Name:_____________________ Date:______

Regents Physics Mr. Morgante

UNIT 4B

Magnetism

Magnetism

-Magnetic Force exists b/w charges in motion.

-Similar to electric fields, an X stands for a magnetic field line going into the page,

and a ٠ represents a magnetic field line coming out of the page.

-Magnetic properties are due to the electrons orbiting the nucleus of an atom.

-The electrons generate magnetic fields by spinning on their axis.

Magnetic fields are similar to electric fields. The only difference is that electric

fields dealt with negative and positive terminals, and magnetic fields deal with

north and south poles.

Flux Density- measurement of the field intensity per area. The more field lines (as

shown above, the more flux density.

A magnetic field is defined as a region where magnetic force may be detected. If

you take a compass or some other similar device, you should have movement of

the needle in a magnetic field.

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Direction of a magnetic field is the direction in which the N-pole of a compass

would point in the field.

Magnetic field around a bar magnet is mapped by drawing magnetic flux lines

(lines of magnetic force)

The flux lines always form closed paths and they never cross, just the same as you

learned with electric fields lines.

Direction of magnetic fields are tangent to the field line at any point.

Take a look at this website, http://my.execpc.com/~rhoadley/magfield.htm. It has

great diagrams of magnetic fields.

-Remember, the field lines bunched together are called the flux, and the amount of

flux lines in a specific area is called flux density. SI unit for flux is the Weber and

Weber/m2 for flux density.

-Field is strongest near the poles, where the lines are closest together. This

statement corresponds to the flux density definition given earlier.

-If a magnet were broken into pieces, each piece would have a north and south

pole.

Moving electric charges produce magnetic fields!

If you have current moving through a wire, a magnetic field will be generated.

CURRENTS ACT ON MAGNETS, AND VICE VERSA!

Drawing Magnetic Field Lines

Anytime we need to draw vectors pointing into the page (away from you) or out of

the page (towards you) we will use the following symbols…

Tangent Line Tangent Line

Tangent Line

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Into Page Out of Page

1. The “into page” is supposed to look like the feathers on the end of an arrow or

dart that is flying away from you.

2. The “out of page” is supposed to look like the tip of an arrow coming at you.

3. This leads to our third hand rule.

The following right hand rules help determine the direction of magnetic fields,

electric currents and the magnetic forces that develop between the two.

Right Hand Rule #1- Step 1- Point thumb in dir of electric flow

Step 2- Grasp wire w/ right hand

Step 3- The four fingers wrapped around the

wire point in the direction of the magnetic

field!

"When a current-carrying wire is grasped in the right hand, the thumb pointing in the

direction of the electron current, from negative (-) to positive (+), the fingers will point in

the direction of the magnetic induction field."

Right hand rule #2- Use this rule for a coil shaped current

carrying wire, also called a solenoid.

With right hand rule #2, fingers now are direction of e- flow & thumb is direction

of magnetic field. "When a current-carrying coil of wire is grasped in the right hand, the

fingers curled around the coil in the direction of electron flow, from negative (-) to positive

(+), the thumb will point toward the North pole of the electromagnet."

Right Hand Rule #3

What happens when a wire carrying current is within a magnetic field?

This is the Right Hand Rule #3 (for motors also, as will be discussed later).

The middle finger points in the direction of the magnetic field (first - field),

which goes from the North pole to the South pole.

The index finger points in the direction of the current in the wire (second -

current).

The thumb then points in the direction the wire is thrust or pushed while in the

magnetic field (thumb - torque or thrust).

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So, when a wire carrying current sits perpendicular to a magnetic field, a force is

created on the wire causing it to move perpendicular to the field and direction of

current. The greater the current in the wire, or the greater the magnetic field, the

faster the wire moves because of the greater force created. If the wire sits parallel

with the magnetic field, there will be no force on the wire. In between there will be

some force, but a maximum force is only achieved in the perpendicular.

This is what Tesla exploited to make AC motors.

Magnetic Fields Exert Forces on Current Carrying Wires

-When a current carrying wire is placed in a magnetic field, a force is exerted on

the conductor.

-The force is perpendicular to both the field & the current.

