T5C

Capacitance, inductance and power

Say what capacitors and inductors store, write kHz and MHz correctly, define impedance, and calculate DC power.

12 pool questions1 on the exam

Syllabus: Capacitance and inductance terminology and units; Radio frequency definition and units; Impedance definition and units; Calculating power

Capacitors store electric fields, inductors store magnetic fields

Both parts store energy, but in different kinds of field.

Red arrows: electric field. Blue loops: magnetic field.

RememberCapacitance: electric field, farad (F). Inductance: magnetic field, henry (H).

kHz and MHz: mind the capital letters

Radio signals are alternating currents, counted in thousands (kilohertz) or millions (megahertz) of cycles per second.

k is lowercase, M is capital, and Hz is always capital H, lowercase z.

RememberLowercase k, capital M, always capital H and lowercase z.

Impedance: total opposition to AC, in ohms

Resistance opposes any current. Capacitors and inductors add reactance, which opposes only AC. Impedance is the combined opposition to AC, and like both of them it is measured in ohms.

All three are measured in ohms (Ω).

RememberImpedance = opposition to AC current flow. Unit: ohm.

Power is volts times amperes

In a DC circuit, watts = volts × amperes. Picture a rectangle: width is voltage, height is current, area is power.

DC powerP = E × I

Width is volts, height is amperes, area is watts.

Find

Voltage (E)13.8 VCurrent (I)10 A

Power (P = E × I)138W

Find	Formula	Example
Power P	P = E × I	13.8 V × 10 A = 138 W
Power P	P = E × I	12 V × 2.5 A = 30 W
Current I	I = P ÷ E	120 W ÷ 12 V = 10 A

RememberMultiply volts by amperes for watts. To find current, divide watts by volts.

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Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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