19 — Magnetism Flashcards
Magnetic materials
Materials that r attracted to a magnet
Iron, steel, nickel, cobalt
Properties of magnets
- The region where the magnetic attraction is the strongest is known as the poles of a magnet.
- A freely suspended magnet will always point n the North direction (North Pole of magnet to geographic North Pole, but GNP is actually a South Pole)
- All magnets obey the law of magnetism
Law of magnetism
Like poles repel, unlike poles attract
How to determine whether metal bar (unknown) is a magnet?
Bring a real magnet near object and see if it repels.
Induced magnetism (temporary)
- Induced magnetism using a strong magnet — placing a strong magnet near obj. Temporary magnet.
- Stroking method. (Single or double stroke)
- Using an electric current — placing obj in a solenoid (coil of wire)
Methods of demagnetisation
- Heating and hammering
- Using an electric alternating current
-> place magnet in east-west direction in a solenoid carrying an alternating current then gradually reduce current to 0A. When the current is flowing, remove the magnet away from the solenoid. Repeat this action a couple of times.
Why magnets will lose its magnetism over time
- Long periods of heat
- Influence of nearby magnetic fields
- Heating/dropping -> atomic magnets vibrate and settle in preferred orientation -> out-of-alignment -> lose magnetism
Storing magnets + how it works
Using soft iron keepers + diagram mem
1. Magnetic field retained within iron keepers to preserve strength
2. Reduce strong magnetic fields of stored magnets
3. Prevent unwanted magnetism materials from being attracted.
Magnetic field
A magnetic field is a region in which the force of magnetism acts
It has both magnitude (strength) and direction and can be represented by lines of force. (Arrow pointing towards South)
Determining magnetic field pattern for a bar magnet
- Iron filling method
(i) Place magnet below a card.
(ii) Sprinkle some iron filings on top of the card.
(iii) Tap the card a little.
(iv) Iron filings will form a pattern which is the magnetic field pattern for the magnet. - Plotting compass method
Advantage: tells the direction of magnetic field
Rmb diagrams
Iron vs steel
Iron: ‘soft’ as it is easy to magnetism it and loses its magnetism easily
Steel: ‘hard’ as it is more difficult to magnetism it and retains its magnetism for a longer period of time
Iron: requires only a weak magnetic field to magnetise
Steel: requires a strong magnetic field to magnetise
Iron: normally used as an electromagnet
Steel: normally used as a permanent magnet
Permanent magnets do not require the presence of an electric current or a permanent magnetic field to retain their magnetism
How to increase strength of electromagnets
- Increasing the number of coils
- Increase current by adding more batteries
Uses of electromagnet
- Electric bell
(Iron core as electromagnet instead of steel core. Since steel retains magnetism for a longer period of time, it will attract the hammer, opening the circuit and thus causes the bell to ring only once)
Uses of permanent magnets
- Compasses
- Magnetic door catches
- Loudspeakers
- Electric meters
- Motors
Explain why the magnet is attracted to the iron plate.
The iron plate is a magnetic material which becomes an induced magnet when placed in a magnetic field provided by the bar magnet.
The magnetic field from the magnet aligns the randomly arranged domains within the iron plate facing the bar magnet’s North Pole to become an induced South Pole ‘S’ and the opposite side becoming an induced North Pole ‘N’.
