Electric Potential Energy Problems
The specific principles required are indicated in italics at
the beginning of each problem.
- Electric Potential Energy: While sitting in a
restaurant with some friends, you notice that some
"neon" signs are different in color than
others. You know that these signs are essentially just
gas sealed in a glass tube. The gas, when heated
electrically, gives off light. One of your friends, who
is an art major, and makes such signs as sculpture, tells
you that the color of the light depends on which gas is
in the tube. All "neon" signs are not made
using neon gas. You know that the color of light tells
you its energy. Red light is a lower energy than blue
light. Since the light is given off by the atoms, which
make up the gas, the different colors must depend on the
structure of the different atoms of different gases.
Suppose that atomic structure is as given by the Bohr
theory which states that electrons are in uniform
circular motion around a heavy, motionless nucleus in the
center of the atom. This theory also states that the
electrons are only allowed to have certain orbits. When
an atom changes from one allowed orbit to another allowed
orbit, it radiates light as required by the conservation
of energy. Since only certain orbits are allowed, so the
theory goes, only light of certain energies (colors) can
be emitted. This seems to agree with the observations of
your artist friend. You decide to test the theory by
calculating the energy of light emitted by a simple atom
when an electron makes a transition from one allowed
orbit to another. You decide to consider hydrogen since
you know it is the simplest atom with one electron and a
nucleus consisting of one proton. You remember that the
proton has a mass 2000 times that of an electron. When
you get home you look in your textbook and find the
electron mass is 9 x 10-31 kg and its charge is 1.6 x
10-19 C. The radius of the smallest allowed electron
orbit for hydrogen is 0.5 x 10-10 meters, which
determines the normal size of the atom. The next allowed
orbit has a radius 4 times as large as the smallest
orbit.
- Electric Potential Energy: You have a great summer
job working in a cancer research laboratory. Your team is
trying to construct a gas laser that will give off light
of an energy that will pass through the skin but be
absorbed by cancer tissue. You know that an atom emits a
photon (light) when an electron goes from a higher energy
orbit to a lower energy orbit. Only certain orbits are
allowed in a particular atom. To begin the process, you
calculate the energy of photons emitted by a Helium ion
in which the electron changes from an orbit with a radius
of 0.30 nanometers to another orbit with a radius of 0.20
nanometers. A nanometer is 10-9 m. The helium nucleus
consists of two protons and two neutrons.
- Electric Potential Energy: Your job is to evaluate
an electron gun designed to initiate an electron beam.
The electrons have a 20 cm path from the heating element,
which emits them to the end of the gun. This path is
through a very good vacuum. For most applications, the
electrons must reach the end of the gun with a speed of
at least 107 m/s. After leaving the heating element, the
electrons pass through a 5.0 mm diameter hole in the
center of a 3.0 cm diameter charged circular disk. The
disk's charge density is kept at 3.0 µC/m2. The heating
element is a spherical electrode 0.10 mm in diameter that
is kept at a very high charge of -0.10C. There is 1.0 cm
between the heating element and the hole in the disk.
Your first step is to determine if the electrons are
going fast enough. Your boss has pointed that the hole in
the disk is too large to ignore in your calculations.
Using your physics text you find that the mass of the
electron is 9.11 x 10-31 kg.
- Electric Potential Energy - Gauss's Law: You have
landed a summer job working with an Astrophysics group
investigating the origin of high-energy particles in the
galaxy. The group you are joining has just discovered a
large spherical nebula with a radius 1.2 million km. The
nebula consists of about 5 x 1010 hydrogen nuclei
(protons) which appear to be uniformly distributed in the
shape of a sphere. At the center of this sphere of
positive charge is a very small neutron star. Your group
had detected electrons emerging from the nebula. A friend
of yours has a theory that the electrons are coming from
the neutron star. To test that theory, she asks you to
calculate the minimum speed that an electron would need
to start from the neutron star and just make it to
outside the nebula. From the inside cover of your trusty
physics text you find that the charge of a proton (and an
electron) is 1.6 x 10-19 C, the mass of the proton is 1.7
x 10-27 kg, and the mass of the electron is 9.1 x 10 -31
kg.
