Homework 9 Solution

Any complaints email to janovi@wm.edu.

Exercises

5.) A tank circuit consists of an inductor and a capacitor. Give a simple explanation for why the magnetic field in the inductor is strongest at the moment the separated charge in the capacitor reaches zero.

Answer:Capacitors are devices that store charge (remember, you do work on the charge to get it there) which translates to stored energy. Now, when a current is moving through a wire, we know that there is a magnetic field circling the wire. We also know that that magnetic field contains energy. This energy comes from the current. In our situation,the current comes from the charged capacitor. As the charge in the capacitor makes its way into the current, the current and hence the magnetic field increase. So, when all of the charge in the capacitor has left, the current is at a maximum and the magnetic field is strongest.

10.) Cordless telephones often have a switch that allows you to change the radio frequencies they use to communicate with their base units. Why is this feature more important in cities than it is in rural areas?

Answer:It's population density. In a city, you will find more devices, and a higher rate of people using such devices, that produce radio (electromagnetic) waves. Many of these devices will most likely transmit radio waves within similar frequency ranges. In other words, you may pick up your neighbor's cell phone converstation on a baby monitor so your neighbor needs to pick another frequency on which to talk.

13.) When an AM radio station announces that it's transmitting at 950 kHz, that statement isn't quite accurate. Explain why it may also be transmitting at 948 kHz and 954 kHz.

Answer:AM transmission isn't perfect. The actual frequencies will lie within 5 kHz above or below the reported value.

15.) If you tried to communicate with a friend using radios that operated at 2.45 GHz, you'd probably hear lots of noise instead of your friend's voice. Explain.

Answer:Its microwave ovens. There are lots of them in use and the waves they use to heat food oscillate at 2.45 GHz, which is the same frequency at which your radio is transmitting. So the signals you send to your friend will interfere with the microwaves in the background.

18.)The force that a magneic field exerts on a moving charged particle does no work on that particle. Use the definition of work to explain why there's no work done.

Answer:The work done on an object is the force exerted on it times the distance it travels where the force and distance must be parallel. In the case of a charged particle moving through a magnetic field, the field does no work on the particle because the force is perpendicular to the displacement.

26.) Why are most microwave TV dinners packages in plastic rather than aluminum trays?

Answer:The microwaves would reflect off the aluminum and never get into the food to excite the water molecules.

Problems

1.)The frequency of the radio wave emitted by a cordless telephone is 900 MHz. What is the wavelength of the wave?

Answer:We will use the equation v = wavelength * frequency. We know v, its the speed of light. We are given frequency. So solving for wavelength, we get (3 x 10^8 m/s)/(900 x 10^6 Hz) = 0.333 m

3.)The electromagnetic waves in blue light have frequencies near 6.5 x 10^14 Hz. What are their wavelengths?

Answer:We follow the same procedure used in problem 1. The wavelenth is 4.615 x 10^-7 m

Cases

1a.)He gives a hint in saying the liqueur is a water alcohol mixture. Now, im assuming that the meringue contains very little water, so that after being taken from the freezer, the microwave will not heat it. However, the liqueur does contain water so when it is in the microwave, the meringue will NOT absorb the microwaves while the liqueur molecules will and get excited.

1b.) Well, you have a hard dry shell surrounding something containing water. So as the microwaves heat the egg, the water (liquid) will heat and start to expand. However, the dry shell will not expand so core will keep expanding until the pressure is so great the shell will crack. The same will happen to popcorn, which (prior to cooking) is a hard shell containing a moist core.

1c.) Thin metallic sheets have high resistances. If you took a thick piece of metal, it would have a low resistance since there is plenty of space for the electrons to move around. If you make it thinner and thinner, then the electrons will have less space and so start bumping into each other more. When this happens, they transfer their kinetic energy to each other and eventually, the energy gets transferred to the walls.

1.d) To determine why the microwaves will not get through, all we have to do is calculate the wavelength of a microwave and compare that to the diameter of a hole. Using 2.45 GHz for f and using speed of light = wavelength*frequency, we find that the wavelength is about 3 cm(i rounded 2.45 GHz). This is an order of magnitude greater than the size of a hole. The microwave's wavelength is too large so it will reflect from the hole.

5a.)The reciever knows when it recieves the time, and it also knows when the pulses are emitted since they are timed by a standard clock. So the reciever knows the time difference between the emission and reception of the pulse. And since it knows the speed (the speed of light of course), you can use the definition of speed (v = distance traveled / time of travel) and solve for distance traveled.

5d.) The lower frequency is 1.2276 x 10^9 Hz (1.2276 GHz). An antenna's length should be a quarter of the wavelength of the wave it will receive. So the length of this particular antenna should be (3 x 10^8 m.s) / (4 * 1.2276 x 10^9 Hz) = 0.061 m