Basic electrical quantities
Voltage (U) — measure of the electrical "pressure difference" between two points. Unit: Volt (V).
Current (I) — flow of electrical charge through a conductor. Unit: Ampere (A).
Resistance (R) — measure of how strongly a material opposes current flow. Unit: Ohm (Ω).
Ohm's Law
Ohm's Law links the three quantities:
U = R × I (Voltage = Resistance × Current)
Rearranged:
- I = U / R (Current = Voltage / Resistance)
- R = U / I (Resistance = Voltage / Current)
Cockpit relevance:
- At a fixed voltage (e.g. 14 V system) a device with lower resistance draws more current (landing light).
- Faults (high resistance from corrosion in connectors) produce voltage drops and weaker current flow.
Electrical Power (P)
P = U × I (Power = Voltage × Current). Unit: Watt (W).
Derived:
- P = I² × R = U²/R
Example: a 14 V / 100 W landing light → I = P/U = 100/14 ≈ 7.1 A.
Direct Current (DC) vs Alternating Current (AC)
DC (Direct Current):
- Constant voltage and constant direction of current flow.
- Standard in light aircraft electrical systems (14 V or 28 V DC).
- Source: battery and alternator/generator (after rectifier).
AC (Alternating Current):
- Voltage and current direction alternate periodically.
- Characterised by frequency (cycles per second, unit Hz).
- Examples: airliner electrical systems (115 V AC, 400 Hz), public mains (230 V, 50 Hz in Europe).
Frequency (f) — number of cycles per second, unit Hertz (Hz).
Circuits: series and parallel
Series:
- Loads connected one after another.
- Current the same everywhere: I_total = I_1 = I_2 = …
- Voltages add: U_total = U_1 + U_2 + …
- Resistances add: R_total = R_1 + R_2 + …
- Disadvantage: Failure of one load breaks the circuit.
Parallel:
- Loads connected between the same two points.
- Voltage the same everywhere: U_total = U_1 = U_2 = …
- Currents add: I_total = I_1 + I_2 + …
- Reciprocals of resistance add: 1/R_total = 1/R_1 + 1/R_2 + …
- Advantage: Failure of one load does not affect the others.
- Aircraft electrical systems are wired in parallel — safety requirement.
Magnetic field and electric current
Faraday's law: an electric current produces a magnetic field.
Aircraft applications:
- Electromagnets: solenoids in starters, fuel valves.
- Generator/alternator: a conductor moving in a magnetic field generates a voltage (induction).
- Relay: electromagnetic switch — small control currents switch large power currents.
- Speakers and solenoids: current through coil produces motion in a magnetic field.
Conversely: a changing magnetic field induces a voltage in a conductor (Lenz's law, induction) — the principle of a generator.