2014 Tech study guide: electrical principles, Ohm’s Law

As some of you may know, the Tech question pool is being updated this year. That means, of course, that I’ll have to update my study guide. 

Below are the first two sections. These are basically unchanged from the last edition, except that I removed the questions about voltmeter and ammeter from the first section and questions about calculating power from the second. This allows readers to focus on the electrical concepts. We’ll cover the voltmeter and ammeter questions, and the power calculation questions later.

pier-chartsI am considering adding the charts shown at right to aid people in remembering what formulas to use when calculating the various parmaters. What do you think? Should I add them, or would that just muddy the waters?

The study guide will show the correct answers in bold. I don’t know what the deal is, but for some odd reason, the bold text doesn’t really show as bold here on the website.

Electrical principles, units, and terms: current and voltage; conductors and insulators; alternating and direct current; resistance; power

You don’t have to be an electronics engineer to get a Technician Class license, but it does help to know the basics of electricity and some of the units we use in electronics. The most important concepts are current, voltage, resistance, power, and frequency.

Voltage is the force that causes electrons to flow in a circuit. Voltage is sometimes called electromotive force, or EMF. Voltage is the electrical term for the electromotive force (EMF) that causes electron flow. (T5A05) The volt is the basic unit of electromotive force. (T5A11)

The letter V is shorthand for volts. About 12 volts is the amount of voltage that a mobile transceiver usually requires. (T5A06)

Current is the name for the flow of electrons in an electric circuit. (T5A03) Electrical current is measured in amperes. (T5A01) Direct current is the name for a current that flows only in one direction. (T5A04) Batteries supply direct current, or simply DC.

Alternating current is the name for a current that reverses direction on a regular basis. (T5A09) Frequency is the term that describes the number of times per second that an alternating current reverses direction. (T5A12) Alternating current, or AC, is what is available from your home’s wall sockets. Power supplies convert the AC into DC, which is required for most modern amateur radio equipment.

Resistance is the term used to describe opposition to current flow in a circuit. The basic unit of resistance is the ohm. The Greek letter omega (?) is shorthand for ohms.

Conductors are materials that conduct electrical current well, or, in other words, have a low resistance. The copper wires that we use to connect a power supply to a radio are good conductors because copper is a good electrical conductor. (T5A07)

Insulators are materials that that have a high resistance. They do not conduct electrical current very well. Plastics and glass, for example, are good electrical insulators. (T5A08)

The term that describes the rate at which electrical energy is used (or generated) is power. (T5A10) Electrical power is measured in watts. (T5A02)

Ohm’s Law: formulas and usage 

Hams obey Ohm’s Law!

Ohm’s Law is the relationship between voltage, current, and the resistance in a DC circuit. When you know any two of these values, you can calculate the third.

The most basic equation for Ohm’s Law is

E = I × R

In other words, when you know the current going into a circuit and the resistance of the circuit, the formula used to calculate voltage across the circuit is voltage (E) equals current (I) multiplied by resistance (R). (T5D02)

When you know the voltage across a circuit and the resistance of a circuit, the formula used to calculate resistance in a circuit is resistance (R) equals voltage (E) divided by current (I). (T5D03) We can also write this formula as

R = E / I

When you know the voltage across a circuit and the resistance of a circuit, the formula used to calculate current in the circuit is current (I) equals voltage (E) divided by resistance (R). (T5D01) This formula is written

I = E / R

The resistance of a circuit in which a current of 3 amperes flows through a resistor connected to 90 volts is 30 ohms. (T5D04)

R = E / I = 90 V / 3 A = 30 ?

The resistance in a circuit for which the applied voltage is 12 volts and the current flow is 1.5 amperes is 8 ohms.(T5D05)

R = E / I = 12 V / 1.5 A = 8 ?

The resistance of a circuit that draws 4 amperes from a 12-volt source is 3 ohms. (T5D06)

R = E / I = 12 V / 4 A = 3 ?

The current flow in a circuit with an applied voltage of 120 volts and a resistance of 80 ohms is 1.5 amperes. (T5D07)

I = E / R = 120 V / 80 ? = 1.5 A

The current flowing through a 100-ohm resistor connected across 200 volts is 2 amperes. (T5D08)

I = E / R = 200 V / 100 ? = 2 A

The current flowing through a 24-ohm resistor connected across 240 volts is 10 amperes. (T5D09)

I = E / R = 240 V / 24 ? = 10 A

The voltage across a 2-ohm resistor if a current of 0.5 amperes flows through it is 1 volt. (T5D10)

E = I × R = 0.5 A × 2 ? = 1 V

The voltage across a 10-ohm resistor if a current of 1 ampere flows through it is 10 volts. (T5D11)

E = I × R = 1 A × 10 ? = 10 V

The voltage across a 10-ohm resistor if a current of 2 amperes flows through it is 20 volts. (T5D12)

E = I × R = 2 A × 10 ? = 20 V


  1. David Ryeburn says:

    All of your division symbols come out as colons, using two very different web browsers on a Mac. I don’t know if they do any better on Windows machines. Perhaps the colons were intended to be the division sign we all used in grade school which looks like a colon with a horizontal dash between the upper and lower dots of the colon, but a slash / would be preferable to that.

    David, VE7EZM and AF7BZ

    • Dan KB6NU says:

      Thanks for pointing that out. I couldn’t seem to get the division sign to display properly, so I did replace them with slashes.

  2. Fred Becchetti says:

    I dont think the nomogram is very useful.
    Also, EMF causes any charge to flow, not just electrons of course.
    (In a battery and human body includes +ions)
    Also using I=E/R implies current flows in direction of electric field (+ to – direction ie high potential to lower potential)
    and hence direction of a positive charges , NOT the electrons. So that is inconsistent notation (and not one we use in our HS/ college classes/standard textbooks on electronics..) .

    • Dan KB6NU says:

      Fred, thanks for your comments. Since the question pool uses the equation I = E/R, there’s not much that I can do about that.

  3. Dave W8RIT says:

    If I were to use the Ohm’s Laws circles (or triangles), I would do it only after providing a brief explanation on what to do with the info. Like to “cover” up the unknown value you wish to find the answer for, if the data values you have are side-by-side: multiply; if they are above and below one another: divide.

    I think that since everyone learns a little differently, it sure could help some people.

  4. In a class of 50, I usually have 10 who thank me for showing them the ERI and PIE circles and explaining how to use them (the rest understand algebraic notation and don’t need it).

    In your explanation of I=Amps, E=Volts and R=Resistance, you’re being inconsistent. It would be consistent to say R=Ohms, since you’re giving the units of measure.

    Perhaps say it like this:
    I=Current, measured in Amperes.
    E=Electromotive Force or Voltage, measured in Volts.
    R=Resistance, measured in Ohms.

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