Electrical quantities and Ohm's law

Electricity is a drone’s circulation system

Everything on a drone runs on electricity. The battery feeds energy through wires to the ESCs, motors, flight controller, camera and radios. Wire that is too thin runs hot and drops voltage; a reversed connection can destroy expensive equipment instantly. Understanding basic electrical quantities is therefore a matter of both efficiency and safety.

This course moves from fundamentals to real devices. The first module starts with the four quantities used throughout: charge, current, voltage and resistance.

Charge, current and voltage

Electric charge ($Q$, in coulombs, C) is a property of particles; electrons carry negative charge. In metals such as copper some electrons move freely, which is why metals conduct well.

Electric current ($I$, in amperes, A) is the rate at which charge flows past a point:

A current of 1 A is 1 C passing each second, so a 5 Ah battery holds $5\times 3600=18000$ C. In circuit work we use conventional current, flowing from positive to negative outside the source, the opposite of the electrons’ actual motion.

Voltage ($V$, in volts, V) is the energy per unit charge that a source gives to charge, or that charge gives up passing through a component. In a water analogy, voltage is like the pressure from a raised tank, current is the volume of water flowing each second, and resistance is a narrow pipe.

Resistance and Ohm’s law

Resistance ($R$, in ohms, Ω) is how hard it is for current to flow. For many materials at constant temperature, current is proportional to voltage. This is Ohm’s law:

It can be rearranged three ways depending on what you need: $V=IR$, $I=V/R$ and $R=V/I$.

Power and energy

Electrical power is the rate of energy use, in watts (W). Combined with Ohm’s law it can be written three ways:

Use whichever form contains the values you know. Energy is power times time, $E=Pt$, in joules or watt-hours ($1\text{Wh}=3600\text{J}$). The 74 Wh battery from DRT 111 therefore stores $74\times 3600\approx 266000$ J.

Resistor power ratings

A resistor turns electrical energy into heat, so it has a power rating such as 0.25 W or 5 W. Exceed it and the resistor overheats and burns. A common practice is to choose at least twice the calculated power.

Wire resistance

Wires have resistance too, though very little. It depends on material, length and cross-sectional area:

where $\rho$ is the material’s resistivity; copper at 20 °C is about $1.68\times 10^{-8}\Omega \text{⋅m}$. Longer wire has more resistance; thicker wire has less.

Fuses and overcurrent protection

A fuse is a deliberate weak point designed to open when current exceeds a set value, protecting wiring and equipment from heat and fire. Its rating must be above normal operating current but below what the wiring can carry. For bench testing, a smoke stopper limits current at first power-up: if there is a short, its lamp or fuse takes the load instead of the real circuit.

Module lab

Common mistakes

Summary

• Current is the rate of charge flow, $I=Q/t$; voltage is energy per charge; resistance is opposition to current.
• Ohm’s law $V=IR$ and power $P=VI=I^{2}R=V^2/R$.
• Wire resistance $R=\rho L/A$ causes voltage drop and heat, especially at high current.
• Choose resistor and fuse ratings carefully, and use a smoke stopper at first power-up.

Check your understanding

1. A current of 2 A flows for 30 s. How much charge passes?
2. A 47 Ω resistor carries 0.2 A. What is the voltage across it?
3. A 12 V heater draws 36 W. What is its resistance?
4. What is the resistance of 2 m of copper wire with a 1 mm² cross-section?
5. If current in a wire doubles, by what factor does the heat in the wire change?