physics 1b – fall 2009courses.physics.ucsd.edu/2009/fall/physics1b/documents/...chapter 19.4 force...
TRANSCRIPT
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PHYSICS 1B – Fall 2009
Electricity &
Magnetism
Professor Brian Keating
SERF Building. Room 333
Wednesday, November 18, 2009
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I
qB
F
Chapter 19.4 Force on a Current Carrying Wire
For angle θ between L and B
B parallel to direction of wire, θ=0, F=0B perpendicular to direction of wire θ=90o, F= BIL
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIb
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIb
out of page
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIb
0
out of page
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIb
0
BIb
out of page
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIbinto page
0
BIb
out of page
Wednesday, November 18, 2009
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Forces on a loop of current in a uniform B field
B field is uniform and in the plane of the current loop
Find the forces acting on the wires in the loop. (a and b are the lengths)
F1=
F2=
F3=
F4=
BIbinto page
0
BIb
out of page
0
Wednesday, November 18, 2009
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Torque= Force x perpendicular distance
Ch 19.5 Torque on Current
First, a review of Torque
Wednesday, November 18, 2009
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Torque= Force x perpendicular distance
Mass = 1 kg length = 1m
What’s the torque of the ball at the end of the lever?
A. 1 kg
B. 9.8 kg
C.1 N-mWednesday, November 18, 2009
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Last Quiz and Final Exam
• Last quiz is this Friday in class.
• The Final Exam for Physics 1B will be on the Monday of Finals week,
Monday December 7, 2009 from 11:30am to 2:30pm in York 2622
Wednesday, November 18, 2009
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Side view
The current loop in a B field generates a torque around the center proportional to the area of the loop
F1=F2=BIb
Wednesday, November 18, 2009
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Side view
τ =BIA
counterclockwise
A=axb=area of loop
The two forces generatea torque around the center
The current loop in a B field generates a torque around the center proportional to the area of the loop
F1=F2=BIb
Wednesday, November 18, 2009
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Same loop as before…current flowing counter clockwise as viewed from above.
Which picture below has largest torque?
X
B field
A. B. C.
Wednesday, November 18, 2009
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Same loop as before…current flowing counter clockwise as viewed from above.
Which picture below has largest torque?
X
B field
X
A. B. C.
Wednesday, November 18, 2009
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Same loop as before…current flowing counter clockwise as viewed from above.
Which picture below has largest torque?
X
B field
X
X A. B. C.
Wednesday, November 18, 2009
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Normal to the loop plane
Loop makes an angle with B
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=
θx
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θx
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θx
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θ
BIAsinθ
x
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θ
BIAsinθ
x
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θ
BIAsinθ0
x
xx
Wednesday, November 18, 2009
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B
B
B
τ=τ=
τ=BIA
θ
BIAsinθ0
x
xx
The torque tilts the loop so the normal is parallel to B
Wednesday, November 18, 2009
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Loop with N turns of wire
Torque increases with N, B, I and ATorque is maximum when θ=90o,when the loop is parallel to the fieldTorque is zero when θ=0 when loop is perpendicular to the field
N turns of wiretotal current =NI
Wednesday, November 18, 2009
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X
A 3A current wire-loop (with 100 turns) and an area of 0.2 m2 makes an angle of 30o with a magnetic field of 0.3T.
a) Find the torque exerted on the coil. b) What is the direction of rotation? c) What happens if the current is reversed in the coil?
30o
B
Wednesday, November 18, 2009
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X
A 3A current wire-loop (with 100 turns) and an area of 0.2 m2 makes an angle of 30o with a magnetic field of 0.3T.
a) Find the torque exerted on the coil. b) What is the direction of rotation? c) What happens if the current is reversed in the coil?
30o
Θ=60oB
a)
Wednesday, November 18, 2009
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X
A 3A current wire-loop (with 100 turns) and an area of 0.2 m2 makes an angle of 30o with a magnetic field of 0.3T.
a) Find the torque exerted on the coil. b) What is the direction of rotation? c) What happens if the current is reversed in the coil?
30o
Θ=60oB
a)
Wednesday, November 18, 2009
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X
A 3A current wire-loop (with 100 turns) and an area of 0.2 m2 makes an angle of 30o with a magnetic field of 0.3T.
a) Find the torque exerted on the coil. b) What is the direction of rotation? c) What happens if the current is reversed in the coil?