I= Current

B= Mag. Field

F= Force

This is determined by right hand rule #3 where:

B= Index Finger = Mag field dir.

I= Middle Finger = Current dir.

F= Thumb = Mag field dir.

This diagram illustrates right hand rule #3 in two dimensions.

Current Carrying Wires Exert Forces on Each Other

Each current carrying wire exerts a force through the magnetic field that each

creates. This concept is the basis for magnetic levitating trains (MAGLEV)!

Circular magnetic fields are generated around current carrying wires. The strength

of these fields varies directly with the size of the current flowing through the wire

and inversely to the distance from the wire.

In this diagram, the solid purple circle in the

center represents a cross-section of a current-

carrying wire in which the current is coming

out of the plane of the paper.

The concentric circles surrounding the wire's

cross-section represent magnetic field lines.

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If currents pass along two parallel wires, each wire will set up a magnetic field and

the fields will interact according to the rules described in the previous topic.

Two parallel conductors which each carry a current in the same direction will

attract one another, while two such conductors which carry currents in an

opposite direction will repel each other.

Why do parallel current-carrying conductors attract one another?

Conductor B produces a magnetic field, which is perpendicular to the current flow

in A.

The force acting on A will be at right angles to

the current flow and at right angles to the

magnetic field.

Now apply the right hand rule: point the first

finger in the direction of the field, the second

finger in the direction of the current, and then

your thumb will point in the direction of the

direction of the force experienced by the

conductor A.

A similar argument will show that the field due

to conductor A will cause a force on B towards

A. Hence the conductors are attracted to each

other.

Electromagnetism

Electromagnetic Induction - A straight conductor is moved across a magnetic field.

The charge (e-) in the conductor will be acted on by the magnetic force. The

magnetic force will move the charge along the conductor from one end to another.

This will cause a buildup of negative charge at one end and a deficiency of charge

at the other. This produces a potential difference. The potential difference at one

end then develops at the other end of the conductor (high to low concentration

concept). This process keeps on occurring at the opposite ends of the conductor

which when connected to a mechanical device such as an axle creates the basis for

a motor or generator which we will discuss next. The same thing happens if you

move the magnetic field through a stationary wire as in right hand rule #3.

By wrapping an insulated strand of wire around a piece of iron or steel and running

electrical current through the wire, the metal can become magnetized. This is

called an electromagnet. A magnetic field is created around a wire when it has

electrical current running through it, and creating a coil of wire concentrates the

field. Wrapping the wire around an iron core greatly increases the strength of the

magnetic field.

Questions you may have include:

How can you make an electromagnet?

What factors are involved in electromagnetism?

How does an iron core affect the strength?

Making an electromagnet

If you wrap a wire around an iron core, such as a nail, and you send electrical

current through the wire, the nail will become highly magnetized. You can verify

that by picking up small objects or by showing its effect on a compass. This is

called the electromagnet.

Creating a simple electromagnet using a nail

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Insulated wire

Note that the wire must be an insulated wire. A bare wire would short out through

the nail or metal core. In some electromagnets, like in an electric motor, the wire

will look like bare copper. In reality, it is insulated with a thin coating of a clear

material.

Also, if the wire is thin, it may get warm from the resistance to the electricity

passing through it.

Turn on and off

The most interesting feature of the electromagnet is that when the electrical current

is turned off, the magnetism is also turned off. This is especially true if the core is

made of soft iron, which won't become a permanent magnet.

Being able to turn the magnetism on and off has lead to many amazing inventions

and applications.

How electromagnetism works

When electricity passed through a wire, a magnetic field is created around the wire.

Looping the wire increases the magnetic field. Adding an iron core greatly

increases the effect and creates an electromagnet.

Magnetic field

When DC electricity is passed through a wire, a magnetic field rotates around the

wire in a specific direction.

Magnetic field rotating around wire

Compass can show field

This can be demonstrated by connecting a wire to a battery and placing a compass

near the wire. When the current is turned on, the compass needle with move. If you

reverse the direction of the current, the needle will move in the opposite direction.

