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Magnetic field
Compass Needle
Vocabolario:
Compass needle: ago della bussola
True North: Polo nord geografico
Circuit, Loop: circuito (elettrico)
Magnet: magnete
Iron filings: limatura di ferro
Wire carrying an electric current: filo percorso
da corrente elettrica
Magnetic permeability: permeabilità
magnetica
To point: indicare
To exercise: esercitare
Magnets are objects able to attract metallic
materials (items made of iron or steel).
Magnets also attract each other but only if they
are properly aligned.
(If the ends are not properly aligned the force
between them is repulsive rather than attractive).
They show a similarity with an electric dipole
behaviour: each magnet has two poles,
(usually labelled with the letter N and S) and,
as for electric charges, like poles repel each
other while unlike pole attract one another.
Magnetic poles
The magnetic field produced around a magnet
can be easily detected by sprinkling iron filings
near it: the filings line up in the direction of the
magnetic field produced, revealing the field
lines in the neighbourhood of the magnet .
iron filings
Compass Needle
As we can see in the figure below, the magnetic field
produced by a magnet has similar pattern to the electric
field lines produced by an electric dipole. The main
difference is that the magnetic field lines are closed
loop (they have no origin and no ending point) while
electric field lines always originate from positive
charges and end on negative charges.
True North
North magnetic pole
South Magnetic pole
True South
Hans Christian Oersted (1777-1851)
was a Danish physicist and
chemist
100 danske kroner − Danish
banknote from 1970 featuring
Ørsted
Ørsted developed an experiment which provided
evidence that surprised him.
As he was setting up his materials, he noticed a
compass needle deflected from magnetic north
when the electric current from the battery he was
using was switched on and off.
This deflection convinced him that magnetic fields
radiate from all sides of a wire carrying an electric
current, just as light and heat do, and that it
confirmed a direct relationship between electricity
and magnetism.
Oersted’s experiment
Wire carrying an
electric current
Oersted demonstrated that a current produces a
magnetic field
Magnetic
field
wire carrying an electric current
Michael Faraday,(1791 – 1867)
was an English chemist and physicist
magnetic forces are exerted by
magnets on current carrying wires
André-Marie Ampère (1775 –1836),
was a French physicist.
The SI unit of measurement of electric
current, the ampère, is named after
him.
Ampere discovered the existence of force exerted
on one current-carrying wire by another. Ampere
measured the strength of this magnetic force and
studied how it varies with the distance between
the wires and the amount of current flowing in
each.
He showed that the magnetic force between the
wires is proportional to the two currents i1 and i2
and inversely proportional to the distance r
between the two wires.
 0 i1i2l
F

2 r
 0 i1i2l
F

2 r
μ0 = magnetic permeability [ μ0 = 4π × 10-7 N/A2 ]
l = lenght of the wire
If the currents in the wires have the same direction,
the resulting force is attractive; if the currents have
an opposite direction, the resulting force between the
wires is repulsive.