In 1820, Hans Christian Ørsted noticed a compass needle move when it was near a wire carrying a current. This was the birth of Electromagnetism: the discovery that electric currents are the actual source of all magnetic fields.

Audio Explanation

Prefer to listen? Here's how a flow of electrons transforms a piece of metal into a temporary magnet.

Magnetic Field of a Straight Wire

When current flows through a long, straight wire, it creates a magnetic field that circles around the wire like a ring.

[Image of magnetic field lines around a straight current-carrying wire]

The Second Right-Hand Rule (RHR-2)

To find the direction of this circular field:

  1. Thumb: Point your right thumb in the direction of the Current ($I$).
  2. Fingers: Curl your fingers as if grabbing the wire.
  3. Result: The direction your fingers curl is the direction of the Magnetic Field ($B$).

The strength of this field is calculated by: \(B = \frac{\mu_0 I}{2\pi r}\)

  • $\mu_0$: The permeability of free space ($4\pi \times 10^{-7} \text{ T}\cdot\text{m/A}$).
  • $r$: The distance from the wire.

Visual Representation: The Solenoid

If you wrap a wire into a series of loops (a coil), the magnetic fields from each loop add up. This device is called a Solenoid. Inside the solenoid, the magnetic field is incredibly strong and uniform.

A diagram of a solenoid showing how multiple loops of wire create a uniform magnetic field through the center. Ferromagnetic Core B

How to Make a Stronger Electromagnet

An electromagnet is simply a solenoid with a ferromagnetic core (like an iron nail) inside it. You can increase its strength in three ways:

  1. Increase the Current ($I$): More flow equals more field.
  2. Increase the Number of Loops ($N$): More turns of wire add more field layers.
  3. Add an Iron Core: The iron atoms align with the solenoid’s field, magnifying it thousands of times.

Interactive Solenoid Lab

Adjust the number of coils and the voltage of the battery. Observe how the magnetic field (B) inside the coil intensifies as you add loops or push more current through the circuit.

Electromagnet Strength Visualizer

20 loops
3 Amps

Interactive Match: Electromagnet Components

Match the physical action to its effect on the magnetic field.

Why Should I Care?

Electromagnets are the “switch” of the modern world. Because you can turn them on and off, they allow us to control mechanical things with electricity:

  • Maglev Trains: Powerful electromagnets lift and propel trains at hundreds of miles per hour without touching the tracks.
  • Scrap Yard Cranes: Giant magnets pick up tons of steel and drop them just by flipping a switch.
  • Relays: Used in everything from your car’s starter motor to the power grid to safely control high-voltage circuits.

💡 Quick Concept Check:

If you switch the direction of the current in an electromagnet, what happens to the magnetic field?

Click to Reveal Answer
The magnetic field **reverses direction**. The North pole becomes the South pole, and vice versa. This is how electric motors are able to constantly "push" and "pull" to keep spinning.
↑ Back to top