Right hand rule

To find the direction the magnetic field is going, you can use the "right-hand rule"

to determine it. If you take your right hand and wrap it around the wire, with your

thumb pointing in the direction of the electrical current (negative to positive), then

your fingers are pointing in the direction of the magnetic field around the wire. Try

it with the picture above.

Wire in a coil

Wrapping the wire in a coil concentrates and increases the magnetic field, because

the additive effect of each turn of the wire.

Coiled wire increases magnetic field

A coil of wire used to create a magnetic field is called a solenoid.

Iron core

Wrapping the wire around an iron core greatly increases the magnetic field. If you

put a nail in the coil in the drawing above, it would result in an electromagnet at

the north seeking pole on the "N" side.

Using AC electricity

If AC electricity is used, the electromagnet has the same properties of a magnet,

except that the polarity reverses with the AC cycle.

Note that it is not a good idea to try to make an AC electromagnet. This is because

of the high voltage in house current. Using a wire around a nail would result in a

blown fuse in the AC circuit box. There is also the potential of an electric shock.

Strength of electromagnetic field

The strength of the electromagnetic field is determined by the amount of current,

number of coils of wire, and the distance from the wire.

Unit

The unit of magnetic force is called the tesla (T). Near a strong magnet the force is

1-T. Another unit used is the gauss, where 104 gauss (10,000) equals 1 tesla.

Current

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The strength of the magnetic field is proportional to the current in the wire. If you

double the current, the magnetic force is doubled.

Since Voltage = Current x Resistance, you can double the current in a wire by

doubling the voltage of the source of electricity.

Turns of coil

If you wrap the wire into a coil, you increase the magnetic force inside the coil,

proportional to the number of turns. In other words, a coil consisting of 10 loops

has 10 times the magnetic force as a single wire with the same current flowing

through it. Likewise, a coil of 20 loops has 2 times the magnetic force than one

with 10 loops.

Distance

The greater the distance from the wire, the less the force, inversely proportional to

the square of the distance. For example the force at 2 cm. from a wire is 1/4 that of

at 1 cm., and the force at 3 cm. is 1/9 the force at 1 cm.

Effect of iron core

When the coil is wrapped around an iron core, the strength of the electromagnetic

field is much greater than the same coil without the iron core. This is because the

atoms in the iron line up to amplify the magnetic effect. The orientation of the

atoms in the iron is called its domain.

Current

When you increase the current, the magnetic strength increases, but it is not exactly

linear as it is with the coil by itself. The characteristics of the core cause the curve

of magnetic strength versus current to be an s-shaped hysteresis curve.

The shape of this curve depends on how well the material in the core becomes

magnetized and how long it remains magnetized. Soft iron loses its magnetism

readily, while hard steel tends to retain its magnetism.

In conclusion

By wrapping a wire around an iron core and applying an electric current through

the wire, you create an electromagnet. This device is magnetic only when the

current is flowing. The iron core greatly increases the magnetic strength. Other

main ideas include:

- A conductor that cuts the lines of flux creates a potential difference in the

conductor.

-If the conductor is connected to a circuit, a current will flow.

- The direction of the current that is induced is such that its magnetic field opposes

that of the field that induced the current!

The Generator Principle

A conducting loop rotating in a uniform magnetic field experiences a continual

change in the # of flux lines crossing the loop. This change induces a potential

across the ends of the loop, which alternates in direction & varies in magnitude

(b/w none and maximum). When the plane of the loop is perpendicular to the

field, there is no induced potential. When the plane of the loop is parallel to the

field, the involved potential is maximum.

-Magnitude of induced potential is proportional to:

1) Velocity perpendicular to the field.

2) Intensity of magnetic field.

- A current will be created in a circuit as a result of the induced potential.

-Since the induced potential alternates b/w both ends, the current is an alternating

current.

-Transformer= A device used to increase or decrease voltage. Does not affect

power generated.

Loop is spun

If the wire is made into a loop that is then spun or rotated in the magnetic field, you

can have continuous current. Since each side of the loop is going in a different

direction in the magnetic field, the current flows around the loop, depending on

which direction it is rotated.

