Magnetic Field Basics

Wrap a wire into a coil, run current through it, and it becomes a magnet strong enough to pick up nails. Turn off the current and the magnetism vanishes. This is an electromagnet, and it demonstrates the key principle: every electric current produces a magnetic field.

The magnetic field around a straight wire forms concentric circles. The field is stronger close to the wire and weaker farther away. Coiling the wire into loops concentrates the field, because each loop's field adds to the next. More loops, stronger field. More current, stronger field. Inserting an iron core inside the coil amplifies it further, because iron's atomic magnetic domains align with the external field.

This principle works in both directions. A current creates a magnetic field (electromagnet). A changing magnetic field creates a current (generator). The symmetry is called electromagnetic induction and it is the operating principle of every transformer, generator, and electric motor on the power grid. Transformers use it to step voltage up or down. Generators use it to convert mechanical rotation into electricity. Motors use it to convert electricity back into rotation.

Worked Example

A simple electromagnet uses 100 turns of wire carrying 2 amps of current. A second electromagnet uses 200 turns carrying 1 amp.

• Identify the key variable. Magnetic field strength in a solenoid is proportional to the number of turns times the current (N x I).
• Compare. First: 100 x 2 = 200. Second: 200 x 1 = 200.

Which electromagnet produces the stronger field?

They are equal. The product N x I is the same. You can achieve the same field strength by using more turns at lower current or fewer turns at higher current. This tradeoff matters for designing efficient motors and transformers.

Every design choice in a motor or transformer, more turns versus more current, is really a trade against this one product: N times I.

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