Download Magnetism Review

Physics
Name: __________________________
Date: _____________ Period: _______
Magnetism
(Unit Review)
Key Vocabulary
Coronal mass ejection
Diamagnetic
Differential rotation
Electric motor
Electromagnet
Electromagnetic induction
Faraday’s Law of Induction
Galvanometer
Gauss
Generator
Hard magnet
Large hadron collider
Magnetism
Magnet
Magnetic material
Magnetic force
Magnetic poles
Magnetic domains
Magnetic field
Paramagnetic
Permanent magnet
Right-hand rule
Source magnet
Solar wind
Solenoid
Soft magnet
Transformer
Turbine
1. List the four fundamental forces at work in the universe. Number them from strongest and weakest (1 =
strongest, 4 = weakest).
the strong force (1), the electromagnetic force (2), the weak force (3), and the gravitational force (4).
2. Explain the difference between a magnet and a magnetic material.
A magnetic material is any material in which for a short time the domains are aligned in same direction.
Magnets have more permanently aligned magnetic domains. However, magnets are made from
magnetic materials, so these materials (typically metals) can be magnetized. Even if they are not
magnetized, they will be attracted to a magnet and prolonged contact can create a temporary magnet.
3. At an atomic level, what makes a material magnetic?
Remember the all magnetism must come from an electric current, so at the atomic level the electric
currents come from the motions of the electrons. From here quantum mechanics quickly becomes
abstract, but some rudimentary understanding is helpful. Remember in the video we watched that every
electron can be thought of as a tiny magnet. The overall magnetism has to do with pairing of the
electrons (up spin vs down spin). In an element with a filled electron shell, the electrons pair up
causing their magnetic fields to cancel out, but in half filled shells the unpaired electrons magnetic
fields add up making at least the atom itself magnetic. Lastly in order for a material made from
magnetic atoms to be magnetic, the magnetic fields must be aligned, but this depends on the crystal
structure of the material and while some crystals align all of the magnetic fields, others alternate and
cancel out, while others are mostly random and “somewhat” magnetic.
4. Explain the magnetic domains model of magnetism.
A magnetic domain is region in which the magnetic fields of atoms are grouped together and aligned.
The idea allows us to simplify the quantum mechanical description of atomic behavior in crystal
structures into a proxy that simplifies the basic understanding of why magnetic atoms may or may not
form magnetic materials based on the orientation of groups of atoms within the magnetic material. If
the domains are aligned it is magnetic, but if these “warring factions” are misaligned their fields cancel
out the magnetic properties.
5. What force is exhibited between these magnetic poles?
a. N and S attract
b. N and N repel
c. S and S repel
6. In what ways are electricity and magnetism the same? How are they different?
Electricity and magnetism are essentially two aspects of the same thing, because a changing electric
field creates a magnetic field, and a changing magnetic field creates an electric field. This is why
physicists usually refer to "electromagnetism" or "electromagnetic" forces together, rather than each
term separately. Remember the electricity is really a poor choice of term as charge can either be
stationary (static electricity) or flowing (current). A current, or flowing electric field creates a magnetic
field and a moving magnetic field creates an electric field (causing current to flow).
One creates the other and vice-versa. So in reality they are two parts of the same thing the
electromagnetic force.
7. Explain the difference between temporary and permanent magnets (HINT: Don’t just say how long they
last…explain why)
When the magnetic field in a material is strong enough that it keeps the magnetic domains aligned
despite the innate motion of the atoms themselves, it is said to be a permanent magnet. Remember that
no magnet is truly permanent and in this case is simply a long lasting (sometimes REALLY long
lasting) organization of magnetic domains. Temporary magnets however are just that, temporary.
Their magnetic domains quickly lose their organization (either because they are paramagnetic or soft
ferrromagnets).
8. Is it possible to isolate a north or south pole on a magnet like you did with a positive or negative
charge? Explain why.
No. If you cut one magnet into two pieces you get two smaller magnets, each with a north pole and a
south pole. What you are trying to produce is called a magnetic monopole and doesn’t
exist. Remember that at the atomic level the configuration of the atoms and magnetic domains create
the magnet and breaking them down smaller does not remove their magnetic properties, but execute
them over a smaller area.
