Simple phenomena of magnetism
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Magnetic Materials Ferromagnetic materials (iron, steel, nickel, cobalt) can be magnetised to form magnets. In their unmagnetised state, they are attracted to magnets. Magnetic materials Materials that can be magnetised by a magnet; in their unmagnetised state they are attracted by a magnet (e.g.
iron, steel, nickel, cobalt). Non-magnetic materials Materials that cannot be magnetised and are not attracted by a magnet (e.g. aluminium, wood, plastic, copper). Magnetic Poles The poles are the places in a magnet to which magnetic materials are attracted.
They are near the ends of a bar magnet and occur in pairs of equal strength . North pole (N pole) – points roughly towards Earth's geographic north South pole (S pole) – points roughly towards Earth's geographic south FIG 4.1.1: Magnetic Poles Three carbon isotope nuclei side by side: C-12 with 6 protons (red) + 6 neutrons (grey), C-13 with 6 protons (red) + 7 neutrons (grey), C-14 with 6 protons (red) + 8 neutrons (grey).
All with same electron shell configuration around them. Law of Magnetic Poles Like poles repel, unlike poles attract. The force between magnetic poles decreases as their separation increases. Induced Magnetism When an unmagnetised magnetic material touches or is brought near a permanent magnet, it becomes a magnet itself.
The magnetism is said to be induced . FIG 4.1.2: Induced magnetism A permanent bar magnet (labelled S-N) positioned near an unmagnetised iron bar. The iron bar shows induced poles: S pole at the end nearest the N pole of the permanent magnet, and N pole at the far end.
Arrows indicate attraction between unlike poles. Key Point A N pole in the permanent magnet induces a S pole at the near end of the magnetic material (and N pole at the far end). Temporary vs Permanent Magnets Property Temporary Magnets (Soft Iron) Permanent Magnets (Steel) Ease of magnetisation Easy to magnetise Harder to magnetise Retaining magnetism Loses magnetism easily Retains magnetism Description "Soft" magnetic material "Hard" magnetic material Magnetic Fields Magnetic field A region in which a magnetic pole experiences a force.
Direction of magnetic field The direction of the force on a N pole at that point. Field lines point from N to S. FIG 4.1.7: Magnetic field around a bar magnet A bar magnet with N pole on right, S pole on left.
Curved field lines emerge from N pole, loop around externally, and enter S pole. Arrows on lines point from N to S. Lines are closest together (strongest field) near the poles. Field Strength The magnetic field is stronger where field lines are closer together and weaker where they are further apart.
Magnetic forces arise from the interaction of magnetic fields . Plotting Magnetic Field Lines Method 1: Plotting Compass Place bar magnet on paper Position plotting compass near one pole Mark positions of compass needle poles with dots Move compass so its S pole is over the previous N pole mark Continue until reaching the other pole of the magnet Join dots and add arrows pointing N → S Method 2: Iron Filings Place paper over bar magnet Sprinkle iron filings evenly onto paper Tap paper gently – filings align with field lines Quick method but cannot show direction; not useful for weak fields.
Electromagnets An electromagnet is a coil of wire wound on a soft iron core . It is a temporary magnet that can be switched on and off. Increasing Electromagnet Strength Increase the current in the coil Increase the number of turns on the coil Move the poles closer together (horseshoe shape) FIG 4.1.11: C-core (horseshoe) electromagnet A C-shaped soft iron core with wire coils wound around each arm.
Current direction shown with arrows. Magnetic field lines pass through the core and across the gap between the two poles (N and S). The poles are close together for maximum strength. Uses of Permanent Magnets and Electromagnets Permanent Magnets Electromagnets Compass Cranes for lifting scrap iron Electric motors Electric bells Generators Magnetic locks Loudspeakers Relays Hard disk drives Circuit breakers Key Difference Permanent magnets: No current needed; field cannot be varied.
Electromagnets: Require current; strength can be varied; can be switched on/off.
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