Current Electricity
When charges flow, magic happens. A river of electrons moving through a wire creates the current that powers civilization.
Start with the simplest version: this lesson is about Current Electricity. If you can explain the core idea to a friend using everyday language, examples, and one clear reason why it matters, you have moved from memorising to understanding.
When charges flow, magic happens. A river of electrons moving through a wire creates the current that powers civilization. Current electricity is the movement of electric charge, and it's fundamentally different from static electricity—instead of charges sitting still, they're on the move. This chapter explores how charges flow, what forces make them move, and how to control their flow using resistance and voltage.
From Static to Dynamic
In Chapter 1, we studied electric-charges-and-fields at rest. Now those charges are moving. When you flip a light switch, you're creating a pathway for electrons to flow from one terminal of a battery to the other. The potential difference from electrostatic-potential-and-capacitance is what pushes them along, like how altitude difference makes water flow downhill.
Electric Current
Electric current is defined as the charge flowing past a point per unit time:
I = Q/t
Measured in amperes (A). One ampere means one coulomb of charge passes a point every second. Current is like the flow rate of a river—not the total water content, but how much water passes a fixed cross-section each second.
There's a conceptual subtlety: in a metal wire, electrons flow from negative to positive terminal (from lower to higher potential). But by convention, we define current as flowing from positive to negative—the direction that positive charges would move. This is historical accident, but it's universal, so we stick with it.
Drift Velocity vs. Electric Field
Inside a conductor, electrons don't zoom through at the speed of light. They drift slowly through the material, typically a few millimeters per second. This drift velocity depends on:
I = nAve
Where:
- n is the number density of free electrons
- A is the cross-sectional area of the wire
- v is drift velocity
- e is the electron charge
Despite the slow drift speed, current flows instantly because the electric field (not the electrons) propagates at nearly the speed of light through the wire.
Ohm's Law and Resistance
Electrons collide with atoms as they drift through a conductor. This resistance opposes the flow. Ohm's Law relates voltage (potential difference), current, and resistance:
V = IR
Or equivalently: R = V/I
Resistance is measured in ohms (Ω). One ohm is the resistance of a conductor where 1 ampere of current flows when 1 volt is applied.
Resistance depends on the material's properties:
R = ρL/A
Where:
- ρ (rho) is resistivity (material property)
- L is the length of the conductor
- A is its cross-sectional area
Thinner wires have more resistance (smaller A), longer wires have more resistance (larger L), and different materials have different resistivities.
Electrical Power and Energy
When charge moves through a potential difference, power is dissipated:
P = VI = I²R = V²/R
This power is often converted to heat. A 100-watt bulb converts 100 joules of electrical energy to heat and light every second. This is why thick wires are used for high-current applications—they have lower resistance, so less power is wasted as heat.
Energy consumed: E = Pt
Your electricity bill measures energy consumption in kilowatt-hours (kWh)—1000 watts running for 1 hour.
EMF and Internal Resistance
A real battery isn't perfect. It has electromotive force (EMF) that drives current, but also internal resistance that opposes it:
V = EMF - Ir
Where r is the internal resistance. This explains why a car battery voltage drops when you start the engine—the huge current flows through internal resistance, reducing the voltage available to the starter.
Kirchhoff's Laws
Networks of resistors follow two fundamental rules:
- Junction Rule: Current into a junction equals current out (charge conservation)
- Loop Rule: Sum of potential differences around any closed loop equals zero (energy conservation)
These let you solve complex circuits with multiple resistors and batteries.
Related Topics
electric-charges-and-fields | electrostatic-potential-and-capacitance | moving-charges-and-magnetism | alternating-current
Socratic Questions
- Why does heating a metal wire increase its resistance, even though the electrons have more thermal energy to move? What's happening at the atomic level?
- If we could reduce the resistivity of a material to zero (create a perfect conductor), what would happen to Ohm's Law? Would circuits work the same way?
- Why do high-voltage power lines use thin wires instead of thick ones? What economic principle is at play (hint: think about I²R losses)?
- In a Series circuit, current is the same everywhere, but in a parallel circuit, voltage is the same everywhere. Why do these rules emerge from conservation of charge and energy?
- When you touch a 12-volt car battery, you feel almost nothing, but household 110 volts can be dangerous. Why is voltage alone insufficient to determine electrical danger—what else matters?