Chap 21 & 22: Magnets & Magnetic Fields objectives Did this –Charges –Force between charges &...
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Transcript of Chap 21 & 22: Magnets & Magnetic Fields objectives Did this –Charges –Force between charges &...
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Chap 21 & 22: Magnets & Magnetic Fields
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objectives
• Did this– Charges
– Force between charges & Electric Field
– Moving charges, current
• will do this– Magnetic Field
– Force on moving charges in magnetic field
– Generation of magnetic field by moving charges
– Generation of current by moving magnetic field
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Examples of Magnets
• Compass• Earth• Hi-Fi speakers• Fridge magnets• Electric motors• Scrap yards• Cupboard doors
• video/audio tapes
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Properties of magnets
As with charges we find that there are attractive and repulsive forces.
We find that magnets stick to certain non-magnetised materials
We find that magnets can both attract and repel each other
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EARTH’S MAGNETIC FIELD
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Aurora Movie
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Magnetic Induction
Ferromagnetic materials such as iron, cobalt, gadolinium and dysprosium can become permanently magnetic
Paramagnetic materials such as steel can become magnetised but this will only last for a short time
Magnetism can be induced in materials by rubbing with another magnetised material
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Magnetic Induction
A permanent magnetised ferromagnet can thus be attracted to paramagnetic material by inducing magnetism in that material
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Magnets are Cool!
NS
+-
• North Pole and South Pole– Opposites Attract– Likes Repel
Lets Break it!
• Magnetic Field Lines– Arrows give direction – Density gives strength– Looks like dipole!
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Permanent Magnets
• North Pole and South Pole– Opposites Attract– Likes Repel
• Magnetic Field Lines– Arrows give direction – Density gives strength– Looks like dipole!
NS
NS NS
Lets Break it!
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Field Lines of Bar Magnet
S N
Complete the lines
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Magnetic Poles
In electrostatics there are two types of charges: positive and negative
Similarly there are two types of “poles”:North and South
The North pole of a compass needle points to the geographical north pole.
Like poles repel Dislike poles attract
By convention:
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Quick Quiz
The Geographical North pole is defined where the axis of rotation of the earth goes through the arctic
North
Is this:
• Exactly the north magnetic pole
• Nearly the north magnetic pole
• Exactly the south magnetic pole
• Nearly the south magnetic pole
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MonopolesUnlike with electric charge no isolated magnetic pole or monopole has ever been discovered
A north pole is always found with a corresponding south pole
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No Magnetic Charges
• Magnetic Fields are created by moving electric charge!
• Where is the moving charge?
Orbits of electrons about nuclei
Intrinsic “spin” of electrons (more important effect)
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Magnetic Field
Magnetic Field like the Electric Field is another example of a vector field
It is defined everywhere
It has a magnitude Units: 1ms C
N Tesla (T)
It has a direction, the direction that a compass needle would point
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Magnetic Field Lines
If we move a compass around and record the direction it points everywhere we can map out the direction of the magnetic field lines
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Magnetic Field Lines
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Magnetic Field Lines
Experiments of Pierre de Maricourt mapped out the field lines on naturally magnetic sphere
Demonstrated that they all pointed to two diametrically opposed points or “poles”.
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Moving charges in a magnetic field
Moving charges in a magnetic field experience a magnetic force
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Magnetic Field
N S
NS
B
Magnetic Field, B, is in direction compass needle points Magnitude is defined in terms
of force on moving charges
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Moving charge in magnetic field
Experiments show
FB
v
+
B
B
B
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Electric vs Magnetic Field Lines
• Similarities– Density gives strength– Arrow gives direction
• Leave +, North
• Enter -, South
• Differences– Start/Stop on electric charge– No Magnetic Charge, lines are continuous!
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Difference betweenElectric & Magnetic Forces• acts in the direction of
the electric field • acts on a charged
particle regardless of whether the particle is moving
• does work in displacing the particle
• acts perpendicular to the magnetic field
• acts on a charged particle only when the particle is moving
• does no work in displacing the particle
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Force on a moving charge
BvF qB
BvF qB
The result of all of these experiments can be summarised by this equation
We can use this relationship to define the magnitude of B
Units: 1ms C
N Tesla (T)
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Work & Energy
Magnetic force does no work in
displacing a moving particle Kinetic energy
of particle cannotchange
speed cannot changebut velocity and
direction can
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Charged particle in uniform magnetic field
Magnetic field into boardBvF qB
+
v v
FB
+
+
++
+
+Note speed never changes
but direction does
Force is always to v
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Charged particle in uniform magnetic field
Magnetic field into board
BvF qB
v
FB
+
+Since force is always radial it acts to keep particle moving in a circle
qvBFB r
mv2
r
mvqB
qB
mvr
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Right hand rule
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Review RHR• Force on moving (+) charge
in Magnetic field – Thumb….. gives F on + charge– fingers……. I(or v), – palm….. B (field)
•
Palm out of page.
