Lesson#34Lesson#34Topic: Exploring MagnetsTopic: Exploring Magnets

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. Play with magnets and write observed Play with magnets and write observed properties of magnets.properties of magnets.

2.2. Hypothesize explanations for how magnets Hypothesize explanations for how magnets work.work.

1/2/07

Assignment: Research 5 important uses of magnets or magnetic fields

Warm Up: What are some important uses of magnets or magnetic fields?

Lesson#35Lesson#35Topic: Explaining the Magnetic ForceTopic: Explaining the Magnetic Force

Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)1.1. Briefly describe relativity and how it relates to the transfer Briefly describe relativity and how it relates to the transfer

of “information”.of “information”.2.2. Explain how the magnetic force is replaced in absence of Explain how the magnetic force is replaced in absence of

the magnetic force. the magnetic force. 3.3. Derive the first equation of the magnetic force from our Derive the first equation of the magnetic force from our

previous equation for the electric force.previous equation for the electric force.4.4. Describe how the electric force and the magnetic force Describe how the electric force and the magnetic force

are really two parts of the same underlying fundamental are really two parts of the same underlying fundamental electromagnetic force.electromagnetic force.

1/3/07

Assignment: Describe how an accelerating charge will disrupt the surrounding electric and magnetic fields and create an electromagnetic wave.

Warm Up: Is it possible for information to travel instantaneously from source to receiver?

Crash course relativityCrash course relativity

NOTHING NOTHING can travel faster than the speed of can travel faster than the speed of light. light.

Not even fundamental information.Not even fundamental information.

Example: Say hello to a friend. Your friend will Example: Say hello to a friend. Your friend will never hear you say hello the instant that you never hear you say hello the instant that you say it. say it.

Information (like everything else) has to travel Information (like everything else) has to travel through both time and space before being through both time and space before being received (before action takes place).received (before action takes place).

This causes a disturbance in the electric force This causes a disturbance in the electric force between charges.between charges.

Crash course relativityCrash course relativityThis effect is minimal at low (non-relativistic This effect is minimal at low (non-relativistic speeds)speeds)But is considerable at high speeds.But is considerable at high speeds.Consider two point charges at rest.Consider two point charges at rest.

Now consider two point charges moving at Now consider two point charges moving at relativistic speeds (at the speed of light).relativistic speeds (at the speed of light).It is impossible for the charges to It is impossible for the charges to “communicate” with one another.“communicate” with one another.

Explaining the Magnetic ForceExplaining the Magnetic ForceWhen traveling at the speed of light, When traveling at the speed of light, these like charges do not repel.these like charges do not repel.

At high speeds they repel, but less than At high speeds they repel, but less than they would at lower speeds.they would at lower speeds.

In order for Newton’s laws to hold true, In order for Newton’s laws to hold true, then there must be some force acting then there must be some force acting opposite the electric force.opposite the electric force.

This force is present when charges are This force is present when charges are moving.moving.

This is the magnetic force.This is the magnetic force.

Explaining the Magnetic ForceExplaining the Magnetic ForceThe direction of this force is perpendicular The direction of this force is perpendicular to the velocity of the charge (current).to the velocity of the charge (current).

But it is also perpendicular to the direction But it is also perpendicular to the direction of the magnetic field. of the magnetic field.

The magnetic field forms in concentric The magnetic field forms in concentric circles around the direction of the moving circles around the direction of the moving charge.charge.

Lesson #36Lesson #36Topic: Magnetic Field Topic: Magnetic Field Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

1.1. Describe the direction of the magnetic field Describe the direction of the magnetic field surrounding a magnet or a wire carrying current.surrounding a magnet or a wire carrying current.

2.2. Describe the direction a magnet will force another Describe the direction a magnet will force another magnet or a wire with current.magnet or a wire with current.

3.3. Describe the direction a wire with current will Describe the direction a wire with current will force a magnet or another wire with current.force a magnet or another wire with current.

4.4. Describe the magnetic field created with a Describe the magnetic field created with a solenoid.solenoid.

1/8/07

Assignment: Holt p781 #4, 8, 9, 13, 14, 17, 19, 22 - 27, 32, 35

Warm Up: Sketch a picture of the magnetic field lines surrounding a bar magnet. Sketch what happens if the magnet gets broken in half.

Magnetic FieldsMagnetic FieldsThe magnetic field is the direction that the The magnetic field is the direction that the northnorth pole of a magnet will be forced. pole of a magnet will be forced.

When a magnet is placed near another When a magnet is placed near another magnet’s field, it is easy to see the direction magnet’s field, it is easy to see the direction that the magnet will be forced.that the magnet will be forced.

When a magnet is placed near a wire with When a magnet is placed near a wire with current, it is more difficult to predict the current, it is more difficult to predict the direction the magnet will be forced.direction the magnet will be forced.

When two wires with current are placed near When two wires with current are placed near one another, it is also difficult to predict the one another, it is also difficult to predict the direction that they will be forced.direction that they will be forced.

Observe the following diagrams.Observe the following diagrams.

