Magnetism is an interaction between moving charges.

• Magnetic forces, similar to electric forces, are due to the action of magnetic fields.

• A magnetic field $\vec{B}$ is created by a moving charge.

• Magnetic interactions are understood in terms of magnetic poles: north and south.

• Magnetic poles never occur in isolation. All magnets are dipoles, with two poles.

• Practical magnetic fields are created by currents—collections of moving charges.

• Magnetic materials, such as iron, occur because electrons have an inherent magnetic dipole called electron spin.

We will develop and use three important magnetic field models.

A charged particle moving in a magnetic field experiences a force perpendicular to both $\vec{B}$ and $\vec{v}$. The perpendicular force causes charged particles to move in circular orbits in a uniform magnetic field. This cyclotron motion has many important applications.

《 LOOKING BACK Sections 8.2–8.3 Circular motion

《 LOOKING BACK Section 12.10 The cross product

Currents are moving charged particles, so:

• There’s a force on a current-carrying wire in a magnetic field.

• Two parallel current-carrying wires attract or repel each other.

• There’s a torque on a current loop in a magnetic field. This is how motors work.

Magnetism is much more important than a way to hold a shopping list on the refrigerator door. Motors and generators are based on magnetic forces. Many forms of data storage, from hard disks to the stripe on your credit card, are magnetic. Magnetic resonance imaging (MRI) is essential to modern medicine. Magnetic levitation trains are being built around the world. And the earth’s magnetic field keeps the solar wind from sterilizing the surface. There would be no life and no modern technology without magnetism.