It all depends...
Engineers are always trying to make things as simple as possible. The problem is that sometimes they make things so simple that they don't make
sense! Unfortunately, scientists haven't helped much. As a result, electronics is saddled with some bits of ‘jargon’ which get used a lot, but can be
confusing unless you are familiar with all the unspoken assumptions.
The most common bits of electronic engineering jargon appear when we talk about the voltage on something. For example, with regard to
the circuit shown opposite, it is common to read, or hear, a comment like,
“The voltage on the resistor R7 is 3 V”.
The question is —
What does this mean?
- Does it means that at one end of the resistor the voltage, V1, equals 3 V?
- Or does it mean the voltage at the other end, V2?
- Or does it mean the difference in potential, V1 -V2, between the ends of the resistor, is equal to 3 V?
Strictly speaking, the answer is 'Who knows?'...
An experienced engineer will usually be able to decide the correct answer - they will make an educated guess. In most cases the voltage on
a resistor or capacitor means the voltage difference between its two leads. However, when we are talking about applying an input voltage from some
kind of generator it can mean the voltage nearest the place in the circuit where the input is appearing.
When faced with this kind of problem you can do one of two things:
- ask the person who made the statement just what they meant.
- get a voltmeter and measure the voltage at each end of the resistor (and end to end) and try to decide what value is being stated.
It is not your fault if you are puzzled and irritated by this, the problem is the fault of whoever made the ambiguous statement. Alas, this kind of comment
is very common.
The problem here is the use of a word like ‘on’. In practice it is clearer to use ‘at’ to mean a voltage with respect to zero volts (i.e. ground), and ‘across’ to mean the difference in voltage between the two leads of a component.
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)