Voltage drop greater than voltage supply

Thread Starter

JackBidds

Joined Jan 10, 2021
16
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
 

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ericgibbs

Joined Jan 29, 2010
11,937
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
 

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bertus

Joined Apr 5, 2008
21,168
Hello,

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

Bertus
 

Thread Starter

JackBidds

Joined Jan 10, 2021
16
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
11,937
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
 

Thread Starter

JackBidds

Joined Jan 10, 2021
16
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
11,937
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
 

Thread Starter

JackBidds

Joined Jan 10, 2021
16
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
7,414
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.
 

Thread Starter

JackBidds

Joined Jan 10, 2021
16
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
7,414
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
605
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.
 
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