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.
Understanding Magnetism
At the microscopic level, magnetism comes mainly from electrons. Each electron behaves a little like a tiny magnetic dipole because of its motion and an intrinsic property called spin. In most materials, these tiny effects point in many different directions.
Their fields cancel, so the object has no strong overall magnetism. In iron, nickel, cobalt, and some alloys, groups of atoms can line up in regions called magnetic domains. An unmagnetised piece has domains facing different ways.
When many domains become aligned, the piece becomes a magnet. Heating, hammering, or dropping a magnet can disturb this alignment and make it weaker.
Electric current produces magnetism because a current is made of moving charges. Around a straight wire, the field forms circles rather than travelling straight outward. The right hand grip rule helps predict the direction.
Point the thumb of the right hand in the direction of conventional current. The curled fingers show the field direction around the wire. A coil of wire concentrates these circular fields into a stronger field through its centre.
Adding more turns, increasing the current, or placing an iron core inside the coil makes an electromagnet stronger. This is why scrapyard cranes can switch magnetic lifting force on and off.
A magnetic field does not push every charge in the same way. A stationary charged particle feels no magnetic force from a steady magnetic field. The force appears when the particle moves across the field.
Its direction is sideways to both the motion and the field direction. As a result, the magnetic force often bends a particle's path instead of speeding it up. This idea is used in particle accelerators and mass spectrometers.
In an electric motor, forces on current carrying wires create a turning effect. In a generator, moving a wire through a magnetic field creates an electric current. These two devices work through related effects, though their energy changes go in opposite directions.
Earth has a magnetic field that reaches far into space. It is produced by moving liquid iron in the outer core. The field helps guide compass needles, which turn until they lie along the local field direction.
A compass does not point exactly to geographic north in every location. The difference is called magnetic declination, and it changes with place and time.
Earth's field also deflects many charged particles arriving from the Sun. Some particles enter near the polar regions and produce auroras when they collide with gases high in the atmosphere.
When studying magnetism, keep field direction separate from force direction. Field lines are a model for showing direction and relative strength. They are not physical threads in space.
Remember that a compass responds to the field at its own position, not to a whole diagram at once. In calculations, identify the direction of current or particle motion first. Then identify the field direction.
Finally use a right hand rule carefully, paying attention to whether the problem describes a positive charge, a negative charge, or conventional current. A negative charge experiences a force in the opposite direction from a positive charge moving the same way.
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 .
- The magnetic field around a long straight wire is .
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 in a magnetic field alone.
Practice Questions
- 1 A straight wire carries a current of . Find the magnetic field from the wire using , where .
- 2 A particle with charge moves at perpendicular to a magnetic field of . Calculate the magnetic force using .
- 3 Two bar magnets are brought close together with north facing north. Describe what happens and explain it using magnetic field interactions.