- Electric Potential Energy, Gauss's Law: You are
working in cooperation with the Public Health department
to design an electrostatic trap for particles from auto
emissions. The average particle enters the device and is
exposed to ultraviolet radiation that knocks off
electrons so that it has a charge of +3.0 x 10-8 C. This
average particle is then moving at a speed of 900 m/s and
is 15 cm from a very long negatively charged wire with a
linear charge density of -8.0 x 10-6 C/m. The detector
for the particle is located 7.0 cm from the wire. In
order to design the proper kind of detector, your
colleagues need to know the speed that an average
emission particle will have if it hits the detector. They
tell you that an average emission particle has a mass of
6.0 x 10-9 kg.
- Electric Potential Energy, Heat Energy (Heat Capacity,
Latent Heat): You are reading a newspaper
report of a lightning strike in Jackson, Wyoming. Two men
were sitting at a table outside a small cafe on a
beautiful 30 oC day when a thunderstorm approached.
Suddenly, a bolt of lightning struck a large aspen tree
near their table. Needless to say, the men were very
startled. One of the men remarked, "It just about
scared the espresso out of me." They reported that
when the bolt hit the tree and there was a loud hiss and
a release of much steam from the tree. The lightning had
boiled away some of the tree's sap. You are curious, and
wonder how much water could be evaporated in this manner.
So you study your physics book and make a few estimates
and assumptions. You estimate that the electric potential
difference between the tree and the thunderhead cloud was
about 108 volts, and the amount of charge released by the
bolt was about 50 Coulombs. You also assume that about 1%
of the electrical energy was actually transferred into
the sap, which is essentially water. The specific heat
capacity of water is 4200 J/(kg oC) and its heat of
vaporization is 2.3 x 106 J/kg.
- Electric Potential Energy, Gravitational Force:
NASA has asked your team of rocket scientists about the
feasibility of a new satellite launcher that will save
rocket fuel. NASA's idea is basically an electric
slingshot that consists of 4 electrodes arranged in a
horizontal square with sides of length d at a height h
above the ground. The satellite is then placed on the
ground aligned with the center of the square. A power
supply will provide each of the four electrodes with a
charge of +Q/4 and the satellite with a charge -Q. When
the satellite is released from rest, it moves up and
passes through the center of the square. At the instant
it reaches the square's center, the power supply is
turned off and the electrodes are grounded, giving them a
zero electric charge. To test this idea, you decide to
use energy considerations to calculate how big Q will
have to be to get a 100 kg satellite to a sufficient
orbit height. Assume that the satellite starts from 15
meters below the square of electrodes and that the sides
of the square are each 5 meters. In your physics text you
find the mass of the Earth to be 6.0 x 1024 kg.
- Electric Potential Energy, Mechanical Energy: You
have been able to get a part-time job in a University
laboratory. The group is planning a set of experiments to
study the forces between nuclei in order to understand
the energy output of the Sun. To do this experiment, you
shoot alpha particles from a Van de Graaf accelerator at
a sheet of lead. The alpha particle is the nucleus of a
helium atom and is made of 2 protons and 2 neutrons. The
lead nucleus is made of 82 protons and 125 neutrons. The
mass of the neutron is almost the same as the mass of a
proton. To assure that you are actually studying the
effects of the nuclear force, an alpha particle should
come into contact with a lead nucleus. Assume that both
the alpha particle and the lead nucleus have the shape of
a sphere. The alpha particle has a radius of 1.0 x 10-13
cm and the lead nucleus has a radius 4 times larger. Your
boss wants you to make two calculations:
(a) What is the minimum speed of such an alpha particle
if the lead nucleus is fixed at rest?
(b) What is the potential difference between the two ends
of the Van de Graaf accelerator if the alpha particle
starts from rest at one end (from a bottle of helium
gas)?