1. The Magnetic Field

2. Magnetic Field
Every magnet sets up a magnetic field in the space around it.
Magnetic fields exert forces on moving charged particles and electric currents.

3. Measuring the Magnetic Field
An electric dipole experiences a torque when placed in an electric field.

4. Measuring the Magnetic Field
A magnetic field exerts a torque on the compass needle, causing the needle to point in the field direction.

5. Measuring the Magnetic Field
Because the magnetic field has both a direction and a magnitude, we represent it using a vector, .
B represents the magnitude or strength of the field.

6. Magnetic Field Vectors and Field Lines
Mapping out the field of a bar magnet using compasses:

7. Question 1
A compass is placed at the black dot. In which direction will the compass point?
Answer: B

8. Question 1
A compass is placed at the black dot. In which direction will the compass point? B

9. Question 2
A compass is placed at the black dot. In which direction will the compass point?
Answer: D

10. Question 2
A compass is placed at the black dot. In which direction will the compass point? D

11. Magnetic Field Vectors and Field Lines
Drawing the magnetic field lines of a bar magnet:

12. Magnetic Field Vectors and Field Lines
A single bar magnet (closeup):
Closer to the magnet, we can more clearly see how the field lines always start on north poles and end on south poles.

13. Magnetic Field Vectors and Field Lines
Two bar magnets, unlike poles facing:
With more than one magnet, the field lines still start on a north pole and end on a south pole. But they can start on the north pole of one magnet, and end on the south pole of another.

14. Magnetic Field Vectors and Field Lines
Two bar magnets, like poles facing:
With two like poles placed nearby, the field lines starting on the north poles curve sharply toward their south poles in order to avoid the north pole of the other magnet.

15. The Magnetic Field of the Earth

16. The Magnetic Field of the Earth

17. Magnetic Field Vectors and Field Lines
Magnets can have more than one pair of north- south poles, and the poles do not need to be at the ends of the magnet.
For example, the magnetic field of a refrigerator magnet:

18. Magnetic Field Vectors and Field Lines
The pole structure of a refrigerator magnet can be revealed by a special film that contains fine iron filings.