Table of Contents
IGCSE Coordinated Science: Magnetism
Magnetism
For this unit, you’ll need to:
Describe the properties of magnets
“A magnet is a material or object that produces a dipole magnetic field (i.e. one with 2 poles), which pulls on other ferromagnetic materials and repels other magnets”
– has 2 poles, N->S
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– Field movement from N->S
– Can be create with an induced electric current
Types of magnetic materials: Ferromagnetic (ferrous metals e.g. steel, iron – strongly attracted); Paramagnetic (e.g. Pt, Al, O2 – weakly attracted to magnets); Dimagnetic (e.g. plastic, wood, copper – repelled by both poles; non-magnetic)
Give an account of induced magnetism. (Electromagnetic Induction)
- By moving a wire up and down inside a magnetic field, it intersects the field perpendicularly, thus generating a current that is registered in the galvanometer. When the wire is moved in the opposite direction (e.g. up instead of down), the current flows the other way, creating an Alternating Current.
- By moving a magnet inside a solenoid, the same effect is achieved (field and wire still intersect perpendicularly). In this case, it is the field moving and not the wire.
– The speed of movement affects the magnitude of the current.
– Direction of movement affects the dir. of current
– Amo. of coils in solenoid affects the magnitude of the current.
Identify the pattern of field lines round a bar magnet
– The direction of field movement is always North -> South
Distinguish between the magnetic properties of iron and steel.
– There are 2 types of iron: Soft Iron and Hard Iron. (Describes Magnetic properties [Coercivity]. If you were hit on the head with a soft iron bar, it would still feel very hard).
– Soft iron is easy to magnetize, but does not retain its magnetism in the absence of a magnetic field. (The field aligns the electrons in the iron, grouping different particle spins together). Soft Iron has high susceptibility and low retentivity. This makes soft iron ideal for applications where the magnetic field can be turned on and off, e.g. as the core of an electromagnet.
– Hard Iron and Steel retains its magnetism in the absence of a magnetic field. However, unlike Hard Iron, Steel is harder to magnetize – therefore Hard Iron has high susceptibility and high retentivity, whereas Steel has low susceptibility and high retentivity
– The best magnetic materials are Ferrous (i.e. compounds of Iron). The reason why Iron is easier to magnetize than Steel is because it is 100% Fe, whereas Steel is a compound with a lower amo. of Iron than pure Iron.
Distinguish between the design and use of permanent magnets and electromagnets.
– Electromagnets are temporary: a result of the electric field (flow of electrons). When the pd. is removed, the magnetism goes away.
– Permanent magnets have their atoms aligned to produce a constant magnetic field.
– (Actually, the electrons are aligned according to spin [+-0.5] in each atom, and the proportion of +/- determines the poles. After a ferromagnetic material reaches a certain temp. [Curie Temperature], it becomes paramagnetic since the energy is transferred to the atoms, throwing them out of alignment. Purely for geeky fun, you don’t have to know this.)
– Use of Permanent magnets: Compasses, Apple’s MagSafe chargers, fridge magnets, generators, dynamos, magnetic screwdrivers, door holders
– Use of Electromagnets: relays, speakers/microphones, motors, LHC, generators, dynamos, CRT, doorbells, electromagnets to pick up scrap metal in junkyards, Maglev trains, MRI
