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.
I 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