Oersted's Experiment

In 1820, Oersted found the initial relationship between electricity and magnetism that even caused speculations. What did he find in his experiment? Did he really discover the findings by chance? In this article, we will talk about Oersted's findings and how he came up with Oersted's law.

Oersted Discovering the Relationship between Electricity and Magnetism

Oersted's law is a physical law in electromagnetism that states that an electric current produces a magnetic field. Hans Christian Oersted (1777–1851), a Danish scientist, discovered this on April 21, 1820, when he noted that the needle of a compass close to a wire carrying current rotated perpendicular to the wire. While he defined the magnetic field produced by a straight current-carrying wire, he also discovered the initial relationship between electricity and magnetism.

The needle of a simple compass deflects when brought near a current-carrying wire, Wikimedia Commons CC BY-SA 3.0

In 1820, Oersted announced his finding that a nearby electric current deflected a compass needle from magnetic north, demonstrating a direct link between electricity and magnetism. There was a myth that he discovered the finding by chance during one of his lectures, however, he had been hunting for a relationship between electricity and magnetism since 1818. He was just not sure about the meanings of the findings.

His first assumption was that magnetic properties radiate from all sides of a wire carrying an electric current like light and heat radiate. He conducted more intense research three months later and published his findings soon after. He demonstrated that an electric current creates a circular magnetic field as it travels along a wire. Oersted also noted that the strength of the magnetic field \(B\) was directly proportional to the magnitude of the current \(I\)

\(B \propto I\)

and the strength of the magnetic field \(B\) was inversely proportional to the distance from the current \(r\).

\(B \propto \frac{1}{r}\).

Oersted's Experiment Diagram

The figure below is that of a circuit diagram showing Oersted's setup. A battery is connected to a magnetic compass and a resistor. The switch is initially open, no current passes through the compass, and its needle does not deflect.

In the image, there is a magnetic compass placed in a circuit that involves two batteries and a resistance. Because the switch is off, the wire is not carrying current and there is no change in the compass. JavaLab

In the image below, we can see that the switch is now closed. A current is now passed through the compass and its needle deflects.

Now the switch is on. The wire starts carrying current and there is a deflection in the needle of the compass. JavaLab

Why is there a deflection in the compass needle while the switch is on only? Let's elaborate on that!

Oersted's Experiment Simulation

When current travels through the circuit, the electrons flowing through the wire create a magnetic field surrounding it. The north of the needle is deflected by positioning the magnetic compass, as indicated in the image above. This is because there is a magnetic field produced by a current carrying wire. The direction of the magnetic field can be shown with the right-hand rule which we will discuss in a bit.

The Direction of the Magnetic Field of a Current-Carrying Wire

The image below is a simple illustration showing how the direction of the current can be determined if the direction of the magnetic field is known, and vice-versa.

According to the right-hand rule, the thumb shows the direction of the current, and four fingers curl around the wire and show the direction of the magnetic field, Creative Commons

The right-hand rule, as it is called, may be used to determine the direction of the magnetic field at a given position, as well as the direction of the arrowheads on magnetic field lines, which is the direction in which the compass needle's "north pole" points. The fingers curl around the wire in the direction of the magnetic field if the right hand is wrapped around the wire with the thumb pointing in the direction of the current.

Oersted's Experiment Examples

Let's apply the right-hand rule to a simple example to determine if we understand how it may be used to find the direction of the magnetic field. In the figure below, a current-carrying wire is passing through the hand with the current flowing up and to the right. We know from Oersted's experiment that this current will generate a magnetic field, but in what direction will this field point?

A current passes in a direction that is up and to the right. The right-hand rule can be used to find the direction of the magnetic field, adapted from image by SVGguru CC BY-SA 4.0

The right-hand rule can be used to determine the direction in which the magnetic field lines will point. That is, the thumb follows the direction of the current; up and to the right. A straight current firstly indicates that the magnetic field will be curling around the wire. Then, the knuckles that are facing us (out of the page) indicate that at that point the magnetic field lines are pointing toward us, and the fingertips pointing away from us (into the page) indicate that at that point the magnetic field lines are pointing away from us. The image below then gives us the complete picture of the direction of the magnetic field (drawing only one field line for simplicity).

The current passing through a wire in the direction identified in red will produce a magnetic field in the direction indicated in blue, Wikimedia Commons CC BY-SA 4.0

If we were to place a compass in the vicinity of this wire we would notice that the north pole of that compass will deflect to point in the direction of the field lines. Inverting the direction of the current will invert the direction in which the curling magnetic field lines will point. This is essentially what Oersted had discovered in his experiment and has formed the basis of electromagnetism.

A Summary of Oersted's Experiment of Electromagnetism

We can now summarise Oersted's discoveries from his experiment. For a straight wire carrying a constant direct current (DC), Oersted discovered that:

• The current-carrying wire is encircled by magnetic field lines.
• The magnetic field reverses direction when the direction of the current is reversed.
• The magnitude of the current was directly proportional to the strength of the magnetic field.
• The strength of the magnetic field was inversely proportional to the distance from the current.