Physics & RF
B10 Ohm's Law & Electricity — Study Guide
This guide covers Ohm's Law & Electricity as part of Physics & RF. Start with the vocabulary, then try to answer each question before revealing the answer. Connect these facts to a real radio link or electrical circuit and practice the subject in the arcade when you are ready.
Acronyms first
| Term | Meaning |
|---|---|
| V | Voltage |
| I | Current (from French 'intensité du courant') |
| R | Resistance |
| P | Power |
| DC | Direct Current |
| AC | Alternating Current |
| E | Voltage (EMF) |
| PF | Power Factor |
| VA | Volt-Amperes (apparent power) |
| VAR | Volt-Ampere Reactive |
| SWR | Standing Wave Ratio |
Key facts
In circuits, the letter L stands for…
Inductor / inductance
In circuits, the letter C stands for…
Capacitor / capacitance
Unit of voltage:
Volt (V)
Unit of current:
Ampere (A)
Unit of resistance:
Ohm (Ω)
Unit of power:
Watt (W)
Unit of capacitance:
Farad (F)
Unit of inductance:
Henry (H)
Water analogy: voltage is like…
pressure in a water tank
Water analogy: current is like…
water flowing in a pipe
Water analogy: resistance is like…
a narrowing of the pipe that reduces flow
Who published the law linking V, I and R, and when?
Georg Simon Ohm, 1827
1 amp equals how many electrons per second passing a point?
6.28 × 10¹⁸
Voltage is also called…
electric potential difference
Charges at rest with no conductor are called…
static charge
Ohm's law equation:
V = I × R
To find current, cover I in the triangle. You get:
I = V ÷ R
To find resistance:
R = V ÷ I
Scenario: a 12 V battery drives a 4 Ω load. What current flows?
3 A
Scenario: 2 A flows through a 10 Ω resistor. What's the voltage across it?
20 V
Scenario: 24 V pushes 3 A through a heater. Its resistance?
8 Ω
Basic power equation:
P = V × I
Scenario: a radio draws 2 A at 12 V. Power used?
24 W
How many equations are on the full Ohm's-law power wheel?
12
Transmission line loss formula from the slides:
P_loss = I²R
Why is high current bad for long power lines?
Loss rises with the SQUARE of current, wasted as heat
Who championed DC in the War of the Currents?
Thomas Edison
Who championed AC in the War of the Currents?
Nikola Tesla (with Westinghouse)
In which system does current flow in ONE constant direction?
DC
In which system does current periodically reverse direction?
AC
Scenario: which sources produce DC?
Batteries, solar panels, fuel cells
AC is produced by an alternator — a ____ in a magnetic field.
rotating coil
Main advantage of AC over DC:
Voltage can be stepped up/down with a transformer for efficient long-distance transmission
Main advantage of DC:
Steady, consistent voltage ideal for electronics and battery storage
Scenario: your laptop charger brick converts wall power for the laptop. Which direction?
AC → DC
Conventional current is assumed to flow…
from positive to negative
Electron flow (actual movement in metals) goes…
from negative to positive
Conventional current is still used in circuit diagrams and engineering calculations.
True
Which component opposes current and dissipates energy as heat?
Resistor
Which component stores energy in an ELECTRIC field, like a tiny short-term battery?
Capacitor
Which component stores energy in a MAGNETIC field and opposes changes in current?
Inductor
Which passes AC but BLOCKS DC?
Capacitor
Which blocks high-frequency RF but passes DC — used as an RF choke?
Inductor
Primary purpose of a resistor:
Limit the flow of current
Scenario: a resistor on your EW system smells burnt, is discolored and hot. Likely symptom?
Sudden loss of gain
Scenario: you need a filter that passes some frequencies and blocks others in a jammer. Which pair forms resonant circuits?
Inductors + capacitors (LC network)
ELI means…
Voltage leads current in an inductor
ICE means…
Current leads voltage in a capacitor
In a purely inductive or capacitive AC circuit, the lead is…
90°
Resistive load (heater): phase difference and PF?
0°, PF 1.0
Inductive load (motor, transformer) power factor:
0.6–0.9 lagging
Capacitive load (capacitor banks, long cables) power factor:
0.6–0.9 leading
Maximum power transfer happens when voltage and current are…
in phase
Power factor is the ratio of…
real power ÷ apparent power
Beer mug model: the drinkable beer is…
real power (W)
Beer mug model: the foam is…
reactive power (VAR)
Beer mug model: the whole mug is…
apparent power (VA)
Scenario: your EW team's generator strains and overheats though the wattage looks low. Likely cause?
Poor power factor
Out-of-phase voltage and current in an antenna system cause…
reflected power, heating and high SWR
Practice this module
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