Chapter 4: electromagnetism

Materials that have the ability to attract iron are classified as having a:

strong magnetic force

When a charged particle (electron or proton) is in motion, a magnetic force field ________ to the motion will be created.

perpendicular

what is orbital magnetic moment?

the motion of a charged particle creates a magnetic force field perpendicular to the motion

What is the spin magnetic moment?

the magnetic effect created by electrons spinning on their axes.

the disruption of spin magnetic moment causes:

energy release as it reorients itself permitting magnetic resonance imaging.

what is a magnetic dipole (magnetic domain)?

small magnet created by the electron orbit

how many atoms make up a single dipole?

10^15 atoms or 1 million billion

Dipoles can vary in:

size, they can grow or shrink depending on local conditions.

What is a magnetic field?

A region where other magnets or magnetic materials experience a force

what is a magnet?

any material or object that produces a magnetic field

In nonmagnetic objects:

the magnetic dipoles are randomly arranged and that causes the field to cancel itself out.

If an external force field has the time or strength to orient enough of the dipoles in the same direction and/or cause those dipoles to grow in size, the object exhibits a uniformly.

strong magnetic field and is referred to as a magnet.

The force fields that are created when magnetic dipoles orient to create a magnet are called

lines of force, lines of flux, or the magnetic field.

These lines of force flow not only through the magnet itself but also outside the magnetic material, forming a three-dimensional field surrounding the magnet.

The stronger the magnetic field, the more lines of:

flux

The ends of a magnet are defined as?

north and south poles

Lines of force outside a magnet always flow from:

north to south

Lines of force within a magnet is called?

south to north

It is important to remember that lines of force never:

intersect and can be parallel

many circumstances may cause, the magnetic field to:

contract or expand; Under these conditions, the lines of force bend closer or further from one another, making them no longer truly parallel.

The stronger the magnetic field, the greater the number of:

lines of flux or the greater the flux density.

Flux density is determined by both:

field strength and by the area in which the lines of flux are located.

magnetic flux= field strength/area

There are two primary units used to measure the strength of magnetic fields:

Tesla and gauss

The units for magnetic flux density are?

Tesla (T) and gauss(G)

1 tesla = 10,000 gauss.

1 T = 1 Wb/m', or 1 Wb per square meter

The SI unit for magnetic flux is

the weber, represented by the symbol Wb.

1 Wb = 10° lines of flux

Magnets can be classified by:

type of production as natural, artificial permanent, and electromagnets.

Natural magnets are called

Lodestones

how are the natural magnet, lodestones, created?

iron oxide (magnetite) remains in the earth's magnetic field for ages, slowly orienting the magnetic dipoles in the same direction.

An artificial permanent magnet is manufactured from a steel alloy called

alnico, which is made from a combination of aluminum, nickel, and cobalt.

While it is hot, alnico is subjected to the field of a strong commercial magnet to permit easier orientation of the magnetic dipoles. Upon cooling, the magnetic field becomes relatively permanent.

what is an electromagnet?

A solenoid with an iron core; temporary magnet produced by moving electric current. (a magnet whose magnetic field can be turned on and off with an electric current)

Because the electrons comprising the flow of current create magnetic fields in exactly the same manner as do the orbiting and spinning atomic electrons, any flow of current produces a________ ________.

magnetic field

When the current ceases flowing, the magnetic field _______, making these electrical magnets temporary.

collapses

It is possible to create a fourth type of magnet by;

vibration.

What are the 3 laws governing magnetism?

1. Repulsion-Attraction

2. Inverse Square Law

3. Magnetic Poles

what is the repulsion- attraction law as it pertains to electromagnetic?

Like poles repel; unlike poles attract.

like lines of force repel and unlike lines of force attract, when placed within each other's force fields

what is the inverse square law as it pertains to electromagnetic?

The force between two magnetic fields is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them.

