Question

Which of the following statements are true? (Select all that apply.)

(A) At any time, the electric and magnetic field lines are parallel to each other.
(B) All moving electric charges give rise to electromagnetic waves.
(C) Electromagnetic waves can exert forces on charged particles that they encounter.
(D) The intensity of the wave drops off as 1/r.
(E) Electromagnetic waves are produced when changing electric fields which produce magnetic fields which are also changing, thus producing a changing electric field, and so on.
(F) The direction of v can be found using the right-hand rule. Put your four fingers along E, sweep them in the direction of B and your thumb will point along v.
(G) Electromagnetic waves carry energy and momentum.

Answer 1

(B), (C), (E), (F) and (G) are true statements for electromagnetic waves.

Explanation:

• An electromagnetic wave has both electric and magnetic fields perpendicular to each other and hence (A) is false.

• Statement (B) is true since moving charges back and forth are accelerating and they give rise to oscillating electric and magnetic fields that travel at the speed of light.  

• The produced electric field is capable of exerting forces on other charged particles. Ans so, (C) is true.

• Intensity of an EM wave falls off by $\frac{1}{r^2}$ and hence (D) is false.

• A changing magnetic field gives rise to a changing electric field and vice versa. Hence, (E) is true.

• Statement (F) states the right hand thumb rule. Put your first four fingers excluding thumb straight in the direction of electric field and curl them inwards towards the direction of magnetic field; the direction of force $V = (E \times B)$ is their cross product and its direction is given by the direction to which thumb points.

• Even though EM waves do not carry mass; they carry energy and momentum. (G) is true.