Magnetic Field

A compass near a magnet points in a set direction; that invisible region of influence is the magnetic field. Field lines leave the N pole and enter the S pole, and the compass N pole shows the field direction at that point. A current-carrying wire also makes concentric field circles, found with the right-hand rule. Here you can move the compass and change the current to explore fields and electromagnets.

Invisible Force Around a Magnet

🧲 Magnetic Field — A Magnet's Invisible Influence

①Place a compass near a magnet and the needle points a specific way

②This invisible region of force is the 'magnetic field'

③Field lines exit the N pole and enter the S pole

Bar Magnet and Compass

Compass Position (angle)45°

💡 Observation Points

①The red (N) end of the compass needle always points along the field direction

②Near the N pole: needle points away from N

③Near the S pole: needle points toward S

Magnetic Field from Current

Current Strength3A

Right-Hand Rule

Thumb → Current Direction, Fingers → Field Direction

A current-carrying wire produces concentric circular field lines

Electromagnet — Magnet Made by Current

Electromagnet Strength

More coil turns ↑ or higher current ↑ → Stronger field ↑

Electromagnet: passing current through a coil acts like a bar magnet

💡 Electromagnet vs Permanent Magnet

①Electromagnet: cutting current removes magnetism → ON/OFF capable

②Permanent magnet: always magnetic

③Uses: speakers, motors, MRI, maglev trains

Summary

Core Idea of Magnetic Field

Field lines: N → S (outside), Current → Field (right-hand rule)

Field strength: denser field lines mean a stronger field

🎯 Exam Points

①Field lines: leave N → enter S (never cross each other)

②The N end of a compass points along the field

③Right-hand rule: thumb=current, fingers=field

④Electromagnet: more turns ↑, more current ↑ → stronger field

⑤Earth is a giant magnet: a magnetic S pole sits near the geographic North

Examples and Unit Test

Example 1

When current flows upward through a wire, how does the right-hand rule tell you the direction of the magnetic field around the wire?

Point the thumb of your right hand in the direction of the current (upward).

The way your other four fingers curl is the field direction; it forms concentric circles around the wire.

▸ Thumb = current (up); the curling fingers = field (concentric circles)

The field around a straight current forms concentric circles, and its direction is set by the right-hand rule.

Example 2

How can you make an electromagnet stronger, and what is the biggest difference between an electromagnet and a permanent magnet?

Adding more coil turns or increasing the current makes the electromagnet field stronger.

An electromagnet loses its magnetism when the current is cut (ON/OFF capable), while a permanent magnet is always magnetic.

▸ Stronger with more turns or more current; cutting the current removes the magnetism (ON/OFF)

Because its strength and ON/OFF can be controlled, the electromagnet is used in speakers, motors, and MRI machines.

Unit Test

Which statement about magnetic fields is correct?

①Field lines leave the S pole and enter the N pole

②Magnetic field lines cross one another

③The N end of a compass points along the field direction

④An electromagnet keeps its magnetism even after the current is cut

⑤The field around a straight current is a straight line parallel to the wire

▸ ③ The N end of a compass points along the field direction

Field lines leave the N pole and enter the S pole (① wrong) and never cross (② wrong).

An electromagnet loses magnetism when the current stops (④ wrong), and the field around a straight current forms concentric circles (⑤ wrong); the N end of a compass points along the local field (③ correct).

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