The voltage and current relationships shown above apply only to
complete circuits. If the circuit is not complete (an open circuit)
none of the rules for a series circuit apply. Remember, there must be
two things for a voltage difference (voltage drop) to occur across a
resistor, current and
resistance. If there is no current flow, there will be no voltage drop.
If a circuit is incomplete there is no current flow and no voltage drop
across any of the resistors. If you place a volt meter across the
battery you will read the battery voltage (just as in a closed
However, as you move the red probe around the circuit, you will find no
voltage drop as you go around the circuit. If you measure the voltage
across any resistor, the voltage will read zero.
This circuit is incomplete so
there is no current flow.
With no current flow no voltage
can develop across the resistors. A voltmeter will read 0 volts across
Now anchor the black probe at the
negative terminal of the battery. Put the red probe at the positive
terminal. The meter will tell you the battery voltage, which is 30
volts. When you move the red probe to the opposite side of the 10 ohm
resistor you expect a lower voltage, but you get the same voltage. Why
is there no voltage drop? It's because there is no current flowing
through the resistor. No current flow, no voltage drop. You start with
+30 volts, subtract 0 volts and you still have +30 volts. It is the
same across every resistor. Once you reach the end of the circuit
(where the break is) you still have +30 volts.
If the black probe of
the voltmeter is placed on the negative
terminal of the battery there will be no drop in voltage measured as
the red probe is moved around the circuit.
Notice that when you have the red and black probes on opposite
sides of the break that you see the entire battery voltage. This can be
useful to find where the actual break in the circuit is. In the
following circuit the break is in the middle of the circuit. Notice
that when we put the red probe on the side of the break nearest the
positive terminal of the battery we see the battery voltage (30 volts).
However, when we put the probe on the side of the break neares the
negative side of the battery we see 0 volts.
Looking at the voltages on
opposites sides of the break.
Now let's put the probes across the break. Here we see the
battery voltage across the break.
Looking at the voltage across
If we put the probes across the battery we see 30 volts. If we put the
probes across the break we see 30 volts. If we place the probes across
any of the resistors or
any other two points in the circuit (that are not on opposite sides of
the break) we see 0 volts. This tells us where the break is. When we
isolate the two closest points where we see the battery voltage, we
have located the break in the circuit.
Let's find out what is wrong with the following circuit.
Something is wrong here, what is it?
Notice that we see +30 volts on one side of the 15 ohm resistor and 0
volts on the other. Measuring across the resistors we see 0 volts
across the 10 ohm and 5 ohm resistors, but the whole battery voltage
across the 15 ohm resistor. What is happening? Just as above, we see
the battery voltage across the battery and across a break in the
circuit. It looks like the 15 ohm resistor has become an open circuit.
The 15 ohm resistor is open.
Unsticking a sticking point
I have seen people get confused when there is no voltage across a perfectly good
resistor, especially in a working circuit. I think this is because they
had it pounded into them that a resistor will
have a voltage drop. What didn't get pounded in is that this voltage
drop will not exist if there is no current. Let's say you have a 10k
resistor and you measure +10.7 volts on both ends. When you put the
volt meter across the resistor it reads 0 volts. Is the resistor bad?
Maybe, maybe not. If you can prove that there is current flowing
through the resistor then it must actually be zero ohms (if you have
current but no resistance you will get zero volts). Otherwise, the lack
of voltage difference only means there is no current flowing through
Now comes an interesting question. Is there no voltage difference
because there is no current or is there no current because there is no
voltage difference? In the examples above there is no voltage
difference because there is no current. In some circuits, for whatever
reason you may get the same voltage on both sides of a resistor. With
the same voltage on both sides of a resistor, showing that there is no voltage
difference across the resistor, there will be no current flow through
the resistor. Think of it like two people with opposite ends of a soda
straw in their mouths. Each is trying to blow air through the straw.
Unless one blows harder than the other the air goes nowhere. Equal
pressure one opposite ends of the straw means no air flow through the
straw. Equal voltage on opposite sides of a resistor means no current flow through that resistor.