voltage will not generate power.
Open a circuit and no power will
be used in what used to be a
circuit.
The Trick
Question
In Figure 3, we have broken
the circuit at the top. This is
where we had 0.1 amps of
current just before the break. The
trick question is: “With the
voltmeter leads across the break,
what does the meter read?”
Hint: A good quality
voltmeter will have a very high
impedance. It will draw very
little current when operating
to minimize the effect that it
has on the voltage being
measured. In our case, the
voltmeter is perfect and has
an infinite impedance. Nary a
charge shall pass; that is,
current is zero.
Trains of Thought
If you are already sure of
the answer, then this article is
not for you. Some engineers
and technicians will
recognize immediately what
is going on here. If you’re the
least bit unsure, bear with
me. This may be of some
value.
Train A
“There is no current, so there must be no voltage. The
meter would read zero.”
This train doesn’t get you there. This is the most
common rationale that I have seen for this presentation.
The problem is that its premise is false. We can have
voltage with no current. It’s called static electricity. Static
electricity is electricity with no charge flow. Although
batteries will have a small current flow and eventually self-discharge, you can buy a
package of alkaline batteries that
are a year old and still use them
to provide a power source until
they are depleted. Voltage can
be present on the batteries
without current flow all the time
they are in the package.
Train B
“The meter completes the
circuit, but I don’t know what the
current would be.”
This train doesn’t get you
there either. If the meter has an
infinite impedance, then it really
doesn’t complete the circuit. In
this “circuit,” there is zero
current flow. It’s essentially
not a circuit.
Train C
“The resistor is still in the
circuit. It must drop some of
the voltage. However, without
knowing the meter resistance,
we can’t know for sure what
it reads.”
Again, this train doesn’t
get you there. The meter has
an infinite resistance;
therefore, the current through
it and the resistor is zero.
Train D
“The meter resistance is
far higher than the 100 ohm resistor. All the voltage from
the voltage source must drop across the meter. I say 10
volts.”
Train D gets you there. We can have voltage without
current. The meter has a very high impedance (much
higher than the 100 ohm resistor). All the current is
blocked by the meter. The voltage across the meter must
be 10 volts. The entire supply voltage is dropped across
the meter.
Reality
We can get a better feel for why this is
right by setting the trick question up as a
voltage divider as shown in Figure 4.
I also took the liberty of changing the
meter impedance from infinite to a more real
value. Here, we have a one million ohm resistor
where the meter was placed. We can calculate
54 May/June 2018
During troubleshooting, if two devices are in series, we can
sometimes tell the one that is bad by simply measuring the voltage
across them. If the voltage is all across one device, then it must be
preventing current and is possibly the bad device.
Incandescent light bulbs frequently go open-circuit when they
blow. If the supply voltage is measured across the bulb, it's
probably bad.
FIGURE 3. The question.
FIGURE 4. A more real circuit.