Technician: Electromotive Force

Millivolt Meter

Technician: Electromotive Force

This week’s Tech question is from sub-element 5 (Electrical Principles) group A (units) [T5A11]

What is the basic unit of electromotive force?

A. The volt
B. The watt
C. The ampere
D. The ohm

This is one of the fundamental concepts when dealing with electricity, and therefore radio.  The standard units that we use to describe what we’re talking about.  Just like you measure distance in feet or meters, volume in gallons or liters, and temperature in degrees Celsius and Fahrenheit, we use units to describe the electrical properties of a system.

D) The ohm, is the unit of electrical resistance, or the “difficulty” a particular material or circuit has in passing electrical charge through it.  Open air, wood, and rubber, all have a very high resistance, and therefore make excellent insulators.  Most metals and ionic compounds like salt water have very low resistance, meaning electrical current flows through them quite easily.  The ohm is also used to measure reactance and impedance, which are similar concepts.

C) The ampere is the unit of electrical current, technically speaking, the rate that electric charge moves through something.  Objects with high resistance (see above) will typically see lower current than lower resistance objects, when the same voltage (see below) is applied, since its harder to move the charge through it.

B) The watt is a measure of electrical power.  Either power consumed or power supplied.  Mathematically it is the voltage (below) times the current (above.)  For example, a 100W rated lightbulb, at 110VAC, will draw about .9 amps.  (if P=VI, then I=P/V)

A) The volt is then the correct answer.  Personally, I disagree with the term “electromotive force” when used to describe voltage.  I think “electromotive potential” is a more accurate description, though.  Voltage represents how much “force” is available to move those electrons.  A higher voltage, means a higher force, or potential, to get those things moving.

Think of it this way:  A 1 ton boulder at the top of a 1 mile high hill, has a lot of potential.  If you get that boulder rolling down the hill, there’s not much to stop it.  By the time it reaches the bottom of the hill, its going to be packing a lot of energy.  If you take that same boulder, and put it at the top of a 1 foot high ramp, it will still smash whatever is underneath it, but not with the same speed or energy.

While that’s not an exactly accurate metaphor, it sort of fits.  If you connect a lamp to a 1.5VDC battery, then connect that same lamp to a 12VDC battery….  you can guess which one is going to shine brighter, assuming it doesn’t instantly burn out at 12V.

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