Wed May 12th
Key Concept(s) Today: Inductance
Journal:
What is the direction of force to the charged particles below?
1. 2.
+ -
(Direction of Current I)
3. + 4 + 5. -
Positive charge going into paper
Direction of the force on the charged particle for questions
a.
b.
c. d. e. None of the above
Lab: Make you own Demo lab (100+ points)
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Teams of 2 max
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Demo on Friday 28 May
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Team write-up
Focus Today: Inductance
Magnetic Field:
· A magnetic field is caused/created by a moving charge(s) of by changing electric fields.
· A magnetic field will exert a force that is moving perpendicular to the direction of a magnetic field.
· The direction of the magnetic field is the direction in which the north-seeking pole of a magnetic compass would point.
Two right hand rules (“+” flow/charge):
1. The creation of a magnetic field from a current (many moving charges).
-
Thumb = direction of “+” curren
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Curl of finger tips = Direction of Field
2. The force on a moving charge in an existing magnetic field.
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Back of Hand/Index finger = Direction of motion of “+” charged particle
-
Direction of finger tips =
Direction of Field
-
Direction of Thumb =
direction of Force
-
Magnets have two poles (N and S)
-
Magnetics are normally permanent (primary iron, cobalt and nickel) that are caused by alignment of the domains. Or electromagnetic (caused by moving charges).
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Wires affect each other because of creation of magnetic field and the force of a moving charge in a magnetic field.
Practice Problems:
1. What will happen to the two wires in the case below? (arrow points in direction of current)
2. 
What
will happen (direction of force on the wires) to the two wires if the current flows in the opposite
direction?
3. There is a counterclockwise current I in a circular loop of wire situated in an external magnetic field directed out of the page as shown above. The effect of the forces that act on this current is to make the loop?
4. A wire in the plane of the page carries a current I directed toward the top of the page, as shown above. If the wire is located in a uniform magnetic field B directed out of the page, the force on the wire resulting from the magnetic field is?
5.
What is the direction of force to the charged particles below?
1. 2.
+ -
(Direction of Current I)
3. + 4 + 5. -
Positive charge going into paper
Direction of the force on the charged particle for questions
a.
b.
c. d. e. None of the above
L#85: Inductance
Len's Law: The induced current produces an induced magnetic field that opposes the change of flux causing the current.
Any attempt to change a magnetic field within a loop will be resisted by the loop. A voltage will be induced to cause a current to flow to produce a magnetic field that will oppose the change in the present magnetic field.
Initial: Neutral coil wire. As South Pole approaches, (right side of coil sees X’s increase), coil makes right side a South Pole to resist the change (produces dots). When the magnet leaves, just the opposite will happen (coil will produce X's) and the coil will attract the magnet.
1. A metal spring has its ends attached so that it forms a circle. What will cause a current to be induced in the spring?
· Changing the strength of the magnetic field
· Changing the diameter of the circle by stretching/flattening the spring
· Rotating the spring (changing area that x's penetrate)
· Changing the spring’s distance to the magnetic field
I II
2. a. In both I and II, the magnetic fields B are decreasing with time. What is the direction of induced current in both cases?
b. In both I and II, the magnetic fields B are increasing with time. What is the direction of induced current in both cases?
c. In both I and II, the wire loops are being pulled across the uniform magnetic fields with a constant velocity. What is the direction of induced current in both cases?
d. In both I and II, the wire loops are being flattened (smaller area). What is the direction of induced current in both cases?
http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/indcur/index.html
I II
(A) Counterclockwise (A) Clockwise
(B) Clockwise (B) Counterclockwise
(C) to the bottom (not in a circle) (C) to the top, (not in a circle)
(D) Counterclockwise (D) Clockwise
3. Two circular coils are situated perpendicular to the z-axis as shown above. There is a current in the primary coil. What is the direction of current in the Secondary Coil in each of the following?
(A) Increasing the current in Primary
(B) Decreasing the current in Primary
(C) Rotating the secondary coil about a diameter
(D) Moving the secondary coil closer to the primary coil
(E) Decreasing the cross‑sectional area of the secondary coil
(F) Rotating the secondary coil about the z-axis
(A) Clockwise
(B) Counterclockwise
(C) Counterclockwise, zero, then clockwise, zero, (repeat)
(D) Clockwise
(E) Counterclockwise
(F) No current clockwise or counterclockwise, inside part of the ring gets higher in potential and outside gets lower
4. The figure above shows a rectangular loop of wire of width l and resistance R. The entire loop starts out over the magnetic field. Then, the loop is pulled to the right at a constant speed v. What is the direction of the induced current in the loop as: a. it travels across the field? b. as it leaves the field
a. No current: field (DX's) not changing.
b. As soon as the loop reached the edge, it loses loses X’s. The loop produces a clockwise current to counter the loss of X’s (produce more X’s).
