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# Reading Quiz - PowerPoint PPT Presentation

Reading Quiz. 2. What is the shape of the trajectory that a charged particle follows in a uniform magnetic field? Helix Parabola Circle Ellipse Hyperbola. What is the SI unit for the strength of the magnetic field? Gauss Henry Tesla Becquerel Bohr magneton. Reading Quiz.

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• 2. What is the shape of the trajectory that a charged particle follows in a uniform magnetic field?
• Helix
• Parabola
• Circle
• Ellipse
• Hyperbola
• What is the SI unit for the strength of the magnetic field?
• Gauss
• Henry
• Tesla
• Becquerel
• Bohr magneton

• 4. The magnetic field of a straight, current-carrying wire is
• parallel to the wire.
• inside the wire.
• perpendicular to the wire.
• around the wire.
• zero.
• 3. The magnetic field of a point charge is given by
• Biot-Savart’s law.
• Gauss’s law.
• Ampère’s law.
• Einstein’s law.

What is the shape of the trajectory that a charged particle follows in a uniform magnetic field?

• Helix
• Parabola
• Circle
• Ellipse
• Hyperbola

The magnetic field of a point charge is given by

• Biot-Savart’s law.
• Gauss’s law.
• Ampère’s law.
• Einstein’s law.

The magnetic field of a straight, current-carrying wire is

• parallel to the wire.
• inside the wire.
• perpendicular to the wire.
• around the wire.
• zero.
Field Around Magnet
• Use a compass to map the direction of the magnetic field surrounding a magnet.
• White board your results. In particular:
• how does the strength of the field vary with distance from the wire?
• how does the field direction relate to the poles of the magnet?

Magnetism

Activity: Map Field of Magnets
• Use iron filings to map the field of a
• bar magnet
• horseshoe magnet
• White board results
• draw field lines.
• how might magnets generate magnetic fields?

Magnetism

Magnetic Field Lines
• direction of magnetic field, B, is parallel to field line
• number of lines per area is proportional to strength of field
• field lines point
• from N to S
• field lines formclosed loops

Magnetism

Magnetism

No magnetic monopoles!

Magnetism

Ferromagnetism
• Ferromagnetic material
• iron or other materials that can be made into magnets
• You can make a magnet from iron by placing it in a strong B field
• individual domains become aligned with external B field
• Loss of magnetism from:
• dropping
• heating
• Curie temperature
• 1043 K for iron

Preferentially

downwards

Random

Magnetism

Specifying 3 Dimensions
• out of page
• tip of arrow
• into page
• tail of arrow

Magnetism

Force on a moving charge
• Right Hand Rule (#2)
• qv = fingers
• B = bend fingers
• F = thumb
• Find the direction of the force on a negative charge for each diagram shown.

Magnetism

Think-Pair-Share
• Derive an expression for the radius of an e-’s orbit in a uniform B field. Express your answer in terms of me, v, qe, and B. Turn in your solution!

Magnetism

Earth’s Magnetic Field
• magnetic declination
• angular difference between geographic north and magnetic north
• varies with latitude

Magnetism

The Source of the Magnetic Field: Moving Charges

The magnetic field of a charged particle q moving with velocity v is given by the Biot-Savart law:

where r is the distance from the charge and θ is the angle between v and r.

The Biot-Savart law can be written in terms of the cross product as

The Magnetic Field of a Current

The magnetic field of a long, straight wire carrying current I, at a distance d from the wire is

The magnetic field at the center of a coil of N turns and radius R, carrying a current I is

Practice Problems
• Magnetism: Worksheets 1 and 2
• Finish before next class

Magnetism

Magnetic Dipoles

The magnetic dipole moment of a current loop enclosing an area A is defined as

The SI units of the magnetic dipole moment are A m2. The on-axis field of a magnetic dipole is

Ampère’s law

Whenever total current Ithrough passes through an area bounded by a closed curve, the line integral of the magnetic field around the curve is given by Ampère’s law:

where n = N/l is the number of turns per unit length.

The Magnetic Force on a Moving Charge

The magnetic force on a charge q as it moves through a magnetic field B with velocity v is

where α is the angle between v and B.

Magnetic Forces on Current-Carrying Wires

Consider a segment of wire of length l carrying current I in the direction of the vector l. The wire exists in a constant magnetic field B. The magnetic force on the wire is

where α is the angle between the direction of the current and the magnetic field.

Does the compass needle rotate clockwise (cw), counterclockwise (ccw) or not at all?

• Clockwise
• Counterclockwise
• Not at all

Does the compass needle rotate clockwise (cw), counterclockwise (ccw) or not at all?

• Clockwise
• Counterclockwise
• Not at all

The magnetic field at the position P points

• Into the page.
• Up.
• Down.
• Out of the page.

The magnetic field at the position P points

• Into the page.
• Up.
• Down.
• Out of the page.

The positive charge is moving straight out of the page. What is the direction of the magnetic field at the position of the dot?

• Left
• Right
• Down
• Up

The positive charge is moving straight out of the page. What is the direction of the magnetic field at the position of the dot?

• Left
• Right
• Down
• Up

What is the current direction in this loop? And which side of the loop is the north pole?

• Current counterclockwise, north pole on bottom
• Current clockwise; north pole on bottom
• Current counterclockwise, north pole on top
• Current clockwise; north pole on top

What is the current direction in this loop? And which side of the loop is the north pole?

• Current counterclockwise, north pole on bottom
• Current clockwise; north pole on bottom
• Current counterclockwise, north pole on top
• Current clockwise; north pole on top

An electron moves perpendicular to a magnetic field. What is the direction of ?

• Left
• Into the page
• Out of the page
• Up
• Down

An electron moves perpendicular to a magnetic field. What is the direction of ?

• Left
• Into the page
• Out of the page
• Up
• Down

What is the current direction in the loop?

• Out of the page at the top of the loop, into the page at the bottom.
• Out of the page at the bottom of the loop, into the page at the top.

What is the current direction in the loop?

• Out of the page at the top of the loop, into the page at the bottom.
• Out of the page at the bottom of the loop, into the page at the top.
• a or d
• a or c
• b or d
• b or c
• any of a, b, c or d
• a or d
• a or c
• b or d
• b or c
• any of a, b, c or d