Electrical Fundamentals and Circuit Components

Current is simply the flow of electrical charge, whilst potential difference (voltage) measures the energy difference between two points in a circuit. Resistance tells you how difficult it is for current to pass through a component - think of it like electrical friction.

Ohm's Law is your best friend in physics: for ohmic conductors, current is directly proportional to voltage $V = IR$. This means if you double the voltage, you double the current, assuming temperature stays constant. Not all components follow this rule though.

Different components behave in fascinating ways. Filament bulbs contain metal wire that heats up as current increases, which actually increases their resistance. At low currents they follow Ohm's Law, but as current rises, the heating effect kicks in and resistance goes up too.

Semiconductor diodes are like electrical one-way streets. In forward bias, current flows easily, but in reverse bias, they have extremely high resistance and barely any current flows. Superconductors are the opposite extreme - below their critical temperature, they have zero resistance, making them perfect for power cables and maglev trains.

Quick Tip: Remember that resistivity $ρ = RA/L$ is a material property that depends on the substance itself, whilst resistance depends on the specific component's dimensions.

Circuit Analysis and Power

In series circuits, current is the same everywhere, but voltage gets shared across components. In parallel circuits, voltage is the same across each branch, but current splits between paths. These aren't just rules to memorise - they're based on the fundamental principle that charge can't just disappear.

Kirchhoff's Laws make circuit analysis straightforward. His first law says current flowing into a junction equals current flowing out (charge conservation). His second law tells you that all voltages around a complete loop add up to the battery voltage (energy conservation).

Power is energy transferred per second, and you've got three handy equations: P = VI, P = I²R, and P = V²/R. Choose whichever equation uses the values you're given in the question.

Potential dividers are brilliant for creating variable voltages. By putting resistors in series, you can get any fraction of your supply voltage. If one resistor is a thermistor or LDR $light-dependent resistor$, you've got yourself a temperature or light sensor.

Exam Tip: EMF (electromotive force) is the total energy per coulomb that a battery can provide, whilst terminal voltage is what you actually measure across the battery when current flows - the difference is energy lost to internal resistance.