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Magnetism is a fundamental interaction that causes certain materials and moving electric charges to attract or repel each other. A magnet has two poles, called north and south, and the forces are strongest near these poles. Magnetic fields let us describe how magnetic forces act through space around magnets and electric currents. Understanding magnetic fields helps explain compasses, motors, generators, speakers, and many everyday technologies.
A magnetic field is usually shown with field lines that point from the north pole to the south pole outside the magnet. These lines are closer together where the field is stronger, especially near the poles of a bar magnet. Like poles repel and unlike poles attract because of how their magnetic fields interact. Magnetic forces also act on moving charges, so magnetism is deeply connected to electricity in electromagnetism.
Key Facts
- Every magnet has two poles: north (N) and south (S).
- Like poles repel and unlike poles attract.
- Magnetic field lines outside a magnet go from N to S.
- Field strength is greater where field lines are closer together.
- The magnetic force on a moving charge is F = qvB sin(theta).
- The magnetic field around a long straight wire is B = mu0 I / (2pi r).
Vocabulary
- Magnetic field
- The region around a magnet or current where magnetic forces can act.
- Pole
- One of the two ends of a magnet where the magnetic effect is strongest.
- Field line
- A drawn line that shows the direction and relative strength of a magnetic field.
- Ferromagnetic material
- A material such as iron, nickel, or cobalt that can be strongly magnetized.
- Electromagnetism
- The area of physics that studies the connection between electricity and magnetism.
Common Mistakes to Avoid
- Thinking a magnet can have only one pole, which is wrong because isolated north or south poles are not found in ordinary magnets. Cutting a magnet in half makes two smaller magnets, each with both poles.
- Drawing magnetic field lines from S to N outside the magnet, which is wrong because outside a magnet the standard direction is from N to S. Reversing the direction confuses force and field diagrams.
- Assuming magnetic field lines can cross, which is wrong because the field at one point can only have one direction. Crossing lines would imply two different field directions at the same location.
- Believing magnetic force acts on stationary charges, which is wrong because magnetic force depends on motion of charge relative to the field. A charge at rest has magnetic force F = 0 in a magnetic field alone.
Practice Questions
- 1 A straight wire carries a current of 6.0 A. Find the magnetic field 0.020 m from the wire using B = mu0 I / (2pi r), where mu0 = 4pi x 10^-7 T m/A.
- 2 A particle with charge 2.0 x 10^-6 C moves at 3.0 x 10^5 m/s perpendicular to a magnetic field of 0.40 T. Calculate the magnetic force using F = qvB.
- 3 Two bar magnets are brought close together with north facing north. Describe what happens and explain it using magnetic field interactions.