- The inverse square law means that the strength of a force decreases as the distance between the two objects increase. ( as the magnets get farther from each other the force is weaker)

- if you double the distance between two magnets, the force between then becomes 1/4 as stronger ( big decrease) .

This is because the force is inversely proportional to the square of the distance. but directly proportional to the magnitudes of the of the two objects so if you increase the strength of one magnet, the force increase proportionally.

what is true about magnetic poles?

Every magnet has two poles, a north and a south. No matter how much a magnet is divided, even into individual moving electrons, both poles continue to exist

what is magnetic induction?

the process by which materials can be magnetized;

When a non magnetized iron bar is brought within the lines of force of a strong magnet, the dipoles will temporarily align themselves with the lines of force passing through the iron bar. If the bar is removed from the field after a short time, the dipoles will return to their random orienta-tion, thus leaving the bar unmagnetized.

-operates like electromagnetic induction.

magnetic poles are induced through the _______ orientation of the dipoles.

temporary

describe magnetic shielding

The ease with which sensitive devices can be magnetized or demagnetized by stray electrical fields makes magnetic shielding desirable in many situations.

Because there is no magnetic insulator, shielding is accomplished by providing a highly permeable _______ ___________, such as iron, through which stray magnetic fields can be directed, thus protecting sensitive devices from exposure to the stray field. This technique is used to shield some magnetic resonance imaging (MRI) units, although they require radio frequency (RF) shielding as well.

ferromagnetic material

What is permeability in electromagnetic?

The ease with which a material can be magnetized.

what is retentivity?

the ability of a material to stay magnetized

permeability and retentivity are inversely proportional because if it is difficult to orient the dipoles (low permeability), it is also difficult to:

disorient them (high retentivity).

The major classifications of the magnetic properties of materials according to their relative permeabilities are:

1. Ferromagnetic

2. Paramagnetic

3. Diamagnetic

4. Nonmagnetic

What is a ferromagnetic material?

metal such as iron that can be magnetized because it has a strong magnetic field and contains magnetic domains.

what are example of ferromagnetic materials?

iron, cobalt, and nickel; highly permeable and greatly susceptible to induction. These materials have a majority of their dipoles lying in the same direction, thus setting up a natural magnetic field.

what alloys have no ferromagnetic elements and are only designed to enhance permeability and exhibits dramatically increased magnetic field strength.

alnico, permalloy, or mumetal

what is paramagnetic materials?

These materials have only a slight majority of dipoles in the same direction and there is little tendency for the size of the dipoles to grow.

what are examples of paramagnetic?

platinum, aluminum, MRI contrast media are usually paramagnetic.

Diamagnetic materials are

are actually weakly repelled by all magnetic fields, including both north and south poles.

what are examples of diamagnetic materials?

Copper, beryllium, bismuth, water and lead

What are some nonmagnetic materials and Why?

wood, glass, rubber, and plastic, are not affected by magnetic fields and cannot be magnetized.

nonmagnetic materials most often comprise atoms locked into crystalline or molecular patterns, thus forming _______ and _________ bonds and eliminating the ability of electrons to freely orient themselves to external magnetic lines of force.

ionic and covalent

True or false: Electricity and magnetism are actually different aspects of the same force named electromagnetism.

True

Oersted's experiment proves

that when there is no current flowing in the wire, the compass needle aligns itself with the earth's magnetic field. however, when current is flowing, the needle is deflected toward the wire and when the current stops, the needle returns to alignment with the earth's field.

This led to the conclusion that any moving charge produces a magnetic field.

- this applies to even a single electron's negative charge in orbit around an atom.

What is Fleming's hand rule?

several hand rules that are quite useful for remembering various electromagnetic relationships.

The rules are divided into four groups:

(1) hand-thumb rules along a conductor,

(2) hand-thumb rules for solenoid and electromagnet poles;

(3) hand generator effect rules

(4) hand motor principle rules.

it is critical to remember that hand rules that are based on current flow (conventional current direction) and hand rules that are based on electron flow (actual direction of electron movement) will be exact opposites.

