Describe magnetic poles and fields, state that like poles repel and unlike poles attract, and explain that an electric current produces a magnetic field (the basis of electromagnets) (MA STE Introductory Physics, Motion and Forces, HS-PS2-5).

What this topic is asking

This topic covers magnets and the deep link between electricity and magnetism that the Massachusetts Introductory Physics MCAS tests under HS-PS2-5. You must describe magnetic poles and magnetic fields, state that like poles repel and unlike poles attract, and explain that an electric current produces a magnetic field, which is the basis of the electromagnet. The crosscutting idea is cause and effect together with patterns: moving charge (a current) is the cause, a magnetic field is the effect.

Magnetic poles

The pole rule deliberately echoes the charge rule from electric charge and Coulomb's law, and the MCAS tests it directly. Bring two north poles together and they resist; turn one magnet around so north faces south and they snap together. A compass works because its needle is a small magnet whose north pole is pulled toward Earth's magnetic field. Unlike electric charges, magnetic poles cannot be separated: break a magnet in half and each piece has its own north and south pole.

Magnetic fields

The field is how a magnet acts at a distance, the magnetic counterpart of the gravitational and electric fields. The MCAS expects you to read and sketch field-line patterns: lines emerge from the north pole, curve around, and enter the south pole, never crossing. Where the lines bunch up (at the poles) the field is strong; where they spread out the field is weak. Iron filings sprinkled around a bar magnet line up along these field lines, making the pattern visible.

An electric current produces a magnetic field

This is the link between the two halves of the module, and it is the single most important fact here. Moving charge (a current) is always surrounded by a magnetic field. A coil of many turns adds the fields of each loop together to make a strong, bar-magnet-like field with a north and south end. An electromagnet, a coil wound around iron, is the practical result: unlike a permanent magnet, it can be switched on and off, and made stronger by:

• increasing the current,
• adding more turns to the coil,
• using an iron core.

Electromagnets are used in motors, doorbells, speakers, and scrapyard cranes that lift and drop cars by switching the current.

Worked example

Reference-sheet note

The reference sheet does not print anything for magnetism; the magnetism standards are tested qualitatively and through diagrams. What you recall is the pole rule (like repel, unlike attract), that poles come in pairs, the field-line picture (north to south outside the magnet, strongest near the poles), and above all that an electric current produces a magnetic field, which is how electromagnets work and how they are made stronger.

Try this

Q1. State what happens when the south pole of one magnet is brought near the south pole of another. [1]

• Cue. They repel; they are like poles, and like poles repel.

Q2. State what produces the magnetic field in an electromagnet, and one way to make it stronger. [2]

• Cue. The electric current in the coil produces the field; increasing the current (or adding more turns, or using an iron core) makes it stronger.