# Voltage across a resistor

We have a voltage source of 5 V and a resistor of 100 Ω is connected to it. The other end of the resistor is connected nowhere. To simplify terminal “A” of resistor is connected to 5 V & Terminal “B” is not connected anywhere. So my questions are

• What will be the Voltage value at the Terminal “B” of the resistor? and How?
• Will there be a decrease in Voltage from 5 V or it remains 5 V on the both terminals of the resistor?
• What is Voltage? Is it Voltage at the Terminal “B”? or it’s 5 V - Voltage at terminal “B” ?
• The third question is the one you need to answer first. – Marcus Müller Aug 12 '19 at 9:21
• What is the other point in the circuit to which you are making your voltage measurements? The other side of the 5V supply? Terminal "A"? – Peter Jennings Aug 12 '19 at 9:24
• Try drawing the circuit. There is a great tool on this site. It may clarify the situation for you. – Warren Hill Aug 12 '19 at 9:58

Let's turn your word picture into a real picture so we can see what's happening

simulate this circuit – Schematic created using CircuitLab

Voltage is always measured between two points, so when we quote the voltage of a single point, it's always understood that we are measuring the difference to some reference point.

I've taken the liberty of adding the ground or '0V' symbol. You've told us that 'A' is at 5v, which implies that you're taking the negative terminal of your power supply as your 0V reference.

No current flows through R1, so the voltage across it is zero. This means that B is also at 5v.

Voltage is a potential. A good analogy is 'vertical height' in earth's gravity field, as far as energy goes.

In fact, it's such a well ingrained analogy that you'll notice I drew the diagram with height up the page to represent voltage. Most electronic engineers will do this, as it makes any schematic easier to understand.

• Re, "No current flows through R1, so the voltage across it is zero." If you wanted to expand your answer a tiny bit, that would be a good place to mention Ohm's Law. – Solomon Slow Aug 12 '19 at 17:50
• If you draw the resistor downwards, nothing will change. I don't know whether that information is helpful for @Mark or further confuses him. At least he didn't ask for it. I doubt he can deal with this information at his point of understanding. – Thomas Weller Aug 12 '19 at 18:49
• V=I.R and this is a "pull up" resistor. The air gap between B and ground can be considered of sufficient resistance to be infinite. We could redraw the circuit as a voltage divider with ever increasing values between B and ground. – mckenzm Aug 13 '19 at 0:08

Voltage is normally relative to GND (0 V) (see remark of Elliot below).

When you have two terminals A and B and they are not inside a circuit (where there is a route from VCC or some voltage source to GND), there is no voltage difference.

So the terminal A and B would be both 5V, but no electricity will flow, just nothing happens. The resistor does not change anything, it will reduce the current, but since there is no electricity flowing, it will be 0 A.

Things will change as you connect B to GND. Than B will be 0 V, and there is a voltage difference of 5V, and the resistor changes the current, and the voltage difference over the resistor will be 5 V.

• No, voltage is not always relative to ground. Voltage is always measured between two points, and if we don't specify both points then one of them is usually assumed to be a common reference point. The common reference point may be called ground. – Elliot Alderson Aug 12 '19 at 13:02
• @ElliotAlderson Yes true, 'always' was not good to use, but didn't want it to make it more complex than needed; I changed my answer, thanks for the improvement. – Michel Keijzers Aug 12 '19 at 13:55

What is Voltage? Is it Voltage at the Terminal “B”? or it’s 5 V - Voltage at terminal “B” ?

"Potential difference" is the actual meaning of voltage. When you are given two points with different voltage levels, you have the potential difference across those two points. If one point is at 20V and reference point is at 10V, you have the potential difference (voltage) of (20-10=10V). In another scenario you have the same 20V point but reference point is now at 15V. So what's the voltage now? It's 5V.

When we say voltage we mean potential difference. And when someone says 20V (like I just did above), the reference point is 0V.

simulate this circuit – Schematic created using CircuitLab