Voltage is always measured (if it exists) between

**two** points in a "circuit" of any kind, "closed" or "open". Both of your circuit diagrams show "open" circuits because switch SW1 is open in the top diagram and switch SW2 is open in the bottom diagram. Or does your top drawing attempt to show SW1 in the closed position? Either way, read on.

You measure voltage with a voltmeter, and a voltmeter always has

**two** measuring leads. Often, one of the leads is connected to a point called circuit common, and all voltages are measured using the other voltmeter lead. It is then said that these voltage are all measured with respect to circuit common. In your top diagram, the triangle symbols are a standard representation of circuit common. That means if you measure

**Voltage Here (2)** with respect to circuit common the value will always be zero. Any two points attached to circuit common are all at the same potential with respect to any other point and at zero potential with respect to each other.

In the top diagram with SW1, R1 and DC Voltage 5V,

**none** of the labels

**Voltage Here (1)** and

**Voltage Here (2)** have any meaning because these labels refer to

**single** points. Voltages are never measured at a

**single** point. Voltages are always measured between

**two** points.

In the top diagraom, if SW1 is closed, then

**DC Voltage 5 V** will appear across R1 between the

**Voltage Here (1)** point and the

**Voltage Here (2)** point because the triangle "ground" symbols both represent the connection to the negative or "-" pole of the

**DC Voltage 5V **source. You would measure zero potential between the two triangle symbols.

In the bottom diagram, If SW2 is closed, the same thing happens,

**DC Voltage 5V** appears across R2. You have two

**identical** **Voltage Here (2)** labels attempting to identify two

**different** points in the circuit. This is not allowed. Each different point must have its own unique label. In any case, it makes no sense to ask "what is the voltage" at either one of the two identically labeled points.

Now let us suppose you connect one lead of your voltmeter to the negative or "-" terminal of the

**DC Voltage 5V** source. You then connect the other voltmeter lead to any of the labeled points in either the top or the bottom diagram. If in the top diagram you measure

**Voltage Here (1)** you are implicitly making this measurement with respect to the negative terminal of the

**DC Voltage 5V** voltage source. If SW1 is closed, you will measure 5 V. If SW1 is open, you will measure 0 V. If in the top diagram you measure

**Voltage Here (2)** you will always measure zero whether SW1 is open or closed.

The bottom diagram behaves exactly the same way, even though it doesn't have a triangle symbol to designate circuit common, because you still connect one lead of the voltmeter to the negative terminal of the

**DC Voltage 5V** voltage source.

As a matter of general principal, you

**must** have at least one two-terminal voltage source in a circuit to have any voltage between any two points of any circuit, no matter how simple or how complicated the circuit. You can also have more than one two-terminal voltage source in any circuit, but if connected in parallel, then both voltage sources must have the

**same** voltage and therefore either one can be replaced by a single source of that voltage.

Just because you have a voltage source in a circuit doesn't mean you will be able to measure any voltages between two points in the circuit. Your example illustrates this: if the switch connected to the voltage source is open, no voltage appears across the resistor. The

**DC Voltage 5V** voltage source doesn't go away. It sits there, still providing a 5 V potential across its two terminals, no matter what else is going on in the circuit.

Quit playing around with Circuit Lab. Learn about Ohm's Law and Kirchoff's Laws from text books or

on-line tutorials **first**. Do some text-book problems. To test your knowledge and understanding, solve for the voltage drops across all the resistors and the currents in the resistors by analyzing the circuit below. Do this with pencil and paper and perhaps a desk calculator. Do not simulate this circuit in Circuit Lab. Write your calculations on a sheet of paper, scan or photograph it, and upload the images here.