P221 Hints.

Quiz hints (a.k.a. Study questions) for Physics P221

Don't forget, Quizzes also include:

Questions about the lab exercises.
Questions you missed on the last quiz.
Questions on homework assignments.
Questions similar to the ones we did in class.

We will probably not get to everything on this list. Check the assignments page (and come to class!) to find out what exactly what we have and have not covered.

Electric fields

Which of the following are units of the electric field? Vm eV V/m N/C J/C Nm/C V/W
It is possible to demonstrate the electrical attraction between two charged objects very easily as was shown in class. Why is it much more difficult to demonstrate the gravitational attraction between two small objects?
Explain why the electric field must be zero in a conductor under electrostatic conditions but not if current is allowed to move.
If the electrical force is so much stronger than the gravitational force, why does gravity and not electrical force hold the moon in orbit around the earth?
The equation relating the force on a charge to an external electric field is q E = F (notice that E and F are vectors). If the charge q is a proton, which direction will the force act if the electric field points north? If the charge q is an electron and the electric field points north, which direction will the force act?
In the definition of an electric field, why must the test charge be very small?
If you rub a balloon with rabbits fur it stuck to the wall. Explain how this works.
Why is it not a good idea to use silk (or wool or rabbit fur) to wipe the dust off a record or television screen?
Can two lines of force ever intersect? Explain your answer.
In Coulombs law, what change would there be in the force of attraction between an electron and a proton if the proton were to have a negative 1.6x10^-19 C of charge and the electron a positive 1.6x10^-19 C of charge instead of the way it really is?
All of chemistry is based on the electrical attraction between negatively charged electrons and positively charged protons. What would be the result in chemistry if every proton in the universe became negatively charged and every electron became positively charged but with the same magnitude of charge as they have now?
Suppose an electric field points from the floor to the ceiling in this room. If a proton is shot from the back of the room towards the front, which way will the path curve?
Suppose an electric field points from the floor to the ceiling in this room. If an electron is shot from the back of the room to the front, which way will the path curve?
A person standing on an insulated stool touches a charged insulated conductor. What happens?
Suppose you form a Gaussian surface outside a point charge which does not include the charge. Why doesn't this prove that the field around a point charge is always zero?
What is the total flux through the following closed surfaces if a charge q is placed inside each: sphere cube cone hemisphere
Prove the following statement using Gauss' law and electrostatic conditions: The excess charge on a conductor resides on the outer surface of the conductor.
A charge q is placed at the center of a spherical Gaussian surface. What happens to the flux through the surface under the following changes: The radius is doubled. The charge is moved off center. The charge is doubled. The charge is replaced with a charge of -q.
What happens to the electric field in each of the four cases above?
What two things do you want to think about when choosing a Gaussian surface?
Two point charges of unknown magnitude and sign are a distance d apart. The electric field is zero at a point on a line between them. What can you conclude about the charges?
Why is Gauss' law not useful in calculating the field due two three charges located at the corners of a triangle?
What is meant by the statement "charge is quantized" ?

