Technical Basics
The three quantities
Ohm's law and power are the backbone of this domain. Learn V = I × R, P = V × I, and the two derived forms P = I²R and P = V²/R. Most numerical questions are one substitution away. Watch units: milliamps must become amps, and kilohms must become ohms, before calculating.
Series and parallel
Resistors in series add. Resistors in parallel give a value smaller than the smallest resistor; for two resistors, R = (R1 × R2)/(R1 + R2). The trap is assuming parallel always halves - it only does so for two equal resistors.
AC, frequency and wavelength
Alternating current reverses direction periodically; frequency is cycles per second in hertz, and period is 1/f. Wavelength follows λ = 300 / f(MHz) in metres. So 145 MHz gives about 2 m, and 7 MHz gives about 40 m. A frequent trap is mixing metres and centimetres, or using frequency in kHz in that formula.
Worked example
A 12 V supply feeds a 100 Ω resistor. Current I = V/R = 12/100 = 0.12 A (120 mA). Power P = V × I = 12 × 0.12 = 1.44 W, or V²/R = 144/100 = 1.44 W. If that resistor were in parallel with another 100 Ω, the pair would be 50 Ω and the current would double.
Decibels and the dB trap
A 3 dB change doubles or halves power; 10 dB is a factor of ten. Questions ask for the ratio, not the absolute power, so read carefully whether the answer is in dB or watts.