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Review Activity
Unit 7 – Electrostatics & Magnetism
1.
Name ____________________________
Period ____
Again, which one of these (mass or force) is a vector and why? Which of these, mass or force, is weight?
Why? What is the unit of force in the mks system?
Force is the vector because it has direction. Weight is not mass. It is a force that acts on mass. Weight has a
direction, down toward the Earth, is a force, and is measured in newtons.
2.
What is the rule for magnetism? What is domain theory? How does the alignment of electron spin in
clusters of atoms (usually iron) appear? Is it random? Aligned? What is the Curie Point?
Opposite poles attract and like poles repel. Domain theory simply states that a magnet is made of magnetized
regions called domains. It is these domains that have the same alignment. When many of the domains are all
aligned, the material is said to be magnetized. The Curie Point is a temperature above which magnetization is
lost and the material is no longer a permanent magnet.
3.
What happens when you break a magnet? Draw a bar magnet split in half in both perpendicular directions.
Do the two halves attract or repel in each case? What elements can be made into a permanent magnet?
What is the difference between ferromagnetic, diamagnetic, and paramagnetic materials? Why can a
paperclip, attracted to a magnet, become magnetized and pick up another paperclip? What is magnetite?
Because of domain theory, it is easy to see that when broken, a material will retain is magnetic alignment.
Because like poles repel, breaking a magnet parallel to its alignment results in a magnet that repels itself.
Breaking a magnet perpendicular to the alignment does not result in anything significant. The two resulting
magnets are attracted to themselves in the original orientation. Ferromagnetic materials can be made into a
permanent magnet. This includes, but is not limited to: iron, nickel, and cobalt. Paramagnetic materials are
weakly attracted in a magnetic field and diamagnetic materials are weakly repelled by a magnetic field. A
paperclip in a magnetic field becomes a temporary magnet when the magnetic field it is placed in aligns the
domains within the paperclip. Magnetite (and hematite) are iron oxides that are naturally occurring and
magnetic.
4.
Who was the first to publish a book on electricity and magnetism and theorize that the Earth is a giant
magnet? What were Benjamin Franklin’s contributions in the field of electricity? What did Michael Faraday
contribute with respect to magnetism and electrical current? What did Charles Dufay contribute to our
present understanding of electricity? When did each of these individuals make their contributions?
William Gilbert. Ben Franklin did experiments to determine that lightning was indeed electricity. He also
invented the lightning rod and is credited as giving the two types of charge the names positive and negative.
Faraday discovered and explored the relationship between moving charges and magnetic fields, as well as
force acting on electric charges from changing magnetic fields. Dufay worked with conductors and insulators
and explored basic rules of electric attraction and repulsion. A lot of what he did seems like the most basic
understanding today, but he laid the foundation in a time when nobody knew what charge was. Gilbert did his
work in the 1500’s, Dufay in the 1700’s, Franklin in the late 1700’s, and Faraday in the 1800’s.
5.
What do the field lines of a bar magnet look like? What direction do they go? Do they go from north to
south or south to north? What do the field lines of a horseshoe magnet look like?
They emanate out of the north pole of a magnet and connect to the south pole. They go from north to south
outside the magnet and south to north inside the magnet. In a horseshoe magnet, it is still north to south
between the two poles because this is outside the physical material. Field lines do not cross, and are closer
together when the field strength is stronger.
6.
What is a motor? What is a generator? How are they similar? How are they different?
A motor is a device that uses Faraday’s law to take energy from moving charge (electricity) and make some
part of it move (usually turn) through the interaction with a magnetic field. A generator uses the same
principle, but uses mechanical energy of something moving (usually turning) in the presence of a magnetic
field to produce electrical current (electricity).
7.
What happens when a loop of wire is moved relative to a magnet? What happens when a magnet is moved
relative to a loop of wire? What is the name of the sensitive instrument used to measure current?
In both situations, a force acts on the charge. If the force is in the direction of a conducting material (wire), a
current can be produced. A galvanometer is a device to measure electric current and uses the exact same
principle to deflect the needle.
8.
What happens when a strong magnet falls through a copper pipe? Why? Is copper magnetic? What is Lenz’
Law? How does a ring shooter work? How does the iron bar make it work better? Why does it not work
when a ring of plastic is used or if the aluminum ring has a cut through it?
The changing magnetic field from the falling magnet induces a current in the pipe. That current creates a
magnetic field that opposes the falling magnet, slowing it down. This is known as Lenz’ law. The ring shooter
works by creating a very large magnetic field inside a conductive ring. The changing magnetic field creates a
large current in the ring that produces a strong magnetic field that opposes the original field, repelling it and
shooting the ring.
9.
What does conservation mean? What does it mean when charge is conserved?
Conservation of charge means that charge is not created or destroyed.
10. How can an object become charged? Explain how an object can be charged by friction, conduction,
induction and induction by grounding.
Charging an object can be done by adding or taking charge away, or by separating the charge. Friction
separates charge. Conduction allows excess charge to transfer by contact. Induction does not involve contact,
so charge cannot be transferred. Induced charge can be accomplished by bringing a charged object near, then
touching (or grounding) while the charged object is nearby. This separates the charge and allows charge to
transfer to or from the object, leaving it charged.
11. Where does the word electron come from? What do resinous and vitreous mean?
The greek ‘elektron.’ Resinous and vitreous refer more to amber and glass than anything specifically related to
charge. However, amber (resinous from its origin of tree sap) is associated with negative charge and vitreous
(referring to glass) is associated with positive charge. It’s not clear what made that original connection.