Magnetic fields

All magnetic fields can be said to be caused by _________________? Explain.
What is the difference between magnetic flux and magnetic field?
A proton is shot from the back of the room towards the front of the room and a magnetic field points from the floor towards the ceiling. Which way will the proton's path bend?
A piece of iron which originally unmagnetized is heated while oriented in a north-south direction. When it is cooled it is found to have a permanent magnetic field. Explain what happened.
A small piece of metal will accelerate towards a magnet when placed near enough. Doesn't this contradict conservation of energy? Where does the energy come from?
List several similarities and several differences between electric and magnetic fields.
It is found that charged particles from space called cosmic rays strike the earth more frequently at the poles than at the equator. Explain why.
If a charged particle moves in a straight line through a region of space where there is no electric field, can you say that the magnetic field in that region is zero? Explain.
An electron is shot from the back of the room towards the front and a magnetic force causes the path to bend towards the right (if you are facing the blackboard).
What direction does the magnetic field point that causes the force which causes the bending?
Charged articles from the sun go into orbit around the earth because of the earth's magnetic field. If the particles end up orbiting East to West, what charge is on them?
A proton is shot from the back of the room towards the front of the room and a magnetic field points in the same direction. Which way will the path of the proton bend? <>
<>An electron is shot from the back of the room towards the front of the room and a magnetic field points from the ceiling towards the floor. Which way will the electron's path bend?
<>Explain how a velocity selector for charged particles works.
<>Suppose a velocity selector is set up for electrons coming from a source in the back of a TV picture tube. What would happen if the electrons were replaced with protons? Explain?
<>A piece of flexible wire is anchored with the two ends at the same point. If a current flows through the wire, will the magnetic forces act to cause the wire to bunch up or spread into a loop? (The answer is not neither.) Explain.
<>A current is flowing north along a power line. Which direction does the magnetic field point above the wire?
<>The earth's magnetic north pole is the south pole of an internal magnet. One theory for explaining this magnetic field says that it is caused by a current moving in the iron core of the earth. Describe the orientation of this current loop and the direction the current would flow. <><>
<><>What is the law of Biot and Savart used for?
<><>Explain the physical meaning of each term of the law of Biot and Savart<><>and explain what the law says.
<><>Explain the physical meaning of each term in Ampere's law<><>and explain what the law says.
<><>A hollow pipe carries a current along it's length. What can you say about the magnetic field inside the tube and how do you know this?<><><>
<><><>In a particle accelerator a beam of particles travels down a long evacuated tube before colliding with a target. Explain why the beam will change diameter and tell whether the diameter gets bigger of smaller. Assume no external magnetic or electric fields.
<><><>Suppose we wish to apply Ampere's law to a cable with 12 wires each carrying current I. If 7 wires carry this amount of current in one direction and the other wires carry this amount in the opposite direction, what current is to be used in the right hand side of Ampere's law?
<><><>A current is sent through a vertical spring from whose lower end a weight is hanging. How will the magnetic effects of the current change this arrangement and why?
<><><>Use a law of physics you have recently learned to prove that the earth's magnetic field cannot come from charges circling above the earth's surface.
<><><>A loop of current carrying wire is in the x-y plane. The current is clockwise as viewed from the z axis. If a magnetic field is turned on in the +x direction, Which direction will the loop turn? In the last question, which direction does the torque vector point?

Electric potential

If the ceiling is at a higher electrical potential than the floor and a proton is shot form the back of the room towards the front, which way will the path curve?
Suppose you have a small positive test charge in the field around a point charge. Which path could you move this test charge without doing work on it?
Two point charges of unknown magnitude and sign are a distance d apart. The electric potential is zero at a point on a line between them. What can you conclude about the charges?
If the potential difference between two points is zero, must the electric field also be zero at both points? Give an example.
What can be said about the electric field in a region of space where the electrical potential is constant?
What is the difference between the electric field and the electric potential?
It was reported in a newspaper account that an electrical worker was accidentally electrocuted when " he accidentally touched a high voltage cable and 20,000 V of current surged through his body". What is wrong with this description?
Which of the following are vector fields? a) The gravitation field. b) The electric field. c) The electric potential. d) Magnetic flux. e) Electric flux.
Birds sit on high-tension wires and do not get electrocuted, even when the wire is bare, yet a squirrel which steps from a bare wire to a pole or to another wire dies instantly. Why?
How can the principle of conservation of charge be true if rubbing a rubber rod with fur makes it charged?
The electron volt is a unit which measures what?
A coulomb of positive charge is made to move from a potential of +10V to a potential of -10V. Does the charge gain energy or lose energy? How much energy is gained or lost?
An electron is pushed into an electric field where it acquires a 1V electrical potential. Suppose instead that two electrons are pushed the same distance into the same electric field. What would the potential of the two electrons be?
Suppose you move a charge from point A to point B and find you have done no work. Does this mean there is no electric or magnetic field between A and B? Explain.
A molecule whose charge distribution is not perfectly symmetrical is called a _____________________ .
Suppose an electric field points upward. What orientation of a dipole would have the a) greatest potential energy? b) lowest potential energy?
What is an equipotential?
What is the difference between the electric field and the electric potential?

Electromagnetic properties of matter

All magnetic fields can be said to be caused by _________________? Explain.
Explain the difference between paramagnetism and diamagnetism.
What do we have to change in Maxwell's equations in order to apply them inside material that is dielectric?
What do we have to change in Maxwell's equations in order to apply them inside material that is paramagnetic?
Theories in particle physics lead us to believe that the neutron (which has no net charge) is made of three quarks (two with a charge of +1/3 e and one with a charge of -2/3 e). How does this help to explain that neutrons have a magnetic moment even though they are neutral in charge?
Methyl alcohol is a polar molecule. How does this help explain why it has a large dielectric constant?
Explain the difference between ferromagnetism, paramagnetism and diamagnetism.
What happens to an electric field in the presence of a dielectric and why?
What happens to an applied magnetic field in the presence of a diamagnetic material and why?
What happens to an applied magnetic field in the presence of a ferromagnetic material and why?
What is the difference between a ferromagnetic and a paramagnetic material?
Explain why the field inside a dielectric is less than the field in a vacuum if the external charge is constant.
What is the difference between free and bound surface charge density?