Transfer current

There must also be some way to transfer the current to the rest of the circuit. In an

AC generator, this is done by having a ring on each end of the wire. A metal

contact or brush rubs or slides against each ring, allowing the electricity to flow

through the circuit. In a DC generator, this is done using one split-ring called a

commutator. An AC generator uses two slip rings.

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Comparison of DC and AC loops and rings

Generator in action

The following picture shows an AC generator. As one side of the loop moves to

the other pole of the magnetic field, the current in it changes direction. The two

slip rings of the AC generator allow the current to change directions and become

alternating current.

Simple AC generator

(Image from the PBS American Experience series: Inside the AC Generator)

In a DC generator, the split-ring commutator accommodates for the change in

direction of the current in the loop, thus creating DC current going through the

brushes and out to the circuit.

Note that the DC current is not a steady value. Rather, it is a "bumpy" signal, with

zero voltage at the break in the ring. The power from the current could be

mathematically described as a sine wave squared. Since most DC generators have

many more than one loop, the "bumps" even out and are not noticed.

The faster the wire passes through the magnetic field, the greater the current.

Full-sized generators

Instead of having a single loop, generators used to supply electricity to homes and

businesses have multiple magnets and loops consisting of wires wound around an

iron core, similar to an electromagnet. The more loops of wire passing through the

magnetic field, the higher the voltage that is created.

Large generator with multiple windings

Generators used to provide electricity to the community are huge. The rotor can be

well over 10 feet in diameter.

Can be used as motor

Note that when a generator has its wire wound around a iron core, it can also be

used as an electric motor. Instead of rotating the loops through a magnetic field to

create electricity, a current is sent through the wires, creating electromagnets. They

will then be repelled by the outer magnets, thus rotating the shaft as an electric

motor.

If the current is DC, the split-ring commutators are required to create a DC motor.

If the current is AC, the two slip rings are required to create and AC motor.

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Examine an unplugged electric motor to see how both a motor and generator looks

inside.

In conclusion

Electrical current is generated by moving wire through a magnetic field. Electrical

generators rotate a coil of wires through a magnetic field. The difference between

an AC and a DC generator is that the AC generator uses slip rings to transfer the

current to the electrical circuit, while the DC generator uses a split-ring

commutator. Very large generators create electricity for the community. An

electric motor is very similar to a generator, except that power is provided to turn

the rotors.

NAME________________________________ DATE________

Regents Physics Mr. Morgante

Magnetism Notesheet

Raise your hand if you are tired of doing my Regents Physics Notesheets-

Good! Do your own for a change!

Directions: Read Review Book pages on Magnetism. Create some sort of outline that

encompasses bold print, sketches, units, vectors, + / - charges, etc.

Be ready to explain your outline in an interview.

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Name:____________________ Date:________

Regents Physics Mr. Morgante

Magnetism Problems

1. A wire conductor is moved at constant speed perpendicularly to a uniform magnetic field. If the strength of the magnetic

field is increased, the induced potential across the ends of the conductor

a) remains the same c) can't be determined

b) decreases d) increases

2. Which diagram below best represents the magnetic field near a bar magnet?

a) b) c) d)

3. The diagram below shows a bar magnet. Which arrow best represents the direction of the needle of a compass placed at

point A?

a) b) c) d)

4. In order to produce a magnetic field, an electric charge must be

a) stationary b) negative c) positive d) moving

5. The diagram below represents a 0.5-kilogram bar magnet and a 0.7-kilogram bar magnet with a distance of 0.2 meter

between their centers. Which statement best describes the forces between the bar magnets?

a) Gravitational force

and magnetic force are

both attractive.

b) Gravitational force and

magnetic force are both

repulsive.

c) Gravitational force is

attractive and magnetic force is

repulsive.

d) Gravitational force is

repulsive and magnetic force

is attractive.

