Video Transcript
Which of the following could be a source of radio waves? (A) Decaying atomic nuclei. (B) Direct electric currents. (C) Alternating electric currents. (D) High-speed electrons impacting on a metal plate. Or (E) none of the answers is correct.
First off, let’s recall that radio waves is a type of electromagnetic radiation characterized by having the lowest range of energy and frequency in the electromagnetic spectrum. Now, answer choice (A) says decaying atomic nuclei, which refers to the nucleus of an atom breaking apart, which releases energy in the form of electromagnetic radiation. This radiation typically has a very high energy and frequency, falling within the range of what are known as 𝛾 rays. 𝛾 radiation is on the other extreme end of the spectrum, so decaying atomic nuclei is not a possible source of radio waves.
Next, answer choice (B) says direct electric currents. We can recall that electromagnetic radiation is essentially caused by the acceleration of electric charge. And while it is true that an electric current is the movement of electric charge, a direct current has a basically constant value of charge movement over time. So once a direct current is established, the charge no longer accelerates and therefore won’t emit electromagnetic radiation such as radio waves.
Now, answer choice (C) says alternating electric currents. So once again, we’re talking about the movement of electric charge, but an alternating current has a continuously changing value over time. This means alternating electric currents accelerate electric charge over and over again. This process emits electromagnetic radiation on the lower energy end of the spectrum, including radio waves. So option (C) looks pretty good, and we can eliminate (E).
But let’s go ahead and check out option (D). It turns out fast-moving electrons hitting a metal plate accelerates electrons in the plate really rapidly, which releases electromagnetic radiation typically in the form of X-rays, which are much higher energy than radio waves. So let’s eliminate answer choice (D). And we know (C) is the correct answer. Alternating electric currents are a possible source of radio waves.
