Place compasses here and there around a bar magnet and each needle points a different way. The space where the magnet's force reaches is its magnetic field.
The direction of the field is where a compass's N pole points, and the field lines that trace those directions leave the N pole and enter the S pole outside the magnet.
Moving the compass around the magnet in the diagram above, you can watch the needle turn to follow the field lines.
Even without a magnet, a current in a wire makes a ring-shaped field around it, and a current in a coil turns it into an electromagnet that behaves like a bar magnet.
The Invisible Space Around a Magnet
🧲 Finding the Field with a Compass
①A compass placed near a magnet turns to point in a definite direction
②The space where the magnet's force acts is called the magnetic field
③The field's direction at a point is where the N pole of a compass placed there points
④Sprinkling iron filings on paper over a magnet shows the shape of the field lines directly
Bar Magnet and Compass
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Field lines around a bar magnet and how the compass needle points differently at each spot
45°
💡 What to Look For
①The red end (N) of the needle follows the field line at that spot
②Near the N pole the needle points away from it
③Near the S pole the needle points toward it
④Beside the middle of the magnet the needle lies parallel to it, pointing toward the S pole
The Field Made by a Current
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Ring-shaped field around a wire carrying current out of the screen
3A
Right-hand rule
Thumb → current, curled fingers → field
A current in a straight wire makes a circular field centered on the wire, and a larger current makes a stronger field
Electromagnets: Magnets Made by Current
Strength of an electromagnet
More turns or more current → stronger field
A current in a coil gives it an N pole and an S pole at its ends, like a bar magnet. An iron core inside makes it much stronger
💡 Electromagnets vs Permanent Magnets
①An electromagnet loses almost all its magnetism when the current stops, so it can be switched on and off
②Reversing the current swaps its N and S poles
③A permanent magnet stays magnetic without any current
④Uses of electromagnets: speakers, electric motors, scrapyard cranes, maglev trains
Summary
Magnetic Fields at a Glance
Field lines run N → S outside a magnet; a current's field follows the right-hand rule
Where field lines are closer together, the field is stronger
🎯 Common Test Points
①Field lines leave the N pole and enter the S pole, and never cross or split
②The direction a compass N pole points is the field's direction
③Right-hand rule: thumb along the current, curled fingers along the field
④An electromagnet is stronger with more turns and more current
⑤Earth is a giant magnet with a magnetic S pole near the geographic North Pole, which is why a compass's N end points north
Examples and Unit Review
Example 1
When current in a straight wire flows out of the screen, as in the diagram below, how does the right-hand rule give the direction of the field around it?
1
Point your right thumb along the current, that is, out of the screen toward you.
2
Your fingers wrapped around the wire curl in the direction of the field, so seen on the screen the field forms rings turning counterclockwise.
▸ A ring-shaped field around the wire, turning counterclockwise
If the current flows into the screen (⊗) instead, the field turns clockwise.
Example 2
How can an electromagnet be made stronger, and what is its biggest difference from a permanent magnet?
1
More turns in the coil or a larger current make the field stronger; so does an iron core.
2
An electromagnet loses almost all its magnetism when the current is cut, whereas a permanent magnet stays magnetic all the time.
▸ More turns or more current make it stronger, and unlike a permanent magnet it can be switched on and off with the current
That is how a scrapyard crane lifts scrap metal with an electromagnet and drops it by cutting the current.
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
1
Field lines leave the N pole and enter the S pole (① wrong) and never cross (② wrong).
2
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).