Electricity and Magnetism

A series circuit gives current exactly one path — through every component in a single loop — so the current is the same everywhere. A parallel circuit splits into branches, each a complete loop back to the battery, so the current through the battery equals the sum of the branch currents. Adding bulbs in series increases resistance and drops current; adding bulbs in parallel opens more paths, so total resistance falls and total current rises.

Current (I) is the rate of flow of charge, measured in amperes with an ammeter. Voltage (V) is the 'push' driving that current, measured in volts with a voltmeter. Resistance (R) measures how much a component opposes current flow, measured in ohms, and links to the other two through $R = \dfrac{V}{I}$.

Conductors, mostly metals like copper, have free electrons that drift easily through the material, allowing current to flow. Insulators, like plastic, rubber and glass, hold their electrons tightly bound to atoms, so no current can pass. Copper is the standard wire material because it conducts well and is far cheaper than silver or gold.

Every magnet has a north and south pole, with field lines running from N to S outside the magnet, strongest right at the poles. Like poles repel and unlike poles attract — the rule behind every magnet-interaction question. Only ferromagnetic materials like iron, nickel and cobalt respond strongly, and a magnet can attract unmagnetised iron by temporarily inducing opposite poles in it.

An electromagnet is a coil of insulated wire (a solenoid) carrying current, which creates a magnetic field just like a bar magnet, with N and S poles forming at each end. Adding a soft iron core inside the coil makes the field much stronger because the iron itself becomes magnetised while current flows. Unlike a permanent magnet, this field only exists while current flows, which is the key link between electricity and magnetism.