30o
Θ=60oB
counter clockwise direction
a)
b)
Wednesday, November 18, 2009
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X
A 3A current wire-loop (with 100 turns) and an area of 0.2 m2 makes an angle of 30o with a magnetic field of 0.3T.
a) Find the torque exerted on the coil. b) What is the direction of rotation? c) What happens if the current is reversed in the coil?
30o
Θ=60oB
counter clockwise direction
the torque will have the same magnitude but in the opposite (clockwise) direction,
a)
b)
c)
Wednesday, November 18, 2009
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Electric motors (not same as ‘engines’)A current loop in a magnetic field produces a torque
Problem
A dc current does not produce complete rotation
X
B field
Wednesday, November 18, 2009
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Electric motors (not same as ‘engines’)A current loop in a magnetic field produces a torque
Problem
A dc current does not produce complete rotation
X
B field
X
Wednesday, November 18, 2009
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Electric motors (not same as ‘engines’)A current loop in a magnetic field produces a torque
Problem
A dc current does not produce complete rotation
X
B field
X
Xτ=0
Wednesday, November 18, 2009
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Electric motors (not same as ‘engines’)A current loop in a magnetic field produces a torque
Problem
A dc current does not produce complete rotation
X
B field
X
Xτ=0
dc current only rotates coil until it is perpendicular to the field
Wednesday, November 18, 2009
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Solution with direct current source is to use a commutator. Split-ring commutator reverses the current direction when τ=0.
Wednesday, November 18, 2009
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19.7 Magnetic field due to a current carrying wire
Magnetic field due to currentAmpere’s LawForce between current carrying wires
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Hans Oersted 1820
Magnetic fields are produced by an electric current
No Current Current turned onWednesday, November 18, 2009
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Magnetic field lines around a current in straight wire - circle with radius R
IB
R
= permeability of free space=4πx10-7T·m/A
B decreases with distance
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Direction of the field- Right hand rule
IB
R
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Direction of the field- Right hand rule
IB
R
Thumb-along I
Fingers- around I point along B
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Ampere’s Law Andre Marie Ampere (1775-1836)
General relation between current and magnetic field
Iclosed path
ΔL - line segmentB - magnetic fieldBll – component of B parallel to ΔL
B
BllΔL
sum over all segments in the closed loop
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The magnetic field around a straight wire calculated from Ampere’s Law
IB
R
The B field has a constant value at a constant radius R.B and ΔL are in the same direction
Therefore, from Ampere’s Law
from Ampere’s Law
Wednesday, November 18, 2009
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Application of Ampere’s Law
A coaxial cable has an inner conductor carrying current in one direction and an outer conductor carryingan equal current in the opposite directionFind the B field due to the currents at a radius R outside the coaxial cable.
I
I
R
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Application of Ampere’s Law
A coaxial cable has an inner conductor carrying current in one direction and an outer conductor carryingan equal current in the opposite directionFind the B field due to the currents at a radius R outside the coaxial cable.
B=0 since the total current is equal to zero. The B fields due to the two currents cancel
I
I
R
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field B=0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from aboveWednesday, November 18, 2009
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23
Wednesday, November 18, 2009
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PHYSICS 1B – Fall 2009
Electricity &
Magnetism
Professor Brian Keating
SERF Building. Room 333
Wednesday, November 18, 2009
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Today
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
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Today • Last quiz of course on Friday- covers Ch 19.
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
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Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 52: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/52.jpg)
Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.• Ch 19 HW solutions are on the web as are all
lectures in Ch 19 after class today.
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 53: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/53.jpg)
Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.• Ch 19 HW solutions are on the web as are all
lectures in Ch 19 after class today.• No books, multiple pages, laptops etc.
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 54: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/54.jpg)
Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.• Ch 19 HW solutions are on the web as are all
lectures in Ch 19 after class today.• No books, multiple pages, laptops etc.• Magnetic field by wires review
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 55: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/55.jpg)
Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.• Ch 19 HW solutions are on the web as are all
lectures in Ch 19 after class today.• No books, multiple pages, laptops etc.• Magnetic field by wires review• New: current loops
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 56: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/56.jpg)
Today • Last quiz of course on Friday- covers Ch 19.• You CAN bring in 1 page (8.5” x 11”) notes for
this quiz, as well as calculators.• Ch 19 HW solutions are on the web as are all
lectures in Ch 19 after class today.• No books, multiple pages, laptops etc.• Magnetic field by wires review• New: current loops• New: permanent magnets
Don’t forget the Problem session Thursday nightWednesday, November 18, 2009
![Page 57: PHYSICS 1B – Fall 2009courses.physics.ucsd.edu/2009/Fall/physics1b/documents/...Chapter 19.4 Force on a Current Carrying Wire For angle θ between L and B B parallel to direction](https://reader034.vdocuments.net/reader034/viewer/2022051919/600c4f9a7c5d70093a3a17d5/html5/thumbnails/57.jpg)
A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
Wednesday, November 18, 2009
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
B=Bearth
Wednesday, November 18, 2009
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
B=Bearth
Wednesday, November 18, 2009
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
B=Bearth
Wednesday, November 18, 2009
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
B=Bearth
Wednesday, November 18, 2009
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A 5A current passes through a wire downward in the vertical direction. a) At what distance R from the wire will the magnetic field equal the earth’s field Bearth =0.5x10-4 T.
