M7-S1: Maxwell's Contribution to Electromagnetism
- In the 1830s, Michael Faraday proposed that changing magnetic fields produced electric fields
In Module 6: Electromagnetism, we learnt about Faraday’s Law of induction whereby when a conductor experiences change in magnetic flux (changing magnetic field), a current is induced. The induction of current resembles an electric field as electrons are directed in a certain direction that is parallel to the orientation of the electric field.
- In 1861, Scottish James Maxwell extended and corrected Faraday’s proposal by postulating that changing electric fields produced magnetic fields and in fact the two phenomena should be perceived as a single entity.
- This means oscillating electric fields would produce magnetic fields. Oscillating magnetic fields would produce electric fields.
- A moving electric charge would thus produce a magnetic field due to the presence of its intrinsic electric field.
- Maxwell’s Classical Theory of Electromagnetic Waves
- Maxwell’s classical model of light (Rayleigh and Jeans) proposes that the energy contained within light or any other electromagnetic wave is continuous and is inversely proportional to its wavelength.
- In his 1864 paper ‘Electromagnetic Theory of Light’, Maxwell used experimental findings and differential equations to calculate the speed of electromagnetic waves.
- Maxwell’s contribution to electromagnetism allowed for different and more accurate methods to be used to calculate the speed of light.
Rosa and Dorsay in 1907 indirectly determined the speed of light through using the electric and magnetic permeability of air.
- This positive finding verified the existence of electromagnetic waves and confirmed that light in fact does have electromagnetic properties.
- The value attained from this experiment had to be later corrected because speed of electromagnetic waves was deduced to be independent of the medium they propagate in.
- The German physicist Heinrich Hertz was the first to generate and detect certain types of electromagnetic waves in the laboratory. He performed a series of experiments that not only confirmed the existence of electromagnetic waves, but also verified that they travel at the speed of light.
Figure 2: Hertz's experimental set-up.
Hertz used an AC circuit that resonates at a known frequency and connected it to a loop of wire as shown in Figure 2. High voltages induced across the gap in the loop produced sparks that were visible evidence of the current in the circuit and that helped generate electromagnetic waves.
- Frequency of alternating current = frequency of oscillating electric field = frequency of oscillating magnetic field = frequency of electromagnetic wave produced.
Across the laboratory, Hertz had another loop attached to another circuit, which could be tuned (as the dial on a radio) to the same resonant frequency as the first and could, thus, be made to receive electromagnetic waves. This loop also had a gap across which sparks were generated, giving solid evidence that electromagnetic waves had been received.
Hertz also studied the reflection, refraction, and interference patterns of the electromagnetic waves he generated, verifying their wave character.
- The contribution of Maxwell to electromagnetism inspired Heinrich Hertz in 1888 to discover the Photoelectric Effect.
However, he was unable to fully explain why only sparks of certain frequency can induce sparks at the receiving coil, regardless of their intensity.
Next Section: Speed of Light