BI
F
+ v+ + +
x
Thumb out, fingers up, palm left.
•Magnetic field produced by moving charge.
–Thumb I, fingers where you want it, palm gives B
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Direction of Magnet Force on Moving Charges
Velocity B Force
out of page right
out of page left
out of page top
out of page down
Right Hand Rule• Thumb ___, Fingers ___, palm ___
• Negative charge has opposite F!
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Preflight
What is the direction of the magnetic field in region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
1 2
v = 75 m/sq = +25 mC
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory.
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Force on moving charge in magnetic Field
• The magnitude of the magnetic force FB
exerted on the particle is proportional to the charge q and to the speed of the particle v
• The magnitude and direction of the force FB
depend on the velocity of the particle v and the magnitude and direction of the magnetic field B
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Force on moving charge in magnetic Field
• When the particle moves parallel to the magnetic field vector, the magnetic force acting on the particle is zero
• When the particle’s velocity vector v makes an angle with the magnetic field the magnetic force acts in a direction perpendicular to both v and B i.e. F is to the plane formed by v and B
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Force on moving charge in magnetic Field
• The magnetic force exerted on a positive charge is in the opposite direction of the force exerted on a negative charge moving in the same direction
• The magnitude of the magnetic force exerted on the moving particle is proportional to sin where is the angle the particles velocity vector makes with the direction of B
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Charged particle in uniform magnetic field
qB
mvr
m
rqBv
m
qB
r
v
rBq
mv
q
p
v
rB
q
m
Bubble chamber
Mass spectrometer
angular velocity
velocity
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Direction of Magnet Force on Moving Charges
Velocity B Force
out of page right
out of page left
out of page up
out of page down
Right Hand Rule• Thumb ___, Fingers ___, palm ___
• Negative charge has opposite F!
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Preflight
What is the direction of the magnetic field in region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
1 2
v = 75 m/sq = +25 mC
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory.
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• The magnetic force on a charge depends on the magnitude of the charge, its velocity, and the magnetic field.
• F = q v B sin()– Direction from RHR
• Thumb (v), fingers (B), palm (F)
– Note if v is parallel to B then F=0
BV
Magnitude of Magnet Force on Moving Charges
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Example
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
1) Which particle experiences the greatest magnetic force?
1) 1 2) 2 3) 3 4) All Same
2) The force on particle 3 is in the same direction as the force on particle 1.
1) True 2) False B
1
2
3
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Electric vs Magnetic
Electric MagneticSource: Charges Moving ChargesAct on: Charges Moving ChargesMagnitude: F=Eq F = q v B sin()Direction: Parallel E Perpendicular to v,B
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Velocity SelectorDetermine magnitude and direction of
magnetic field such that a positively charged particle with initial velocity v travels straight through and exits the other side.
v
Ex x x x x x x x x x x x x x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x x
What do you need to change if want to select particles with a negative charge?
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Motion of q in uniform B field
• Force is perpendicular to B,v– B does no work! (W=F d cos )– Speed is constant (W= K.E.)– Circular motion
x x x x x x x
x x x x x x x
x x x x x x x
x x x x x x x
x x x x x x x
x x x x x x x
x x x x x x x
x x x x x x x
Uniform B into page
v F• Force is perpendicular to B,v
• Calculate R
R
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Preflight
What is the speed of the particle in chamber 2.
1) v2 < v1
2) v2 = v1
3) v2 > v1
1 2
v = 75 m/sq = +25 mC
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity v1=75 m/s up, and follows the dashed trajectory.
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Preflight
Compare the magnitude of the magnetic field in chambers 1 and 2
1) B1 > B2
2) B1 = B2.
3) B1 < B2
1 2
v = 75 m/sq = +25 mC
Each chamber has a unique magnetic field. A positively charged particle enters chamber 1 with velocity 75 m/s up, and follows the dashed trajectory.
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Magnitude of Field inside of solenoid : B=0 n I
n is the number of turns of wire/meter on solenoid.
Solenoids
Direction Thumb direction of I, fingers point toward center, palm gives direction of B.
![Page 64: Chap 21 & 22: Magnets & Magnetic Fields objectives Did this –Charges –Force between charges & Electric Field –Moving charges, current will do this.](https://reader035.fdocuments.net/reader035/viewer/2022062321/56649ed05503460f94bdef50/html5/thumbnails/64.jpg)
What is the net force between the two solenoids?
a)Attractive
b) Zero
b)Repulsive
Look at field lines, opposites attract.
Look at currents, same direction attract.