For example, the right-hand thumb rule applies to conventional current flow. This is identical to the left-hand thumb rule applied to actual electron flow.

The hand thumb rule for a straight conductor

applies to remembering the relationship between the direction of current along a straight conductor and the direction of the resulting magnetic lines of force field.

explain the Fleming's right-hand thumb rule for a straight conductor.

states that if the right hand is used to grasp a conducting wire with the thumb in the direction of the current flow, the fingers will indicate the direction of the magnetic field lines of force surrounding the conductor

If the thumb points in the direction of actual electron flow, then a left-hand thumb rule must be used to show the direction of the magnetic field lines of force surrounding the conductor.)

right hand thumb rules for solenoid and electromagnet poles

thumb = direction of north pole

fingers = conventional current or electron flow

right hand generator rule

thumbs = movement of conductor or armature

index finger = magnetic lines of force field

middle fingers = current or electron flow

left hand motor rule

Thumb- movement of conductor

Index finger-magnetic lines of force

Middle finger-direction of current flow

Using the right-hand thumb rule to determine the direction of the magnetic lines of flux, it is apparent that inside the loop the magnetic fields from both sides join to double the magnetic flux density. Creating a ______.

coil; the flux density is greatly increased.

What is a solenoid?

A coil of current-carrying wire that produces a magnetic field

what is electromagnet?

a solenoid with a ferromagnetic core; the flux density can be increased further still by adding a ferromagnetic core.

The strength of solenoids and electromagnets is determined by the.

1. Number of loops (or turns) of wire

2. The current strength

3. The permeability of the core

What rule can be applied to a solenoid or electromagnet to determine the location of the magnetic poles?

The right hand rule; If the fingers point in the direction of the current, the thumb will point toward the north pole

If the fingers point in the direction of actual electron flow, then a left-hand thumb rule must be used for the thumb to point toward the north pole.

Both the solenoid and the electromagnet demonstrate magnetic properties only while electric current is ?

flowing

If the current stops flowing, the magnetic properties ____, and if the current is adjusted up or down, the magnetic strength changes accordingly.

Vashin

What factors governs the effectiveness of solenoids and electromagnets?

the diameter of the coil, its length, and the current passing along the coil.

Electromagnets are used as remote control devices in circuit breakers and in the temporary locks, sometimes called _____, on radiographic equipment?

detents

Solenoids are often used as

detent locks on the overhead crane of x-ray tubes.

The detent can be activated by a switch at the x-ray tube controls. The switch activates the magnetic properties of the solenoid, which in turn attracts a metal latch or bolt. This temporarily locks the tube to the center of the x-ray table or at the proper distance from the receptor.

An electromagnetic relay is similar to circuit breakers. They are used to protect radiographers from?

electrical shock by isolating control buttons on the x-ray console from the actual circuit in which high voltage is flowing.

The simple presence of a magnetic field is not sufficient to cause electrons to move along a wire. What properties must have a motion relative to each other to induce an electrical current?

(Faraday discovered) magnetic lines of force and the wire must have a motion relative to each other to.

There are three ways to create the motion between lines of force and a conductor. Name the three. ( faradays)

1. Move the conductor through a stationary, unchanging strength magnetic field.

2. Move magnetic lines of force through a stationary conductor with an unchanging strength magnetic field.

3. Vary the magnetic flux strength from a stationary magnet through a stationary conductor. As the flux strength varies, the lines of force will expand and contract, in effect causing the relative motion necessary to induce current.

There are two primary laws that govern the induction of current by magnetic fields. The two are:

1. Faraday's law (sometimes called the First Law of Electromagnetics)

2. Lens law ( sometimes called the Second Law of Electromagnetics)

What does Faraday's law state?

states that four factors regulate the strength of induced current when magnetic lines of force and a conductor are in motion relative to one another.

