### Electromagnetism guidance notes

Electricity and Magnetism

## Electromagnetism guidance notes

for 14-16

The following guidance notes cover these practical collections:

### Oersted, electric current and magnetism

Electromagnetism
Electricity and Magnetism

## Oersted, electric current and magnetism

Teaching Guidance for 14-16

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, heating and lighting were connected.

Scientists seek to ‘unify’ apparently different phenomena by finding their underlying causes.

Question: How can we show that an electric current has a heating effect, and can also result in light?

Heating effect of a current.

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.

Question: What does this suggest?

Scientists draw on evidence from everyday life; they try to understand the evidence through experimentation.

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.)

Experiment: Set a compass pointing north-south; place a wire above it, lying east-west. Connect to a 1.5 V cell. There should be no effect.

Preconceptions (existing ideas) may make it difficult to make progress.

Oersted’s thinking
Oersted imagined an electric current ‘struggling’ through a wire. As it flowed, this ‘conflict’ gave rise to 'heat' (or infra-red radiation, as we now understand it) and light, which radiated away from the wire. Might it not also result in a magnetic field, radiating away?

One idea leads to another, which can then be tested.

Question: How would you draw magnetic field lines to represent Oersted’s idea?

Question: How should a compass needle behave, if this idea is correct?

Answer: 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.

Experiment: Set up the previous experiment, but with the wire lying north-south above the compass, parallel to the compass needle. Switch on – the needle rotates to lie east-west.

Oersted's experiment.

Oersted reversed the current; the needle moved the other way.

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.

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.

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.

Experiment:

Magnetic field due to an electric current in a wire.

He went on to show that a thicker wire produced a greater effect. He also showed that materials placed between wire and compass had no effect.

Question: Why?

Answer: Thicker wire has less resistance, so greater current. Non-magnetic materials have no effect on a magnetic field.

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.

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.

In principle, scientific results are checked by other scientists who repeat the experiments, to see if they get the same results.

Oersted had observed and described an experimental phenomenon. Ampere took it further by writing a mathematical equation to account for it.

Question: Oersted had shown that an electric current has a magnetic field around it. What practical applications were developed from this idea?

Answer: Electromagnets, motors, dynamos, transformers, etc.