SP211
Worksheet 32
33.1-33.3: Electromagnetic Waves
Problem 1
The figure below shows the electric field at some point in space of an electromagnetic wave and some instant
instant. The wave it traveling to the right (the +x direction).
(a) What is the magnetic field vector at this instant? Give both the magnitude and direction.
(b) What is the Poynting vector at this instant? Give both the magnitude and direction.
Problem 2
An electromagnetic wave has an oscillating magnetic field given by
B = (0.0125 T) sin[(9.67 × 106 radians/m)x − (2.90 × 1015 radians/s)t].
(a) What is the intensity of this light?
(b) What is the wavelength of this light?
Problem 3
A light bulb acts as a point source with a 120 W power output. Assume that all of this power is output as
light.
(a) What is the amplitude of the oscillating electric field of the light given off by this bulb at a distance of
4.5 m away?
(b) What is the amplitude of the oscillating magnetic field at this position.
(c) What is the rms (root mean squared) value of the electric field at this position?
Problem 4
A square sheet of paper is 2.5 m long on each side. The paper is held perpendicular to a bright light with
an intensity of 1500 W/m2.
(a) If the paper is black and absorbs all of the incident light, what is the magnitude of the force pushing on
the paper?
(b) Instead, assume that the paper is shiny and perfectly reflects all of the incident light. What is the
magnitude of the force pushing on the paper?
HINT: When light is perfectly reflected there is a double momentum kick from against the surface just like
when a pall bounces off a wall.