How will the compass needles be deflected?
Bearth
I into the page
R
view from above
B=Bearth
Wednesday, November 18, 2009
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A power line carries a current of 100 A from east to west. a) Find the magnitude of the B field due to the wire at a position 20 m below the line due to the current. b) Find the direction of B.
I
Wednesday, November 18, 2009
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A power line carries a current of 100 A from east to west. a) Find the magnitude of the B field due to the wire at a position 20 m below the line due to the current. b) Find the direction of B.
I
this is much smaller than the earth’s magnetic field 0.5x10-4 T
a)
Wednesday, November 18, 2009
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A power line carries a current of 100 A from east to west. a) Find the magnitude of the B field due to the wire at a position 20 m below the line due to the current. b) Find the direction of B.
I
this is much smaller than the earth’s magnetic field 0.5x10-4 T
a)
b) South- by the right-hand rule.
B
Wednesday, November 18, 2009
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Force between two current carrying wiresof length L, parallel at distance R.
I1
I2 RL
Wednesday, November 18, 2009
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Force between two current carrying wiresof length L, parallel at distance R.
I1
I2 R
I1 produces a field B12 at the position of wire 2.L
B12
Wednesday, November 18, 2009
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Force between two current carrying wiresof length L, parallel at distance R.
I1
I2 R
I1 produces a field B12 at the position of wire 2.
I2 experiences a force due to B12
L
B12
F
Wednesday, November 18, 2009
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Force between two current carrying wiresof length L, parallel at distance R.
I1
I2 R
I1 produces a field B12 at the position of wire 2.
I2 experiences a force due to B12
The force between the two wires is
L
B12
F
Wednesday, November 18, 2009
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Force between two current carrying wiresof length L, parallel at distance R.
I1
I2 R
I1 produces a field B12 at the position of wire 2.
I2 experiences a force due to B12
The force between the two wires is
L
Directions
B inward
F attractive
B12
F
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
I1
I2 R
Current in opposite directions
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
B12
F
I1
I2 R
Current in opposite directions
Force - Attractive
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
B12
F
I1
I2 R
B12
F
Current in opposite directions
Force - Attractive
Force - Repulsive
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
I1
I2 R
Current in opposite directions
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
B12
F
I1
I2 R
Current in opposite directions
Force - Attractive
Wednesday, November 18, 2009
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Current in same direction-
I1
I2 R
B12
F
I1
I2 R
B12
F
Current in opposite directions
Force - Attractive
Force - Repulsive
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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Two parallel wires 1.0 m in length separated by 4.0 cm each carry a current of 20 A in opposite directions. Find the force exerted between the two wires.
I=20 A
I=20 A
Wednesday, November 18, 2009
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19.9 Magnetic field of a current loop
Current loopSolenoidMagnetic materialsElectromagnetsMotion of a charged particle in a magnetic field
Wednesday, November 18, 2009
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Magnetic field in a current loopB field- out of loop
Straight wire Current loopB field is the sum of fields
Wednesday, November 18, 2009
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B field due to current loop
I
Side view
B into loop
B out of loop
Wednesday, November 18, 2009
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B field due to current loop
I
looks like a magnetic dipole
N
S
Side view
B into loop
B out of loop
Wednesday, November 18, 2009
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B field due to current loop
I
looks like a magnetic dipole
N
S
A current loop creates a magnetic dipole
Side view
B into loop
B out of loop
Wednesday, November 18, 2009
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Motivation. To construct electromagnets, i.e. a device to convert current to magnetic field.