6. The diagram below shows an electron, e, located in a magnetic field. There is no magnetic force on the electron when it

moves

a) toward the right side of the page c) toward the top of the page

b) into the page d) out of the page

7. The diagram below represents the magnetic lines of force around a bar magnet. At which point is the magnitude of the

magnetic field strength of the bar magnet the greatest?

a) A b) D c) B d) C

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8. An electromagnet with an air core is located within the magnetic field between two permanent magnets. The air core of

the electromagnet is replaced with an iron core. Compared to the strength of the magnetic field in the air core, the strength

of the magnetic field in the iron core is

a) the same b) can't be determined c) greater d) less

9. The two ends of a wire are connected to a galvanometer, forming a complete electric circuit. The wire is then moved

through a magnetic field, as shown in the diagram below. The galvanometer is being used to measure

a) potential difference b) temperature change c) resistance d) current

10. The diagram below shows a proton moving with velocity v about to enter a uniform magnetic field directed into the

page. As the proton moves in the magnetic field, the magnitude of the magnetic force on the proton is F. If the proton were

replaced by an alpha particle under the same conditions, the magnitude of the magnetic force on the alpha particle would

be

a) F/2 b) F c) 4F d) 2F

11. The diagram below represents the magnetic field near point P. If a compass is placed at point P in the same plane as the

magnetic field, which arrow represents the direction the north end of the compass needle will point?

a) b) c)

d)

12. The diagram below shows a wire moving to the right at speed v through a uniform magnetic field that is directed into

the page. As the speed of the wire is increased, the induced potential difference will

a) decrease b) remain the same c) increase d) can't be determined

13. Which type of field is present near a moving electric charge?

a) both an electric field and a magnetic field c) an electric field, only

b) a magnetic field, only d) neither an electric field nor a magnetic field

14. The diagram below shows the lines of magnetic force between two north magnetic poles. At which point is the

magnetic field strength greatest?

a) C b) D c) B d) A

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15. A charged particle is moving with a constant velocity. On entering a uniform magnetic field, the particle

a) must change the magnitude of its momentum c) may change its direction of motion

b) may increase in kinetic energy d) must decrease in speed

16. Which diagram correctly shows a magnetic field configurations?

a) b)

c) d)

17. The diagram below shows a compass placed near the north pole, N, of a bar magnet. Which diagram best represents the

position of the needle of the compass as it responds to the magnetic field of the bar magnet?

a) b)

c) d)

18. The diagram below shows a coil of wire (solenoid) connected to a battery. The north pole of a compass placed at point

P would be directed toward point

a) C b) B c) D d) A

19. Two bar magnets of equal strength are positioned as shown. At which point is the magnetic flux density due to the two

magnets greatest?

a) C b) A c) D d) B

20. In the diagram below, steel paper clips A and B are attached to a string, which is attached to a table. The clips remain

suspended beneath a magnet. Which diagram best represents the induced polarity of the paper clips?

a) b) c) d)

21. A magnetic force acts on a charged particle moving at a constant speed perpendicular to a uniform magnetic field. The

magnitude of the magnetic force on the particle will increase if the

a) magnitude of the charge decreases c) speed of the charge decreases

b) time of travel of the charge increases d) flux density of the magnetic field increases

22. Which statement best describes the torque experienced by a current-carrying loop of wire in an external magnetic

field?

a) It is due to the interaction of the external magnetic

field and the magnetic field produced by current in the loop.

c) It is inversely proportional to the length of the

conducting loop in the magnetic field.

b) It is due to the current in the loop of wire, only. d) It is inversely proportional to the strength of the

permanent magnetic field.

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23. The diagram below represents magnetic lines of force within a region of space. The magnetic field is strongest at point

a) A c) D

b) C d) B

24. Which diagram best represents magnetic flux lines around a bar magnet?

a)

c)

b)

d)

25. A student sprinkled iron filings around a bar magnet and observed that the filings formed the pattern shown below. The

magnetic field is strongest at point

a) A b) D c) C d) B

26. Two solenoids are wound on soft iron cores and connected to batteries, as shown in the diagram below. When switches

S1 and S2 are closed, the solenoids

a) neither attract nor repel c) repel because of adjacent south poles

b) repel because of adjacent north poles d) attract because of adjacent north and south poles

27. Which diagram below best represents the magnetic field near a bar magnet?

a) b)

c)

d)

28. The diagram below represents lines of magnetic flux within a region of space. The magnetic field strength is greatest at

point

a) C b) B c) A d) D

Z:\Physics\Regents Physics\Class Material\Unit 4B Magnetism 1-14-10.doc