D
5. A square loop of wire of resistance R and side D is oriented with its plane perpendicular to a magnetic field B. as shown above. What must be done in order to produce a clockwise current I in the loop:
Ans: A clockwise current produces X's. So, you have to have a condition where the wire wants to produce X's. Two conditions will cause a coiled wire to produce X's:
1) A Decrease in the the amount of X's in the magnetic field
a. Wire leaves field
b. Decrease size of field
c. Weaken the field
2) An Increase in the amount of dots (.) in the magnetic field
6. A uniform magnetic field B that is perpendicular to two loops, as shown above. The field is confined to a region of radius a, where a < b. The B field is increasing at a constant rate. What is the direction of current in both loops?
Answer:
Ans. Counterclockwise
I Y
x x x
Z x x x x x x
x x x x x x
x x x
8. In the diagram above, loop “Z” has a steady current that is producing the X’s shown.
a. What is the direction of the current in loop Z?
b. What is the direction of the induced current in loop Y?
C. If the current (I) starts to decrease in loop “Z”, what is the direction of the induced current in loop radius “Y”.
a. Clockwise
b. No induced current (no change)
c. Decrease in “dots”, so directions will be counterclockwise to oppose the loss.
Magnetic Field Worksheet
- What creates a magnetic field?
- 1st right had rule:
- Creation of __________________ because of ___________________________
- Thumb represents ______________
- Finger represents _______________
- 2nd right hand rule:
- Creation of a ______________
- Back of hand represents _______________
- Fingers represents ___________
- Thumb represents ________________
- A permanent magnet (like a bar magnet) has a B field caused by…
a) Electrons spinning around the nucleus of an atom.
b) Electrons hopping from one atom to another.
- A wire has a B field caused by…
a) Electrons spinning around the nucleus of an atom.
b) Electrons hopping from one atom to another.
- What are the 3 materials that can be magnetized and de-magnetized?
· ___________________
· ___________________
· ___________________
- Like poles ______________, unlike poles ____________ .
- Direction of Magnetic field lines, outside a permanent magnet or looped wire with current, are drawn from:
- When drawing magnetic field lines, the strength of the field (B field) is represented by….
- Can magnetic field lines cross each other?
- Ä ¤ which of these represents current into the plane of the paper?
x
which
of these represents a field line into the plane of the paper?
- Draw the field around this wire (I is in direction of arrows)
¾¾¾¾¾¾®¾¾¾¾¾¾®¾¾¾¾¾¾ ¾®
- Which right hand rule did you use to draw the field lines?
- Is there a force on the above wire if it is not in another magnetic field?
- Draw the direction of the force on this wire that has been placed in another magnetic field.
x x x x x x x
¾¾¾¾¾¾®¾¾¾¾¾¾®
x x x x x x x
- Should you have used the 1st or 2nd Right Hand Rule with the above diagram?
x x x x x x a). Draw the force on each part of the wire.
x
x x x x x b). What are the forces
trying to do to the wire
x x x x x x
x x x x x x
x x x x x x
x
x x x x x
19.
a). Draw each force on each part of the wire.
b).
What are the forces trying to make the wire do…
20.
x x x x x x a) Draw the magnetic force on the moving charge.
x x x x x x b) Draw the path of the moving charge.
x
x x x x x
x x x x x x c) What rule did you use?
x x x x x x
x x x x x x
21.
x x x x x x a). As the ring turns on its axis in the field,
x x x x x x what is happening to the current in the ring
x
x x x x x
x x x x x x
x x x x x x
22.
x x x x x x a). As the ring enters the field, what is
x
x x x x x happening to the
current in the ring
x x x x x x
x
x x x x x
x x x x x x
23. What is the direction of force to the charged particles below?
1. 2.
+ -
(Direction of Current I)
3. + 4 + 5. -
Positive charge going into paper
24.
What is the direction of induced current?
Direction of Current I)
25. What is the direction of current in ring B if the current in ring A is decreasing
B
A
Battery
26. What is the direction of current in ring B if the current in ring A is increasing