Question 15 (1 point) An electromagnetic standing wave in air of frequency 750 M

Question

Question 15 (1 point) An electromagnetic standing wave in air of frequency 750 MHz is set up between two conducting planes 80.0 cm apart. At which positions between the planes could a point charge be placed at rest so that it would remain at rest? 1 place: halfway between the plates. 2 places: 30 cm and 50 cm from one of the plates, between the plates. 4 places: 16 cm, 32 cm, 48 cm and 64 cm from one of the plates, between the plates. There is no such spot between the plates where this is possible. 3 places: 20 cm, 40 cm and 60 cm from one of the plates, between the plates. 7 places: 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm and 70 cm from one of the plates, between the plates.

Explanation / Answer

We know that electric field at the nodes is Zero, Now since electrostatic force is given by:

F = q*E

at nodes, E = 0, which means Force would also be zero.

Now location of nodes on a standing wave is

x = nodes = lambda/2

x = c/(2*f)

x = 3*10^8/(2*750*10^6) = 0.2 m = 20 cm

first node will be at x = 20 cm, Now each next node will be at distance of 20 cm from each other, So

Nodes between planes are at = 20 cm, 40 cm, 60 cm

Correct option is E.

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