1. How a Magnet Can Move an Iron Clip Without Touching It
Bring a magnet slowly toward an iron clip on a desk. Before they touch, the clip slides toward the magnet. Nothing visible lies between the magnet and the clip. [1]
For this "force acting without contact," there have long been two ways of thinking. One view is that force reaches directly across empty space; this is called action at a distance. The other view is that force travels through changes in a medium or "field" between objects; this is called action by proximity. [1]
Faraday is said to be the first person to consider action by proximity, inspired by experiments. What did Faraday see that led him to this idea? [1]
2. How Iron Filings on Paper Reveal the Lines Around a Magnet
Faraday conducted an experiment to show the lines around a magnet. He coated paper with thin wax, sprinkled iron filings gently on top, and placed the magnet underneath. The filings arranged themselves spreading out from both ends of the magnet, forming smooth curved patterns. [3]
Faraday called these lines "magnetic lines of force." He explained that the arrangement of iron filings and small compass needles shows the general path of these lines. In 1851, he pasted a diagram of these iron filings into his experimental notebook. [2]
Patterns form even in places not touching the magnet. The space "near" the magnet might not be empty. But are these lines really present in space? [2]
3. Faraday's Question About Magnetic Lines and Maxwell's Idea of a Field
In a 1852 paper, Faraday asked whether magnetism acts directly at a distance or if the force is transmitted through physical interactions in the space between objects. Whether magnetic lines physically exist was the question of the time. [2]
Maxwell tried to explain this question with mathematics. On December 8, 1864, he read "A Dynamical Theory of the Electromagnetic Field" to the Royal Society. He defined the electromagnetic field as the part of space surrounding electrically and magnetically charged bodies, describing its behavior with the concept of a "field" and equations. [4]
One view is that magnetic lines are curves representing the direction and strength of the magnetic field. It is not that physical strings hang in the air. Instead, like how the direction and strength of a ball rolling down a slope are determined at every point, the magnetic field’s direction and strength are determined at every point in the space around a magnet. The magnetic field described here is not a continuously moving wave. However, unlike a slope, the true field is invisible. [4]
4. Why a Magnet's Force Weakens So Quickly as You Move Away
From here, we consider each bar magnet as a magnetic dipole, a pair of N and S poles, separated by a distance much greater than the length of the magnets. This is separate from the force pulling iron like a clip. The way the magnetic field strength around a magnet weakens differs from the way the force between two magnets weakens.
The strength of the magnetic field created by a magnet in the surrounding space weakens roughly in inverse proportion to the cube of the distance. [6] Using a compass and a magnet, this can be investigated in a simple experiment as the relationship between the tangent of the deflection angle of the needle (tan θ) and the distance, with an accuracy of about 10%. [6]
The force acting between two magnets is inversely proportional to the fourth power of the distance, in the range where the magnets are much farther apart than their length. [7] When the distance doubles, the force becomes 1 divided by 2 to the power of 4, which is 1/16th. If it triples, it becomes 1/81st.
Light from a lamp becomes about 1/4th as bright when the distance doubles. The force of magnets weakens more suddenly than that. The reason a magnet pulls strongly when close but feels almost nothing when moved slightly away is due to this rapid weakening. However, this relationship does not hold when magnets are close together.
Magnets have N and S poles. From a distance, the forces of the two poles cancel each other out. Therefore, the force weakens much more suddenly than the force of a single pole (which is inversely proportional to the square of the distance). [7]
5. How Maxwell Showed the Field Also Carries Light
In the same paper, Maxwell stated that light itself is an electromagnetic disturbance propagating as a wave through the electromagnetic field. He predicted that electric and magnetic fields propagate as waves and considered this to be light. [4]
This prediction was confirmed experimentally by Hertz in 1888. [5] By thinking about the familiar phenomenon of a magnet pulling iron, the field concept helped explain light and radio waves.
The way of thinking about the space around a magnet also led to the idea of explaining light as waves of the electromagnetic field. [5]
6. How to Test Force and Distance With a Magnet and a Compass at Home
If you have a magnet and a compass at home, you can observe the relationship between distance and needle deflection. Place the compass on a desk. When the needle stops, place the magnet at a slightly distant position. The needle turns toward the magnet.
Slowly move the magnet backward, measuring the distance with a ruler, and write the needle's deflection in a notebook. Compare how much the needle moves each time you change the distance. Keep the magnet's orientation the same each time. The farther you move it, the closer the needle gets to its original orientation pointing to Earth's north.
Do not place magnets near computers, smartphones, or cash cards. They may be damaged or information may be erased.
This experiment shows how the magnetic field around a magnet weakens. This is different from the 1/16th force between two magnets. Observe with your own eyes how the needle moves greatly when close but stops moving suddenly when moved slightly away. Testing the strength of pulling a clip at different distances is also a good observation.