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Lab 8: Measuring the Speed of Light with a Microwave Oven | CPHY 122, Lab Reports of Physics

Material Type: Lab; Class: Physics II: Elec & Magnetism; Subject: Physics; University: Clark Atlanta University; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

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CPHY 122L
Lab 8
Measuring the Speed of Light with a Microwave Oven
1 Introduction
In Chapter 34 we will study electromagnetic waves. Light and microwaves are
familiar examples. In this lab exercise you will measure the speed of light
using a microwave oven by determining the wavelength of the microwaves
produced in the oven. The relationship between the wavelength and the
speed of light is
c=f; (1)
where fis the frequency.
The heating of foods by microwaves is caused by the transfer of energy
from the microwaves to the food. The key ingredient of food is water. The
energy transfer is made possible by the interaction of the water molecule’s
electric dipole moment with the microwaves. The potential energy of this
interaction is
U=pE;(2)
where p(p= 6:17 1030 Cm) is the dipole moment of a single water
molecule and Eis the time varying electric eld of the microwave.
2 Purpose
Measure the speed of light.
3 Apparatus
Bar of Chocolate (or slice of cheese ): dimensions greater than 18 cm
8 cm.
Standard Consumer Microwave Oven
Paper Towel
Tooth Picks
1
pf2

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CPHY 122L

Lab 8

Measuring the Speed of Light with a Microwave Oven

1 Introduction

In Chapter 34 we will study electromagnetic waves. Light and microwaves are familiar examples. In this lab exercise you will measure the speed of light using a microwave oven by determining the wavelength of the microwaves produced in the oven. The relationship between the wavelength  and the speed of light is c = f ; (1)

where f is the frequency. The heating of foods by microwaves is caused by the transfer of energy from the microwaves to the food. The key ingredient of food is water. The energy transfer is made possible by the interaction of the water moleculeís electric dipole moment with the microwaves. The potential energy of this interaction is U = p  E; (2)

where p (p = 6: 17  10 ^30 Cm) is the dipole moment of a single water molecule and E is the time varying electric Öeld of the microwave.

2 Purpose

Measure the speed of light.

3 Apparatus

 Bar of Chocolate (or slice of cheese ): dimensions greater than 18 cm  8 cm.

 Standard Consumer Microwave Oven

 Paper Towel

 Tooth Picks

 Ruler with centimeter measurement

4 Procedure

  1. Examine the microwave oven, the rear surface as well as inside the door, looking for a label stating the frequency. If there is no label containing this value, the majority of microwave ovens operate at 2 : 45  109 Hz.
  2. If the microwave oven has a rotating turn table remove or disable it.
  3. Remove the packaging from the chocolate and place on top of a piece of paper towel on a plate
  4. Place the plate with the paper towel and the chocolate in the mi- crowave, so that the chocolate is positioned with the longest sides facing the longest sides of the microwave.
  5. Heat the chocolate in the microwave until it barely begins to melt, pausing the oven to check occasionally (the time will vary with the oven, from 20 seconds to 90 seconds)
  6. Once the chocolate has begun to melt remove it from the microwave, probe the surface for a pattern of hot melted spots, about the dimen- sions of a dime.
  7. Use toothpicks to mark the hot-spots, placing them in the center of the hotspots.
  8. Measure the distance between the toothpicks, recording the data. Note that the distance between the toothpicks is one-half the wavelength. Your experimental value of  is found by averaging at least 3 measure- ments.
  9. Compare your results to the speed of light given in your textbook. This means calculate the absolute error.