Magnetic Fields, Magnets and Electromagnets Explained
A magnet has a north and a south pole. Like poles repel and unlike poles attract. The region around a magnet holds a magnetic field B, measured in tesla (T), which turns a compass needle to line up with it.
Magnetic field lines
Outside a bar magnet the field lines run from N to S. Inside the magnet they continue from S back to N, so every line is a closed loop. Unlike electric field lines, magnetic field lines never start or end, because no isolated magnetic pole (monopole) has ever been found. Iron filings sprinkled near a magnet line up along the field, which is the classic way to see it.
Electricity makes magnetism
In 1820 Ørsted found that a current in a wire deflects a compass. A long straight wire is circled by field lines, with strength B = μ₀I/(2πr). Use the right-hand grip rule: point your thumb along the current, and your fingers curl in the direction of the field.
Electromagnets and solenoids
A coil of wire (a solenoid) acts like a bar magnet. Inside a long solenoid the field is uniform: B = μ₀nI, where n is the number of turns per metre. More turns or more current make it stronger; reversing the current swaps N and S. An iron core multiplies the field hundreds of times.
The force on a moving charge
A charge q moving with velocity v through a magnetic field feels the Lorentz force F = q(v × B). This force is always perpendicular to the velocity, so it changes the direction but never the speed: in a uniform field the charge moves in a circle of radius r = mv/(|q|B). Positive and negative charges curve opposite ways.
Worked example
A long wire carries 10 A. At 2 cm away, B = μ₀I/(2πr) = (4π × 10⁻⁷ × 10)/(2π × 0.02) = 1 × 10⁻⁴ T (100 μT), about twice Earth's own magnetic field.
Frequently asked questions
Do magnetic field lines go from north to south?
Outside a magnet, yes. Inside the magnet they go from south back to north, so each field line forms a closed loop.
How can you make an electromagnet stronger?
Increase the current, add more turns of wire (per unit length), or put an iron core inside the coil.
Does a magnetic field change the speed of a moving charge?
No. The magnetic force is always perpendicular to the velocity, so it only changes direction. The charge moves in a circle at constant speed.
What is the right-hand grip rule?
Grip the wire with your right hand with the thumb along the current; your curled fingers show the direction of the magnetic field around the wire.