Reading Comprehension: Changing Fields as a Wave

Changing Fields as a Wave

So far, we have explored different types of waves, including a string wave and radio waves. What exactly happens to the energy when a wave radiates away from a source? What could cause the energy in an electromagnetic wave to decrease as it travels?

Changing Fields Transfer Energy

A charged particle produces an electric field. When this charged particle moves, it causes changes in the electric field around it, and those electric field changes travel away from the source in all directions, like ripples in a pond. But what causes the electric field to radiate outward? Why does the electron have to be moving to cause fields to radiate in this way?

Experiments with electricity and magnetism show that a change in an electric field causes change in a magnetic field, and a change in magnetic field causes change in an electric field. This is what we call electromagnetic radiation: fields changing other fields over and over, transferring energy across empty space.

The light we see, phone signals, and microwave radiation from a magnetron are all made of changing electric and magnetic fields. These electromagnetic waves can travel through empty space forever, but when a wave interacts with matter, such as the electrons in a phone antenna or the food in a microwave oven, it transfers energy to that matter.

As we know from experience, most electromagnetic radiation seems to lose energy with distance. A Bluetooth signal gets weaker the farther you go from your phone, and light from a distant star is much less bright than it would be up close. Much of this change in energy occurs because the wave radiates in many directions. Even when the total energy of electromagnetic waves in all directions does not decrease, the energy transferred in one specific direction tends to become less and less the farther from the source we get.