State the four factors that regulate the strength of induced current when magnetic lines of force and a conductor are in motion relative to one another.

1. the strength of the magnetic field

2. the speed of the motion between lines of force and the conductor

3. the angle between the magnetic lines of force and the conductor.

4. the number of turns in the conducting coil.

This law makes it possible to determine the direction in which the induced current will flow.

what does Len's law (sometimes called the Second Law of Electromagnetics) states?

states that induced current flow sets up a magnetic field opposing the action that produced the original current, or simply, that induced current opposes any

flux change.

what two laws apply to both forms of induction: mutual induction and self induction?

the first and second laws of electromagnetics ( aka, faraday and lenz law)

How does mutual induction occurs?

When two coils are placed in proximity and a varying current supplied to the first coil (as an electromagnet) induces a similar flow in the second coil, mutual induction occurs.

what is the primary coil?

The coil supplied with current

what is secondary coil?

The coil in which the current is induced

The flow occurs in the _______ coil because the primary current alternates.

secondary;

This follows the rule that moving lines of force from a varying intensity current will induce electron flow in the wire through which it passes. In this case, the secondary

In this case, the secondary coil is cut by the varying (and therefore moving) lines of force from the current in the primary coil.

When is alternating current present in self induction ?

when the electrons constantly change direction.

A coil supplied with alternating current permits:

a steady flow of electrons and establishes an electromagnetic effect for half the cycle.

At the instant the current supply reverses, the previously established electromagnetic north and south poles will induce an:

opposing potential difference, thus attempting to induce against the incoming supply of electrons.

what is inductive reactance?

Opposition to current flow in a circuit due to induction.

What is inductive reactance measured in?

measured in ohms of resistance

Self-induction is useful when?

it is desirable to permit direct current to flow while at the same time hindering alternating current.

An important relationship exists between the direction of movement of a wire coil (the armature), the direction of the magnetic lines of force field, and the direction in which the induced current will flow. This relationship is best remembered by the use of another hand rule. What is this hand rule called?

Fleming's right-hand generator rule

What does Fleming's left hand rule represent?

Fleming's right-hand generator rule states that if the thumb points in the direction the conductor (or armature) is moving, and the index finger points in the direction of the magnetic lines of force field, then the middle finger will indicate the direction of the conventional current.

What is a generator?

a device that converts mechanical energy to electrical energy using Faraday's discovery of moving lines of flux in relationship to a conductor to induce current.

This process converts the mechanical energy of the motion (any of the three ways) to electrical energy.

A simple generator comprises of:

conductor and magnets

The conductor is an armature, set between opposing magnetic poles so that it encounters the:

strongest lines of force

If the armature is rotated by a strong source of mechanical energy (such as a steam or water turbine), the generator can produce massive amounts of:

electrical energy

What permits the circuit to remain stationary while the armature rotates without breaking the electrical contact between them?

a set of slip rings and brushes; This allows the electrons to flow without interruption. Each slip ring connects to one end of the armature wire

How does an AC generator work?

Mechanical energy rotates the shaft to which the armature is attached. As the wires of the armature rotate, they cut through the magnetic lines of flux from the magnets and produce electrical current. The slip rings and brushes permit the armature to rotate while maintaining contact with the wires of the circuit.

When the wire's motion is parallel to the lines of force, this produces

no electromagnetic force

the motion of the wire is 90° to the lines of force demonstrating what?

(the maximum angle), resulting in the peak emf (electromagnetic force)

As the wire begins to move into a 270° position (which is a 90° position in the opposite direction), notice that the relationship between the wire's motion and the lines of force has changed. Using Fleming's right-hand generator rule again, it is simple to see that the conventional electrical current has now reversed and is moving out of the page (while the electron flow has reversed to move into the page).

It is at this point that alternating current is produced.

This has occurred because alternating current is produced when the wire's motion relative to the lines of force is reversed.