What is Magnetism?
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Transcript of What is Magnetism?
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Fields
• Model used when force act a distance.
• Quantity / unit measure.
• Direction Too.
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Grav field caused by massg = Fg/mass.
N/kg
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Electric caused by charge E = Fe/q.
N/C
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Magnetic Fields B
• caused by moving charges
• N/C *m/s =
• Tesla T
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What is What is Magnetism?Magnetism?
Magnetism is the force of attraction or repulsion of a magnetic material due to the excess spin of e-.
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Can think of e- circling nucleus are creating bar magnets in magnetic material.
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Magnetic Field, B Direction.Follow compass needle .
Field Strength Strongest near poles. Lines close together.
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Earth’s N geographic pole = S mag pole.
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Sketching Fields
Use arrows to show the direction of compass needle deflection.
Compass needle points away from N and into S mag pole.
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Arrow shows direction in plane of page.
Field into the page X like tail of arrow.
Field out of page … like tip of arrow.
Symbols
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Electric, E & Magnetic, B Fields
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Similarities
• Directional Field = vector• field lines – flux which cannot touch, or bend at
sharp angles, intensity is shown by close spacing.
• Both attract opposite. Repel like.• Drop off with distance inverse square.
• Both Polarize objects.
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Differences
• Magnet feels no force in static E field.
• Charged particle q, no F in static B field.
• For B field, direction compass needle points.
• For E, field direction + test charge would move.
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Reading Magnetism Hamper Ch 6.3
• How are magnetic fields sketched?
• How are Electric fields sketched?
• What is the relationship between charges and magnetic fields?
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Moving charges/ currents induce magnetic fields. In specific directions.
• When a current flows, a magnetic field is generated.
• *Stationary charges do not generate magnetic fields, but do cause electric fields.
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Mag Field Direction around Wire.
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Field circles perpendicular to wire.
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Wire loop – thumb in current direction. Fingers in B direction.
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Solenoid – Loops of wire (can be wrapped around core of iron).
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Solenoids have same field as bar magnets.
And Earth
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Hand Rule: Find Mag N pole in solenoid. Fingers in direction of current, thumb points to magnetic N. Pole (see field lines).
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Charged particles moving in B fields feel a force!!
Force direction determined by hand rule three.
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Direction of mag force on q perpendicular to velocity vector & to field.
For +q place right hand fingers into field, thumb points to v, palm points to mag force.
For –q use left hand or reverse direction of right hand.
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Electron in mag field B.
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Force is proportional to amount of q on particle.
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Magnetic Field Units = Tesla’s
Tesla (T) =
N N C m/s Am
kg m/s2
A m
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Strength/Intensity of Mag Field can bedescribed in terms of force on moving charge q.
Fmag = qvB sin q = charge coulombs
v = velocity m/s
B = Fmag F = force Newtons
qv B = Tesla
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The force on the charge only acts perpendicular to the velocity direction. Charges traveling along field lines feel no force.
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Since arises when the mag field and velocity are perpendicular:
• Fmag = qvBsin.
• Where is the angle between the velocity and the magnetic field direction.
• When = zero the force is zero.• When = 90 the force is maximum.
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Ex 1: A p+ moving east feels a force of 8.8 x 10-19 N upward due to the Earth’s magnetic field. At this location the field strength is 5.5 x 10-5 T north. Find the speed of the particle.
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Fmag = q v B
v = Fmag 8.8 x 10-19 N_ qB (1.6 x 10-19 C)(5.5 x 10-5 T )
= 100,000 m/s.
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If there is a component of velocity parallel to the field is unaffected. There is a helical effect.
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Magnetic Force on q is Fc since it causes curved motion.
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• the magnetic force is causing curved motion so it a centripetal force!
Ex 2: An electron moves at right angles in a uniform magnetic field of 3.0 x 10-3 T. If its speed is 3.0 x 105 m/s, what is the radius of the path of the electron?
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F mag = Fc.
qvB = mv2.r.
. r = mv2 = mv.qvB qB
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5.7 x 10-4 m.
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Force on current carrying wire.
The current in a wire also feels a force in mag field.
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Conventional Current –force direction on wire: fingers into field, thumb in current direction, palm points toward force direction.
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Force on wire proportional to direction of field, current, & length of wire in field.
F = BIl sin F = force (N)B = mag field (T)I = current (A)l = length of wire (m)
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Ex 3: A wire that is carrying a current of 3.5 A east, has 2.00 m of its length in a uniform field of flux density 5.00 x 10-7T directed into the paper.
a) Sketch the wire in the field.b) Find the magnitude and direction of the force.
F = BIl.
(5.00 x 10-7T )(3.5 A )(2.00 m ) = 3.5 x 10-6 N up.
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Ex 4: A 36-m long wire carries 22 A current from east to west. If the maximum magnetic force 4.0 x 10-2 N directed down, find the magnitude and direction of the magnetic field at this location.
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F = BIl
B = F/Il. = 4.0x10-2 N.(22A)(36m)
= 5.0 x 10-5 T. north.
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The current in the wire must be at right angles to the field to feel the force so:
F = BIl sin .
Where is the angle between the wire and the field
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Field & Force Around Parallel Current Carrying Wires
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Since currents generate mag fields, two parallel wires will each generate a field that exerts a force on the adjacent wire. The direction of the force depends on the current direction.
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Currents in same direction exert attractive forces.
Is this real or conventional current?
Hint hand rule for Force - fingers into the B field created by the other wire!
Neg current (real).
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Current in opposite directions, create fields with repulsive forces.
I1 I2
F1 F2
By Newton’s 3rd Law, F2 = -F1.
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The Ampere (A) is a unit of rate of current flow, but is based on the force felt between 2 current-carrying wires.
The Ampere, not the Coulomb, is considered a fundamental unit!! Very weird indeed!
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Formal definition of the ampere:
“The constant current which will produce an attractive force of 2 × 10–7 N/m of length between two straight, parallel wires of infinite length and negligible circular cross section placed one meter apart in a vacuum".
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Mag Field interaction Between Current Carrying Wires
These show neg charge flow.
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Do Now: Sketch the E field between parallel plates.Sketch the path of an electron beam shot into the field.
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• An e- is shot into the magnetic field as below. • a. Sketch its path in the field.• b. Suggest a way to use an E field to direct the e- in a straight line.• c. Sketch the electric field that will straighten the e- path.
e-
Crossed Fields
d. Derive an equation that expresses the electric field strength E, required to keep an e- of velocity v, moving through a magnetic field B, in a straight line.
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End Here
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For a straight length of wire the mag field can be expressed as:
B = oI o constant find it. 2r r = dist fr wire m
I = current - A B = field strength T
No longer in curriculum
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Force between two wires proportional to current in wires:
F = oI1I2l 2r
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Ex 4: What is the magnitude & direction of the force btw 5.00 m lengths of wire each carrying 7.5 A of current in the same direction if they are separated by 25.0 cm?
2.25 x 10-4 N toward each other.
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Mag Field Strength around a solenoid:
B = oNI N = number turns of wire
l I = currentl = length of wire in field m
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Hwk Sheet.
Do Holt p 782 with IB ref tablesp 781#19, 21, 22, 24, 25, 33, 35, 41, 42