9. Explain what a magnetic field is and how magnetic field lines work.
In its simplest form a magnetic field is a region connecting the two poles of a magnet. A magnetic
object within the field would experience a force with respect to the position of the other poles in its
field. Like poles repel each other (N to N; or S to S) and opposite poles attract (N to S). Remember
however that the electromagnetic force is infinite and a field is actually a mathematical description of
the relationship between any two (or more) poles expressed at a particular point in space.
10. Compare and contrast a magnetic field with an electric field.
A moving charge always has both a magnetic and an electric field, and that’s precisely the reason why
they are associated with each other. They are two different fields with nearly the same characteristics.
Therefore, they are inter-related in a field called the electromagnetic field. In this field, the electric field
and the magnetic field move at right angles to each other. However, they are not dependent on each
other. They may also exist independently. Without the electric field, the magnetic field exists in
permanent magnets and electric fields exist in the form of static electricity, in absence of the magnetic
field.
The electric field is actually the force per unit charge experienced by a nonmoving point charge at any
given location within the field, whereas the magnetic field is detected by the force it exerts on other
magnetic particles and moving electric charges (even these distinctions are small, more investigation
can be done by checking out the work of James Maxwell).
11. Draw the magnetic field for the following source magnet.
12. Draw the magnetic field for the following pair of source magnets.
This is a N to N, but S
to S field lines are the
same.
13. Relate Earth’s geomagnetic poles with its geographic poles.
The geographical poles represent the ends of the vertical axis around which the planet rotates. The
magnetic poles are the ends of the vertical axis through Earth’s magnetic field. Earth’s geographical
poles do not change much (outside of the wobble), but its magnetic poles are constantly on the move.
(In fact, Earth’s North Pole is currently a south magnetic pole, based on the flow of the magnetic field
lines. Over many years, the magnetic poles can migrate and have been known to switch places.
14. Is earth’s magnetic field permanent? Use your answer to explain how why the earth’s magnetic poles
switch periodically.
The best answer is that the Earth is a temporary magnet and as such does not produce a permanent
magnetic field, however the length of time it has been in a magnetic field and its ferromagnetic
materials suggest that even if the Earth stopped rotating it would still have a small permanent magnet.
As seen in the answer above the magnetic pole does move (somewhat dramatically). The magnetic field
is the result of the liquid metal that makes up the outer core passing through a magnetic field, which
causes an electric current to flow within the liquid metal. The electric current, in turn, creates its own
magnetic field—one that is stronger than the field that created it in the first place. Since the movement
of the liquid core is not constant (since it is a liquid and the geographic pole wobbles), the direction of
the permanent magnetic material fluctuates and thus the magnetic field fluctuates and can even flip
directions.
15. Compare and contrast the magnetic field of a planet (like earth) with a star (like our sun).
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star.
A planetary magnetic is created by the movement of the core. As seen in the question above current is
not consistent as the movements are not consistent. There are many factors affecting magnetic field
including composition of the core, size of the core with respect to the size of the planet, rotational
velocity, etc. The faster the rotation, the greater the current created and hence the stronger the magnetic
field (this is part of why Jupiter has a stronger magnetic field than Earth) – this is related to what they
call the Magnetic Dynamo Theory (see below). Stars are made of plasma and experience differential
rotation. As such, they build up charge pockets whose magnetic field lines snap and realign to reduce
stress on the star’s surface. This along with surface currents/fields produce plasma loops and other
surface features that can lead to events like coronal mass ejections (CMEs).
Magnetic Dynamo Theory
16. What purpose does earth’s magnetic field serve in terms of supporting life on our planet?
The higher energy particle radiation that could endanger life on Earth (mutations etc.) is forced to drift
around the Earth. It acts like a shield from the higher energy particles.
17. Now that we have learned about electromagnetic induction, revisit how earth’s magnetic field is
sustained.
I would answer this in three parts. First the permanent magnet of our core creates current which creates
a magnetic field, so some of it is self-sustaining. Additionally, the current that reaches our planet at the
poles from the funneling of solar wind increases the strength of the magnetic field. Lastly, the moons
irregular orbit and gravitational impact could also contribute to sustaining the magnetic field.
18. Using the right hand rule show the magnetic field for the following circuit.
19. Compare and contrast a solenoid with an electromagnet. Why do we use electromagnets and not
solenoids?
A solenoid is a cylindrical coil of wire whose diameter is small compared to its length. When an electric
current flows through the wire the solenoid generates a magnetic field similar to that of a bar magnet.
An electromagnet is a solenoid wound around a central iron core. The magnetic field generated by the
coil of wire magnetizes the core, increasing the total magnetic field.