Faraday's law

A loop of wire lies in the plane of this page and a constant magnetic field points into the page at 90o. Which direction will the induced current flow?
List and explain some sources of error in the Faraday's Law falling magnet lab. Hint: 'Human error' doesn't count.
A changing magnetic field points from the left side of this page towards the right side. Which direction will the current be induced in a loop of wire lying in the plane of the page? Explain. (trick question)
A loop of wire connected to a resistor is horizontal to the earth and a magnet is dropped through the loop. The magnet will slow down as it falls through the loop. Explain why.
If I drop a magnet through a loop of wire held horizontally, would you expect the magnet to accelerate faster or slower than 9.8 m/s2 ? Explain.
A transformer with 10 turns in the primary and 100 turns in the secondary will convert an AC voltage of 5 V to 50 V. Explain why this doesn't contradict conservation of energy.
Satellites orbit the earth in the earth's magnetic field. This means that an emf is induced from one side of the satellite to the other. What is one reason not to use this emf to do work inside the satellite? (hint: where does the work/energy come form?)
Why don't transformers work with direct current?
In a beam balance scale an aluminum plate is sometimes used to slow the oscillations of the beam. The plate is mounted at the end of the beam so that it moves between the poles of a magnet as the beam slows its oscillations. Why does this arrangement dampen the oscillations?
You drop a magnet through a horizontal loop of wire with the north end pointing down. Looking down on the coil, which direction does the current flow while the magnet enters the coil? Which direction does the current flow as the magnet leaves the other side?
What is a "back" emf?
Give an example of a commonplace device which uses Faraday's law and explain how it works?
In the demonstration of the magnet falling through the copper tube, if the north end of the magnet is pointing downwards, which direction does the induced current flow if you looking from the top of the tube?
What is the primary difference between an electric motor and an electric generator?
In order to trigger a traffic signal you drive your car over a large loop in the road. How does this work? (HINT: It does NOT have to do with the weight of your car.)

Waves and vibrations

What is the difference between longitudinal and transverse waves?
Explain the superposition principle for waves.
Pitch in a sound wave corresponds to which numerical quantity?<>
<>What quality does frequency tell you about for visible light?
<>How are wavelength and frequency related?
What is interference of waves? Give some examples.
What is the Doppler effect? Give some examples of the use of the Doppler effect in every day life.<>
<>What are sound waves?
<>Why don't sound waves travel in a vacuum?
Do radio waves travel in a vacuum? Light waves?
If you hear the sound of thunder 3s after you see the lightening, about how far away is the storm?<>
<>What is resonance?
<>What are beats?
How is an AM signal different from FM?
What is the difference between radio waves and sound waves?
How does Hooke's law figure in simple harmonic motion?
What are some examples of simple harmonic motion?
For the frequency of a mass on a spring in simple harmonic motion the length of the spring does not matter but the mass does but for a pendulum the length matters but the mass does not. Why is that?
Suppose you have a Hooke's law force that looks like F = C ln(a) x/b where C, a and b are constants. What would the frequency be for harmonic motion due to this force?
Under what circumstances do we expect that resonance might become a factor for a vibrating system?
Suppose you have a system that will undergo resonance at a certain frequency. List several ways to prevent resonance for the system while still having it vibrate.

Modern physics

What is meant by the wave-particle nature of an electron? (Aren't these two ways of viewing something incompatible?
What are some of the features of the Bohr model of the atom which cannot be explained by classical physics?
Why does putting a particle in a box cause the energy to be quantized?
What are the four principle quantum numbers and what classical, physical properties does each relate to?
How can the spectrum of hydrogen have many lines (colors) although a hydrogen atom contains only one electron?
What would happen to the hydrogen spectra if all the energy levels were equally spaced?
If Planck's constant were larger then it is would the wave nature of electrons be more or less noticeable than now? Explain.
What is blackbody radiation and why was it important in this chapter?
Explain why the energy of any object becomes quantized when it is put into a box.
Why don't we see a tennis ball in a box as having quantized energy levels?

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