Equipment Compasses Bar Magnets Yellow Wire & power supply.
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Transcript of Equipment Compasses Bar Magnets Yellow Wire & power supply.
Forces and Fields (6)
In the most fundamental equations about the universe, we find fields.
Black holes, the Aurora Borealis, and microwave ovens all are understood in terms of fields.
Fields are abstract, but quite real.
Mr. KlapholzShaker Heights
High School
Magnet basics
• The poles of a magnet are the places where the field (B) is the greatest.
• All magnets have two poles. If you try to break a magnet, you end up with two small magnets (each with a North and a South pole)
Compass• Perform a preliminary exploration.• Keep the compass off of your desktop; free your
needle (it should shiver).• What does your compass do?• Slowly turn the compass’s container; what effect
does this have on the needle? Do the letters or the numbers affect the needle?
• The compass is measuring the Earth’s _ _ _ _ _ _ _ _ _ _ _ _ _ .
Compass• Perform a preliminary exploration.• Keep the compass off of your desktop.• What does your compass do?• Slowly turn the compass’s container; what effect
does this have on the needle? Do the letters or the numbers affect the needle?
• The compass is measuring the Earth’s Magnetic Field.
Compass• Perform a preliminary exploration.• Keep the compass off of your desktop.• What does your compass do?• Slowly turn the compass’s container; what effect
does this have on the needle? Do the letters or the numbers affect the needle?
• The compass is measuring the Earth’s Magnetic Field.
• A compass is a _ _ _ _ _ _ _ _ _ _ _ _ _ detector.
Compass• Perform a preliminary exploration.• Keep the compass off of your desktop.• What does your compass do?• Slowly turn the compass’s container; what effect
does this have on the needle? Do the letters or the numbers affect the needle?
• The compass is measuring the Earth’s Magnetic Field.
• A compass is a Magnetic Field detector.
Bar Magnet
• Perform a preliminary exploration at your desk.
• Does the bar have a magnetic field? How do you know?
Yellow Wire• Does a wire make a magnetic field?• Does a current make a magnetic field?• Make the compass point toward the wire (!).• Is the field perpendicular or parallel to the wire?• Compare the field on one side of the current to
the field on the other side. • Inside a current loop, is the field especially weak
(due to subtraction / cancellation) or especially strong (due to addition / enhancement)?
• All magnetism comes from _ _ _ _ _ _ _.
Yellow Wire• Does a wire make a magnetic field?• Does a current make a magnetic field?• Make the compass point toward the wire (!).• Is the field perpendicular or parallel to the wire?• Compare the field on one side of the current to
the field on the other side. • Inside a current loop, is the field especially weak
(due to subtraction / cancellation) or especially strong (due to addition / enhancement)?
• All magnetism comes from CURRENT.
We have 3 big results:
• The magnetic field never points toward the current.
• The magnetic field is perpendicular to current.• The magnetic field on one side of a wire is
opposite in direction to the field on the other side.
How do we make sense of this? And…What does the field look like?
The Magnetic Field (B) due to a current (I)See how it agrees with our data.
Pennsylvania State Univ.Wikipedia
I
B
B
B
B
B
If you placed a compass next to one of the circles, is would align tangent to the circle.
Do you see that this accounts forthe three big results?
The Right-Hand Rule
1. Visualize the field loops around the current.2. Put your right thumb in the direction of the current.3. Your fingers will naturally curl in the direction of the
magnetic field.
Now go back and see how this agrees with the example that we did.
Now tilt this wire so the current is going toward the left. What is the field above and below the wire? [Compare with the compasses A and C]
A
C
Notice that in the plane of the paper, the field is well-defined
Now tilt this wire so the current is going toward the left. What is the field above and below the wire? [Compare with the compasses A and C]
A
C
Notice that in the plane of the paper, the field is well-defined
Same Wire. In what direction is the field between you and the wire?
Which compass is this [A, B, C, D]?
Same Wire. In what direction is the field between you and the wire?
Which compass is this [A, B, C, D]?
Magnetic Force
• The magnetic force acts only on moving charges.
• The force is perpendicular to the movement, and perpendicular to the field.
Motor Effect Examples• Washer / Dryer• Pencil Sharpener• Blender / Mixer / Coffee Grinder• Drip coffee maker• Clock / Watch• Windshield wipers• Car windows• Hard Drive (iPod)• CD spinner• Speakers• Fan (in computer, in AC, in heating a home)
A current in a magnetic field will experience a force.I & B F
To have a magnetic force, the field must come from some source other than the current itself.Example: Put a current in the Earth’s magnetic
field, and a tiny force will push the current.
First Idea / Second Idea
• Recall the Yellow wire and the compass.• An Electric Current makes a Magnetic Field.• I B
• Recall today’s demonstrations.• A current in a field will be forced.• I & B F• {Note: this B is not made by this I.}
Find the Direction of the Force. (Now someone adds a current to the system. The current is not making the
field.)
I
I
BB
The Right Hand Rule for Magnetic Force
1. Put your fingers in the direction of the current (there is more than one way to do this).
2. Fold your fingers in the direction of the magnetic field.
3. Your thumb points in the direction of the magnetic force.
There is no component of I that is perpendicular to B. There is no force.
I B
Also, there is no force if the current is oppositein direction to the magnetic field.
Only the part of the current that is perpendicular to the field makes a force.
B
I I
The Force is Out of the page.
F
A wire (9.4 meters) carries a current of 0.51 A. The wire is in a magnetic field of 0.022 Teslas. How
much force acts on the wire when the wire is: (a) parallel to the field, (b) perpendicular to the field.
A wire (9.4 meters) carries a current of 0.51 A. The wire is in a magnetic field of 22 mT. How much
force acts on the wire when the wire is: (a) parallel to the field, (b) perpendicular to the field.
(a) There is no force.(b) F = BIL = (22x10-6)(0.51)(9.4) = 1.1 x 10-6 N
Contrast between the 2 Right-Hand Rules in Magnetism
Rule 1 Rule 2
How we 1ST saw it
Cause
Effect
Rule
more
Rule 1 - The Rule, in brief
Put your Right thumbin the direction of I.
Your fingers will curlin the direction of B.
Rule 1 - More Details
For example, if I is moving out of the page, in which direction is B at the bottom of the
page?
I
Rule 1 - More Details
I
This will let you narrow the choices (for thedirection of B) down to just 2 choices.
Then, the rule is so helpful.
Rule 1 - More Details
I
The answer istoward the Right.
If you skip thevisualization step,
it’s confusing.
Rule 2 - How we first saw it.
A wire was placed near a
permanent magnet.When we put a current in the wire,
the wire had a force on it.
Rule 2 - The Causes
Current ( I)
AndMagnetic Field (B)
{ Compare to Rule 1 ! }
Note: the B is not made by the I.The B is made by some other source (perhaps it is made by
a permanent magnet or some other current.)
Rule 2 - The Rule, in brief
Put your fingers
in the direction of I.
Fold them in the direction of B
Your thumb will pointin the direction of F.