20. In the following solenoid, determine the north and south poles.
Current Flow
21. Are electromagnets permanent or temporary magnets? How do you know?
A permanent magnet is an object made from a material that is magnetized and creates its own persistent
magnetic field. As the name suggests, a permanent magnet is 'permanent'. This means that it always
has a magnetic field and will display a magnetic behavior at all times. An electromagnet is made from
a coil of wire which acts as a magnet when an electric current passes through it. An electromagnetic
magnet only displays magnetic properties when an electric current is applied to it, so it is a temporary
magnet not a permanent magnet.
22. What are the advantages of electromagnets versus fixed magnets?
You can switch and electromagnet on and off by switching the current on and off. You can switch and
electromagnet’s north and south poles by reversing the direction of the current in the coil. The strength
of and electromagnet’s field can be changed by changing the amount of current in the coil.
Electromagnets can also be much stronger than permanent magnets because they can use large currents.
23. We identified three major factors that impact the strengths of an electromagnet’s magnetic field. What
are they and how does each affect the electromagnet’s strength?
The three major factors are 1) The amount of electric current in the wire, 2) The amount and type of
material in the electromagnet’s core, and 3) The number of turns in the coil. As you increase the
amount of current in the wire, you increase the strength of the magnetic field. The more magnetic the
material, the stronger the field (ferromagnetic, paramagnetic, etc.). The greater the number of turns in
the coil, the greater the magnetic field.
24. What types of things do we use electromagnets for? (Give at least 3 examples)
Motors and generators, transformers, speakers, etc. Just to name a few.
25. Explain how a galvanometer works.
In a galvanometer, the electromagnet is connected to a small
spring. Then the electromagnet rotates until the force exerted by
the spring is balanced by the magnetic forces on the
electromagnet. Changing the current in the electromagnet causes
the needle to rotate to different positions on the scale.
26. Explain how an electric motor works.
Step 1. When a current flows in the coil, the magnetic
forces between the permanent magnet and the coil cause
the coil to rotate.
Step 2. In this position, the brushes are not in contact with
the commutator and no current flows in the coil. The
inertia of the coil keeps it rotating.
Step 3. The commutator reverses the direction of the
current in the coil. This flips the north and south poles of
the magnetic field around the coil.
Step 4. The coil rotates until its poles are opposite the
poles of the permanent magnet. The commutator reverses
the current, and the coil keeps rotating.
27. “All magnetism comes from electric currents.” Is this statement true? Explain.
Not to beat a dead horse, but this fact lies in the structure of the atom and how full the orbitals of
electrons are. The magnetic field produced by natural objects comes directly from these electrons
exerting an unbalanced force on each other which produces a magnetic field that can either be aligned
or misaligned with the crystal structure of the atoms or the magnetic domains in the rest of the material.
Hence the innate ability of have magnetism relies on electric current or the movement of charge. For
electromagnets this is even more pronounced as it is the movement of charge (current) through the coils
that generate the magnetic field which is amplified by the ferromagnetic material placed at the center of
the coil (converting it from a solenoid to an electromagnet) having its magnetic domains aligned. The
magnetic field then is only as strong as the current amplified by alignment of the magnetic domains in
the ferromagnetic material. In principle, the quantum property of magnetism is amplified to the
macroscopic level (REMEMBER the video’s “tiny magnet.”
28. Compare and contrast paramagnetic with diamagnetic.
In many elements the magnetic fields of individual electrons in each atom cancel each other out. This
leaves the whole atom with zero net magnetic field. These materials are called diamagnetic.
In some atoms (like aluminum), the magnetism of individual electrons does not cancel completely.
This makes each atom into a tiny magnet with a north and a south pole. However, due to the random
arrangement of the atoms, the total magnetic field for the object averages to zero over the many atoms.
These materials are called paramagnetic.
29. What is ferromagnetism and how does it relate to the magnetic domains model?
Ferromagnetic metals have very strong magnetic properties. The best examples of ferromagnetic
materials are iron, nickel, and cobalt. Like paramagnetic atoms, electrons in ferromagnetic materials do
not cancel each other’s magnetic fields. However, instead of aligning randomly, the magnetic domains
are aligned and add up producing a relatively strong magnetic field. Diamagnetic, paramagnetic, and
ferromagnetic are all based on the organization of the atoms and on a greater scale the magnetic
domains.
