Currents & Concepts
How Energy Moves, Powers, and Burns in Every Electrical Circuit
Recap: Ohm's Law
Ohm’s Law is the cornerstone of all DC electrical circuits and a foundation for understanding more complex AC and RF behavior.
V = I X R
V = Voltage in volts
I = Current in amperes
R = Resistance in ohms
This equation tells us that if you know any two of these values, you can calculate the third. This is more than math—it’s a predictive model for how circuits behave.
Introducing Power: The Workhorse of Circuits
Definition: Electrical power is the rate at which energy is used or transferred in a circuit.
Unit: Watts (W)
Formula: P = V X I
Where:
P = Power in watts
V = Voltage in volts
I = Current in amps
This tells us how much energy is being consumed or produced at any moment.
Combining Ohm’s Law with Power
Using Ohm’s Law, we can derive two more versions of the power formula:
Substitute V = I × R into the power equation:
P = I² X R
Note: Useful for calculating power loss in resistive components (e.g., wires, resistors).
Substitute I = V / R into the power equation:
P = V² / R
Note: Useful for voltage-drop situations, or when designing circuits around a known voltage source.
Example Table: Real-Life Scenarios
The relationship between these three properties is defined by Ohm’s Law:
V = I × R
Where:
V is voltage (in volts)
I is current (in amps)
R is resistance (in ohms)
Rearranged:
I = V / R
R = V / I
Example Table: Real-Life Scenarios
|
Situation |
Voltage |
Resistance | Current | Power | |
|---|---|---|---|---|---|
| A 13.8V mobile radio with 10A draw | 13.8V | — | 10A | 138W | |
| A 9V battery across 100Ω resistor | 9V | 100Ω | 0.09A | 0.81W | |
| 1A through a 5Ω resistor | — | 5Ω |
1A |
5W |
Why This Matters in Radio
Power Budgeting: Know how much current your rig will pull from your power supply or battery bank.
Heat Management: Resistors, linear amplifiers, and transmission lines dissipate power as heat. Use P = I² x R to calculate that.
Antenna Tuners & Coax Losses: Even a few ohms of loss can eat up watts of RF power. Understand where your watts go!
Fuses & Safety: Overpowering a component = fried gear. Use power ratings to size fuses and components properly.
Bonus Concept: Efficiency
Not all power goes to signal. Much is lost as heat. Efficiency matters:
Efficiency = Output Power / Input Power X 100%
A Class A amplifier may be only 30% efficient.
A Class C or D can be 80%+ efficient, depending on use case.
In ham radio, QRP (low-power ops) often depends on squeezing the most performance out of the least power.
Visual Add-Ons
A printable Ohm’s Law wheel (a diagram showing all variations)?
A power calculator cheat sheet?
A flow diagram showing how power moves through a transmitter to the antenna?
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