12. What is an electroscope? How does it measure electric charge? What are its limitations? What is a
Wimshurst machine? What is the name of the charging method used by the Wimshurst machine where
charge does NOT travel through a conductor, like a wire? How do you charge one? How does a Van de
Graaff generator work? Why is the dome smooth and rounded? What is a Leyden Jar? When was it
invented? By whom? Where? What is a capacitor? How do capacitors work? What is a dielectric?
An electroscope is any device that measure electric charge. The kind that we used in class is fairly common
example where two gold leaves separate when charged. Another type involves a needle that deflects. In both
cases, the limitations are that you cannot get a precise measurement of how much charge is present or what
type of charge is present. A Wimshurst machine is a spinning plate with brushes that induce a charge and
transfer that separated charge to a leyden jar. With this machine, a hand-crank can be used to easily separate
charge. A Van de Graaff generator works by a belt that moves over one type of material on end, picking up
charge, and transferring it to the other end. A large conducting dome is usually at this end, allowing a large
build-up of charge. The dome is smooth so that charge does not build up excessively on any one area where it
th
may leak. A leyden jar is a basic type of capacitor that was invented in the mid 18 century by Pieter van
Musschenbroek. A capacitor two conducting surfaces separated by an insulator that can hold charge (and
energy). By increasing the surface area of the conducting surfaces and bringing the surfaces as close together
as possible (by putting an insulator called a dielectric between them), more charge (and energy) can be stored.
13. When an object is charged, is it the proton or the electron (or both?) that are transported? Why?
The electron moves. The electron is what is transferred between the atoms. In most atoms, most electrons are
not conducting electrons. The atoms (their nuclei) are held in place, while certain electrons are more loosely
held and are conducted.
14. What does it mean for an object to be electrically polarized? How can an object become polarized? What
does it mean for an object to be neutrally charged? Describe how a charged object can attract a neutrally
charged object. What does it mean for an object to be grounded?
Polarization in general simply means that the two ends are different. Electrically polarized objects are charged
negatively on one end and positively on the other. An object that is neutrally charged has equal numbers of
positive and negative charge. A charged object can induce a charge on a neutral object by polarization.
Because the opposite charge will move slightly closer, and the like charge will move slightly farther, the net
effect is that there is less force of repulsion and more force of attraction. There is a net or unbalanced force
that holds the two objects together without actually transferring charge or requiring that the neutral object
become charged. Grounding something means to have it physically in contact with the Earth. Your house is
grounded through the plumbing and (hopefully!) grounding rods driven into the ground near your electric
meter.
15. What is a lightning rod and how does it work? What are the two most important properties of a lightning
rod?
A lightning rod is a device that has sharp points, is located high up, and is attached by a conductor to the
ground. A lightning rod can prevent damage by channeling lightning away from a structure, and can also act to
repel lightning by dissipating charge at the sharp points.
16. What is Coulomb’s Law? How is it similar to universal gravitation? How is it different? Which one appears to
be stronger? What is the inverse-square law? What is the unit of electrical force? What is a coulomb?
Coulomb’s Law is an equation that shows that the force of attraction or repulsion is proportional to the
product of the charge and inversely proportional to the distance between the charges. The unit of ANY force is
18
the newton. A coulomb is collection of charge, 6.2 x 10 charges.
17. What is the main difference between a conductor and an insulator? What makes these materials behave as
they do?
A conductor transfers (gives up) electrons easily and insulators do not. That’s it.
18. What is a superconductor? How does it levitate a strong magnet? What property of a material changes
when it becomes a superconductor? What is the critical temperature?
A superconductor is a conducting material with no resistance. When the temperature of a superconducting
material falls below the critical temperature, it becomes a superconductor.
19. Describe (briefly) the process of the formation of lightning. What is fulgurite? How is it formed? What does
it look like?
Lightning originates when a strong updraft from heat given off from condensation (precipitation) warms the
air and brings with it the water and ice crystals. The friction of collisions in this strong updraft separates charge
and typically leaves the upper part of a thunderstorm positively charged with the bottom negative. The
negative charge of the bottom of a thunderstorm attracts positive (induces a positive charge) at the surface.
Lightning forms when a collection of charge is forced down from the cloud, or up from the earth (more rare).
Lightning regularly travels from cloud to cloud as well and can even travel upward nearly to the ionosphere! A
fulgurite is a glassy mineral formed from a lightning strike on a silica-rich material (like sand). A common type
of fulgurite will be hollow with a glassy inner surface and will branch out like tree roots.
20. Describe five uses for or examples of static electricity.
Research, antistatic sprays, cleaning air (removing particulates by ionizing them), lightning, xerography, paper
handlers, etc…
21. How can you make a simple motor from a wire, magnet, and a battery?
A force acts on a current-carrying loop of wire placed in a magnetic field. If you attach a conducting loop
between the electrodes of the battery, and place a magnet on the battery, a torque acts on the loop, causing
it to spin.
22. How can you tell if an object is charged? How can you tell if it is positively charged or if it is negatively
charged?
Use an electroscope to tell if it is charged. Bring it near a charged object that is known to have either a positive
or negative charge will show you what the charge is.
23. Where is the charge on a charged conductor? How can you remove the charge from a conductor? How can
you charge an insulator? How can you neutralize the charge on an insulator?
The charge resides on the outside of a conductor because it repels itself. Remove the charge by grounding it.
You can charge an insulator by rubbing it with a different material. Neutralize the charge on an insulator by
wiping the insulator with a conductor.
Describe the procedure and purpose of the following demonstrations:
24-43. Descriptions not provided…
Draw the magnetic fields around the two magnets below. Include the direction of the field lines using arrows.