Magnetism 2.notebook

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Magnetism

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A little introduction to magnets...

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A little bit about magnets...Magnets have North & South Poles

If you cut a magnet in half, it creates two new magnets

Monopoles do not exist (only in theory)

Magnetic Field (B)The magnetic field at any location

is the direction in which the north

pole of a compass needle points at

that location

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Soft magnets: easily magnetized but

magnetism gets lost easily

Hard magnets: not easily magnetized but

retains magnetized state

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Magnetic Fields (B)The earth behaves like a big magnet (Geographic North Pole is

actually the South Pole of the inner magnet)

Dip Angle: Angle relative to the horizon of the magnetic field

of the earth

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Magnetic Force on a Moving ChargeB = magnetic field (Tesla T)

v = velocity of charge (m/s)

q = charge (C)

F = magnetic force

θ = angle between B and v

F = qvBsinθB = F

qvsinθB = N/(Cm/s)

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The First Right Hand Rule1) Point fingers of right hand in direction of

velocity

2) Curl fingers towards direction of B

3) Thumb points in direction of force

(reverse if it's a negative charge)

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The First Right Hand RuleFind the direction of F for the following charges:

v

v

v

v

v v

B

B

B

B

B

B

+

+

+

+

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The First Right Hand RuleNotation: o = out of x = into

v

B B B

ov

v

o

o o

oo

o

B

B

Bxv v

x

x

x

xx

x B

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Motion of a Moving ParticleThe magnetic force affects motion of moving particle

Since F acts on a charge q with mass m: F=qvB=ma=m(Δv/Δt)

Since F is to v, the force only changes direction (speed is

constant)

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Motion of a Moving ParticleThe direction of v with respect to B determines resulting

motion of charge

When v is to B, the motion is a circleWhen v is not to B, the motion is a helical

Fc = mac

Fc = mv2/rqvB = mv2/r

r = mv qB

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Quantifying the Force on a Current-Carrying Wire

Fmax = (qvB)(nA )

Fmax = BI

If B and I are NOT perpendicular:

Fmax = BI sinθ

θ is the angle between B and I

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Example 1A wire carries a current of 22.0A from west to east. Assume

the magnetic field of the Earth at this location is horizontal

and directed from south to north and it has a magnitude of

0.500 x 10-4 T. a) Find the magnitude and direction of the

magnetic force on a 36.0 m length of wire. b) calculate the

gravitational force on the same length of wire if it's made of

copper and has a cross-sectional area of 2.50 x 10-6 m2.

Density of copper is 8.92x103 kg/m3.

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Magnetic Fields Around Current-Carrying WiresMoving charges (current) produce magnetic fields

For long, straight wires, the B-field a distance "r" from the wire

is constant

*So the B-field lines form circles around the wire

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Right Hand Rule #2

1) Point thumb in the direction of the current in the wire

2) Fingers curl in the direction of the magnetic field

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Loops and Stacks of Loops

Solenoid (or coil)

Mimics a bar magnet

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Loops and Stacks of LoopsAmount of B-field around a wire is given by:

Bwire = µoI2πr

µo = permeability of

free space

µo = 4π x 10-7 Tm/A

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Example 2

A long, straight wire carries a current of 5.00A. At

one instant, a proton, 4.00 mm from the wire, travels

at a speed of 1.50 x 103 m/s parallel to the wire and

in the same direction as the current. a) Find the

magnitude and direction of the magnetic field

created by the wire. b) Find the magnitude and

direction of the magnetic force the wire's magnetic

field exerts on the proton.

Bwire = µoI2πr

µo = 4π x 10-7 Tm/A

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Total B-Field with Two WiresAdd values of B when in same direction, subtract when opposite direction

P

Q

I1= 2.0A

I2= 1.5A4.0 cm

2.0 cm

Find Btotal at point P at the midpoint

between the two wires as shown.

Find the Btotal for point Q.

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