Download Oersted, electric current and magnetism

Oersted, electric current and magnetism
(adapted from: Nuffield Foundation)
In 1820, Hans Christian Oersted performed an important experiment which showed that there was a
connection between electricity and magnetism. When a current was switched on through a wire, it made a
compass needle turn so that it was at right angles to the wire. The current had produced a magnetic field
strong enough to cause the compass needle to turn.
Background to the experiment
It was already known that an electric current in a wire has a heating effect, and may cause the wire to glow.
This showed that the three phenomena of electricity, heat and light were connected. It was also known
that when lightning, a form of electricity, struck a ship, the ship’s compass might be affected – its polarity
might be reversed. Scientists seek to ‘unify’ apparently different phenomena by finding their underlying
causes.
Question: What did this suggest? There should / could be a link between electricity and magnetism.
How could this link be demonstrated in the lab? Experimenters imagined that, if an electric current flowed
along a straight wire, then the magnetic field it produced would be in the same direction. So they placed
their compass needles at right angles to the wire, thinking they would be deflected by the current so that
they became parallel to the wire. They saw no effect because, as Oersted was to show, the magnetic field
produced is at right angles to the wire. So preconceptions prevented earlier experimenters from seeing the
effect. (Note that the available equipment –voltaic piles, typically – would only produce a small current, so
the effect would in any case have been very weak.)
Oersted’s thinking
Oersted imagined an electric current ‘struggling’ through a wire. As it flowed, this ‘conflict’ gave rise to heat
and light, which radiated away from the wire. Might it not also result in a magnetic field, radiating away?
Question: How would you draw magnetic field lines to represent Oersted’s idea? Radiating out from the
wire.
Question: How should a compass needle behave, if this idea is correct? It might be expected to point
radially to the wire. Try it, and the magnet does not do this. So magnetic field lines do not radiate out from
a current-carrying wire.
Oersted’s experiment
Oersted was giving a public demonstration of the connection between electricity, heat and light. He
connected a voltaic pile to a platinum wire and showed that it became hot and glowed.
As he lectured, his idea about a connection between electricity and magnetism resurfaced in his mind. He
had a compass to hand (for other experiments he intended to demonstrate), so he decided to test his idea
there and then. He could notice a small, but noticeable, movement on the needle! Oersted reversed the
current; the needle moved the other way. Obs. Scientists are often working at the limits of sensitivity of
their instruments. A technique may have to be refined greatly before a phenomenon can be observed
reliably.
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The effect observed by Oersted and his audience was small; no-one was very impressed. Oersted was
aware that other scientists had been distracted by similar elusive phenomena, and it was three months
before he spent any more time on his discovery.
[[[Students should do experiment Here]]]
What Oersted did next
Oersted showed that the magnetic field around a current-carrying wire was circular; i.e. the lines of force
are circles, centred on the wire. He went on to observe that a thicker wire produced a greater effect. He
also noticed that materials placed between wire and compass had no effect.
Question: Why? Thicker wire has less resistance, so greater current. Non-magnetic materials have no effect
on a magnetic field.
Outcomes of publication and next steps of the community in constructing this knowledge
In July 1820, he sent a 4-page paper outlining his results, in Latin, to several scientific journals.
Scientists are expected to share their findings as soon as possible, by publication in journals. In 1820, Latin
was a common (shared) language which allowed scientists of different nationalities to understand each
other’s work.
In principle, scientific results are checked by other scientists who repeat the experiments, to see if they get
the same results.The French scientist André M. Ampere read Oersted’s report, and in the space of a week
had repeated the observations and developed a mathematical theory describing how the magnetic field
depends on the strength of the current and the distance from the wire. Oersted had observed and
described an experimental phenomenon. Ampere took it further by writing a mathematical equation to
account for it.
More about Oersted
Oersted founded the Danish Society for the Dissemination of Natural Science, a society aimed at presenting
scientific ideas to the general public. Oersted also played a key role in the founding of the Technical
University of Denmark, with the intention of improving the scientific basis of engineering.
Many scientists seek to share their work with a wider audience. They may also seek to share the
technological benefits of their discoveries. He described 1820, the year of his great discovery, as the
happiest year of his life. Scientists can get great pleasure from their work.
Prior claim of the discovery
In 1802, an Italian lawyer called Gian Domenico Romagnosi published an account of an observation similar
to Oersted’s. However, his article appeared in a newspaper and it wasn’t taken up by the scientific
community. Romagnosi doesn’t appear to have followed up his preliminary findings. There are many
examples of ‘prior claims’ in the history of science. In practice, the acknowledgement usually goes to:
 the person who publishes a careful, detailed and repeatable account of their observations,
 at a time when other scientists are ready for the idea, and
 in a place it will be read and taken seriously.
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