Some elements of design of electromagnets. The magnetic field due to current through loops or coils of wire.
single coil solenoid multiple turns of wire
Solenoids
• Built up of current loops
Wednesday, November 18, 2009
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Solenoid
side view
current intopage
current outof page
B field lines
NS
Current in a solenoid produces magnetic dipole
Wednesday, November 18, 2009
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Air Core Solenoid vs. Bar Magnet
Wednesday, November 18, 2009
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B field in solenoidHigh field inside solenoidLower fields outsideUniform relatively constant field in central region
Wednesday, November 18, 2009
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B-field in center by Ampere’s Law
1
24
3
N=no. of turns in length L
Only segment 1 contributes becauseBllΔL for other segments =zero
L
i.e. 2 inthe picture
Wednesday, November 18, 2009
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Ampere’s LawAmpere's Law: for a closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.
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Magnetic field in a long solenoid
B=µοnI at centerAmpere's Law: for a closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.
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An electromagnet with has 100 turns of wire wound around an air core with length of 3.0 cm. If a current of 20 A is passed through the wire, what is the B field at center of the magnet.
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Magnetic materials owe their properties to magnetic dipole moments of electrons in atoms.
Applications • permanent magnets, • magnetic core electromagnets• magnetic recording, magnetic tape, computer drives,• credit cards
19.10 Magnetic Domains and Materials
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Classical model for magnetic dipole moment of electron
An electron acts as a magnetic dipole
Spinning charge
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Magnetic properties of matter
diamagnetic
paramagnetic
ferromagnetic
µ/µo
Carbon 1-2x10-5
Iron alum salt 1x10-5
Iron metal 1000-3000
slightly less than vacuum
slightly more than vacuum
much more than vacuum
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Soft magnetic materialse.g. ironEasily magnetized but doesn’t retain magnetization for longUsed as core for electromagnets
Hard magnetic materials e.g. metal alloys Alnico (Aluminum, Nickel, Cobalt)Hard to magnetize but retains the magnetization for a long timeUsed as permanent magnets.
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Magnetic Domains
Magnetism due to magnetic domains.Each domain has millions of atoms with magneticmoments coupledSeparated by domain boundaries
B
domain boundary shifts in B field to givemagnetization along B field direction
Soft magnetic materials-Boundary movement
10-4m
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B
Magnetic dipoles reorient in the domainsto give a net magnetic moment.Harder to do, i.e requires higher B field.but also harder to reverse.
Hard magnetic materials
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Soft magnetic materials e.g. Fe nail can be magnetizedby exposure to a strong B field.
non-magneticmagnetic
Magnetization
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Magnetic material
Solenoid
Magnetic dipoles in iron are aligned by the B field to produce a larger B field
Iron core
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Iron core electromagnet
I
iron-core
solenoid with many turns
V
B=µnI
The B field in the electromagnet is much higher with an iron core than an air core.
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Applications of Iron core electromagnets
Electric motors, loudspeakers, electrical machinery
N S
Magnetic Field
Iron-coreelectromagnet
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Applications of Iron core electromagnets
Electric motors, loudspeakers, electrical machinery
N SNS
Magnetic Field
Iron-coreelectromagnet
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Homework/CAPE
55
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Ch 19 Review
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Magnetic tapeInformation coded in the orientation of magnetic particles
Magnetization can be read on playback to generate a voltage signal
Similar recording for computer hard disks, credit cards.
Information can be erased by magnetic fields.
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Electromagnet
Iron core- magneticfields confined in the core
GapFringe B-fields
Magnetic recordingalternating current
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Electromagnet
Iron core- magneticfields confined in the core
GapFringe B-fields
Magnetic recording
Magnetictape-magnetizedby fringe fields
alternating current
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Motion of a charged particle in a magnetic field
B
(uniform magnetic field)
vF
F is in a plane perpendicular to B
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Motion of a charged particle in a magnetic field
B
(uniform magnetic field)
v’vF
F’
F is in a plane perpendicular to B
After Δt particle is in the same plane
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Motion of a charged particle in a magnetic field
B
(uniform magnetic field)
v’vF
F’
F is in a plane perpendicular to B
After Δt particle is in the same plane
Particle moves in aplane perpendicularto B
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Motion of particle in plane perpendicular to B
The particle moves in a circular path
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A proton with v=1x106 m/s is in a uniform magnetic field of 0.2 T. Find the radius of the trajectory
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A proton with v=1x106 m/s is in a uniform magnetic field of 0.2 T. Find the radius of the trajectory
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A proton with v=1x106 m/s is in a uniform magnetic field of 0.2 T. Find the radius of the trajectory
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Mass spectrometer
Molecular ionsAt velocity v
Ions separated by mass
Application
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