Magnet on MagnetMagnet on MagnetObserve the magnetic field of magnet B and the Observe the magnetic field of magnet B and the direction that magnet A will be forced.direction that magnet A will be forced.

Observe the magnetic field of magnet A and the Observe the magnetic field of magnet A and the direction that magnet B will be forced.direction that magnet B will be forced.

S N N S

A B

B field

FB on A

S N N S

A B

B field

FA on B

Wire on MagnetWire on MagnetObserve the direction that a magnet will be forced when Observe the direction that a magnet will be forced when put in the magnetic field of a wire with current flowing put in the magnetic field of a wire with current flowing into the screen. into the screen.

Newton’s 3Newton’s 3rdrd Law states that if the wire exerts a force on Law states that if the wire exerts a force on the magnet upward, then the magnet exerts the same the magnet upward, then the magnet exerts the same force onto the wire downward.force onto the wire downward.

S N

B field

Fwire on magnet

S N

Fmagnet on wire

B field

Magnet on WireMagnet on WireWe can use the right hand (gun) rule to determine the We can use the right hand (gun) rule to determine the direction of the force acting on a wire. direction of the force acting on a wire. Consider the previous diagram. Consider the previous diagram.

Right hand rule instructions: Right hand rule instructions: 1. Place your index finger in the direction of the current 1. Place your index finger in the direction of the current in the wire. in the wire. 2. Place your middle finger in the direction of the 2. Place your middle finger in the direction of the magnetic field ofmagnetic field of thethe magnet. magnet.3. The direction that your thumb points is the direction 3. The direction that your thumb points is the direction that the wire will be forced.that the wire will be forced.

S N

Fmagnet on wire

B field

Wire on WireWire on WireUse the right hand rule to determine the direction that Use the right hand rule to determine the direction that wire B will be forced due to wire A’s magnetic field and wire B will be forced due to wire A’s magnetic field and vice versa.vice versa.

Use the right hand rule to determine the direction that Use the right hand rule to determine the direction that wire B will be forced due to wire A’s magnetic field and wire B will be forced due to wire A’s magnetic field and vice versa.vice versa.

B field

FA on B

A BB field

FB on A

A B

B field

A BB field

A B

Magnetic field of a solenoidMagnetic field of a solenoidConsider multiple wires where all of the wires Consider multiple wires where all of the wires on top are going into the page and all of the on top are going into the page and all of the wires on the bottom are coming out of the page.wires on the bottom are coming out of the page.

Magnetic field lines will be concentrated inside Magnetic field lines will be concentrated inside the solenoid.the solenoid.

B field

B field

Lesson #37Lesson #37Topic: Calculating Magnetic Force Topic: Calculating Magnetic Force

1.1. Objectives: Objectives: (After this class I will be able to)(After this class I will be able to)

2.2. Calculate the magnitude of the magnetic force Calculate the magnitude of the magnetic force acting on a wire or point charge.acting on a wire or point charge.

3.3. Calculate the magnitude of the magnetic field Calculate the magnitude of the magnetic field acting on a wire or point charge.acting on a wire or point charge.

4.4. Describe the units of magnetic field strength.Describe the units of magnetic field strength.

1/9/07

Assignment: Magnetic Force and Fields

Warm Up: A proton is traveling East in a magnetic field that points North. What is the direction that the proton is forced? What if the proton was traveling North and the magnetic field is also directed North?

Magnetic Force on a wireMagnetic Force on a wireThe greater the current through the wire, the The greater the current through the wire, the larger the magnetic force acting on the wire.larger the magnetic force acting on the wire.The longer the length of wire in the magnetic The longer the length of wire in the magnetic field, the greater the force acting on the wire.field, the greater the force acting on the wire.The stronger the magnetic field that the wire is The stronger the magnetic field that the wire is in, the greater the force acting on the wire.in, the greater the force acting on the wire.The only stipulation is that the current though The only stipulation is that the current though the wire must be perpendicular to the magnetic the wire must be perpendicular to the magnetic field that the wire is placed in.field that the wire is placed in.If the current is parallel to the field, no force will If the current is parallel to the field, no force will be felt.be felt.If the current is not perpendicular, then use the If the current is not perpendicular, then use the component of the B field that is perpendicular.component of the B field that is perpendicular.

Magnetic ForceMagnetic Force

Remember, this is the force exerted by the field Remember, this is the force exerted by the field onto the wire.onto the wire.

The B in this equation is not the B field created by The B in this equation is not the B field created by the wire. It is an already existing B field.the wire. It is an already existing B field.

We can also treat this wire as if it is just a single We can also treat this wire as if it is just a single point charge moving with some velocity.point charge moving with some velocity.

And obtain the equation:And obtain the equation:

122 BlIFm

122 BvqFm

Magnetic ForceMagnetic ForceBoth these equations involve cross products.Both these equations involve cross products.This is used such that we remember to use only This is used such that we remember to use only the perpendicular component of the magnetic the perpendicular component of the magnetic field strength.field strength.To remember this we can transform our equation To remember this we can transform our equation to:to:

Where Where θθ is the angle between the magnetic field is the angle between the magnetic field and the current or velocity.and the current or velocity.

sin122 BlIFm sin122 BvqFm