Question

1.You wish to send a probe to the Moon. The probe has a radio transmitter that you test in the laboratory. When the probe is 76 m from your receiver, the intensity is I₀. When the probe is on the Moon, it sends radio waves to you. If you want to receive radio waves of the same intensity, how much stronger must the probe's electric field be when it is on the Moon? (Treat the radio transmitter as a point source.)

E_{max, moon/}E_{max, 0=}

2. A budding magician holds a 1.23-mW laser pointer, wondering whether he could use it to keep an object floating in the air with the radiation pressure. This might be an idea for a new trick! Assuming the laser pointer has a circular beam 3.00 mm in diameter and the magician rigs up a totally reflecting sail on which to shine the laser, what is the maximum weight the magician could suspend with this technique?

Answer 1

1.

Distance of moon from the earth = 3*10^8 m

Intensity 1/distance^2

Let Emax,0 be the Electric Field radiated from the lab

and Emax,moon be the Electric Field radiated from the moon

Emax Intensity

Emax moon / Emax lab = I0 / 6.4 * 10^-14 I0= 1.5 * 10^13

2.

Diameter = 3mm

Radius = 3/2 mm = 1.5 * 10^-3 m

Spherical area = 4 pi r^2 = (2.8 * 10^-5) m^2

Force = Power* Area = (1.23 * 10^-3) * (2.8 * 10^-5) = 3.444* 10^-8 N

The force exerted by the laser is equal to the weight the magician could suspend.