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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 SI unit for the strength of the magnetic field?

• Gauss

• Henry

• Tesla

• Becquerel

• Bohr magneton

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

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

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

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

QUESTION:

### 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

QUESTION:

### 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

QUESTIONS:

### 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:

The strength of the uniform magnetic field inside a solenoid is

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.

QUESTION:

### Applications

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.

Which magnet or magnets produced this induced magnetic dipole?

• a or d

• a or c

• b or d

• b or c

• any of a, b, c or d

Which magnet or magnets produced this induced magnetic dipole?

• a or d

• a or c

• b or d

• b or c

• any of a, b, c or d