackBidds

Joined Jan 10, 2021
9
Hi J,
There is no voltage, it just a label like A,B,C,D

The 0V is not in the resistor loop, its just a reference point.

Consider you had a car, the chassis of the car is connected to the -neg of the battery, the chassis would be considered the 0V reference.

E
So could I pick any number as the reference point not just 0?

#### djsfantasi

Joined Apr 11, 2010
7,187
So could I pick any number as the reference point not just 0?
Yes, but what is that “any” number in reference to? Voltage is always measured as the difference between two points. If you assign some other number - then there is another point in the circuit which should be labeled 0V. To confuse you further, this point could be internal to a component... but it does exist. Careful selection of values will remain consistent and there will be a physical node that is at 0V.

#### JackBidds

Joined Jan 10, 2021
9
Yes, but what is that “any” number in reference to? Voltage is always measured as the difference between two points. If you assign some other number - then there is another point in the circuit which should be labeled 0V. To confuse you further, this point could be internal to a component... but it does exist. Careful selection of values will remain consistent and there will be a physical node that is at 0V.
Yes but if I can just randomly label anywhere as 0 when it’s not then surely somewhere else in the circuit must be 0

#### djsfantasi

Joined Apr 11, 2010
7,187
Yes but if I can just randomly label anywhere as 0 when it’s not then surely somewhere else in the circuit must be 0
i agree, which is what I said in my post

#### JackBidds

Joined Jan 10, 2021
9
i agree, which is what I said in my post
So I could label the point as 10 volts instead of 0 and everything would still work out the same?

#### djsfantasi

Joined Apr 11, 2010
7,187
So I could label the point as 10 volts instead of 0 and everything would still work out the same?
I think the correct answer is no. The voltage at every other point would be +- 10V, since you changed the definition of your reference point.

This is purely theoretical, as no one but you would make that change. The point I am trying to make is that voltage is defined as the difference between two points. If A is 0V and B is 12V, the voltage between the two points is the same as between A=10V and B=22V.

#### RBR1317

Joined Nov 13, 2010
591
If a circuit has no ground reference for measuring voltage against, then there is a standard notation for expressing voltage drops. Vab is the voltage at point A with respect to point B. So Vab = -Vba.