30. Compare and contrast hard and soft magnets.
A soft magnet is one that is easy to magnetize, but loses its magnetization easily too. Heat, shock, and
other magnets can demagnetize these types of magnets. Materials that make better permanent magnets
are called hard magnets and their domains tend to remain aligned for a long time.
31. What is the relationship between temperature and magnetism? Why is this true?
Temperature is the enemy of magnetism because temperature creates disorder between atoms. So the
higher the temperature, the weaker the magnetic field.
32. What is electromagnetic induction?
Electromagnetic induction is the generation of a current by a changing magnetic field.
33. By moving a magnet near a wire, you can create an _electric current_. Conversely by running current
in a wire, you can create a _magnetic field_.
34. Explain how a simple generator uses electromagnetic induction. Give an example of how we can use
this concept to create electricity in a usable way. What type of current is produced?
A generator uses electromagnetic induction to transform
mechanical energy into electrical energy. We start with a loop (or
coil) of wire positioned between two permanent magnets. As the
loop (or coil) moves past the magnets it creates current, which in
turn creates a magnetic field. As the loop (or coil) moves past the
magnet it produces current in one direction, then passes by the
other magnet producing current moving in the opposite direction.
Thus the standard generator produces AC current by
electromagnetic induction using a moving coil past a permanent
magnet. An example of how we use this is by attaching a
generator to turbines in hydroelectric dams or wind turbines to
generate current. They produce alternating current due to the
configuration of the magnets.
35. AC technology depends heavily on transformer. Explain what a transistor is and draw an example.
(NOTE: There was a typo here as it stated transistors not transformers. These are two VERY different
things and we covered transformers not transistors.) A transformer is a device that increases or
decreases the voltage of an alternating current. A transformer is made of a primary coil and a secondary
coil. These wire coils are wrapped around the same iron core.
36. On the previous page you drew a transformer. Was this a step up or a step down transformer? Explain
each in your answer and when the two types are used.
(NOTE: As with before this is a type and it should read transformer.) The picture above is for a step
up transformer because there are more turns in the secondary coil. This then increases the voltage
output of the secondary wire. A step down transformer will have a greater number of turns in the
primary coil rather than the secondary coil causing a production of fewer volts in the output of the
secondary coil. This relationship can be represented by the equation:
37. Calculate the load current and load voltage in this transformer circuit:
𝑉1
𝑉2
=
𝑁1
𝑁2
28
2390 𝑡𝑢𝑟𝑛𝑠
=
𝑉2
710 𝑡𝑢𝑟𝑛𝑠
28 = 3.37 V2
V2 = 8.318 V
V = IR
8.318 V = I (350Ω)
0.0238 A = I
38. We discussed that physics has symmetry. What does that mean and what is the value of symmetry in
physics?
Symmetry is the idea of being made in two equal parts. Symmetry is the concept that the properties of
particles such as atoms and molecules remain unchanged after being subjected to a variety of symmetry
transformations Symmetry is at the core of both relativity and quantum mechanics. Examples include
things like space and time (sloshing between each other), energy and mass (in relativistic terms mass is
energy), electricity and magnetism (a pair of forces that when in motion create each other).
Supersymmetry is a field of theoretical physics which seeks particles beyond the Standard Model to
balance the bosons with theoretical particles known as fermions.
39. What is Faraday’s Law of Induction? What does it suggest?
Faraday’s Law of Induction states that “the voltage induced in a coil is proportional to the rate of
change of the magnetic field through the coil.” As the magnetic field in a coil changes, the rate of
change reflects in the voltage produced and thus impacts the current flowing through the coil. Slower
or smaller change yields a lower current and thus lower voltage (as resistance stays constant if using the
same wire in the coil), whereas faster or greater change yields a higher current and thus higher voltage
(again assuming constant resistance by using same material in the wire).
40. Explain how a speaker works using our knowledge from the whole year.
In order to translate an electrical signal into an audible sound,
speakers contain an electromagnet. As pulses of electricity pass
through the coil of the electromagnet, the direction of its
magnetic field is rapidly changed. This means that it is in turn
attracted to and repelled from the permanent magnet, vibrating
back and forth. The electromagnet is attached to a cone made of
a flexible material which amplifies these vibrations, pumping
sound waves (longitudinal/compressional) into the surrounding
air and towards your ears. The frequency of the vibrations
governs the pitch of the sound produced, and their amplitude
affects the volume