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DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS

IB PHYSICS

TSOKOS LESSON 5-6 MAGNETIC FIELDS

IB Assessment Statements

Topic 6-3, Magnetic Force and Field

6.3.1. State that moving charges give rise to magnetic fields.

6.3.2. Draw magnetic field patterns due to currents.

6.3.3. Determine the direction of the force on a current-carrying conductor in a magnetic field.

IB Assessment Statements

Topic 6-3, Magnetic Force and Field

6.3.4. Determine the direction of the force on a charge moving in an electric field.

6.3.5. Define the magnetic field and direction of a magnetic field.

6.3.6. Solve problems involving magnetic forces, fields, and currents.

Objectives

Understand the meaning of magnetic field and find its magnitude and direction in simple situations involving straightline conductors

and solenoids

using the right-hand rule

where appropriate

r

IB

2

0

L

NIB 0

Objectives

Find the force on moving charges

and currents,

in magnetic fields and appreciate the definition of the ampere as a fundamental SI unit, using the right-hand rule for forces where appropriate.

sinqvBF

sinBILF

Introductory Video

Electric Field, Magnetic Field

A charge will generate an electric field around itself

Any other charge (small positive test charge) that enters the field will experience a force on it = electric force

Electric Field, Magnetic Field

Same is true for a magnet or an electric current (moving charge)

Each of these produce a magnetic field around them

When another magnet or electric current enters the magnetic field, it will experience a force on it

Direction of magnetic field is determined by relation to magnetic field of the earth

Electric Field, Magnetic Field

Electric Field, Magnetic Field

Magnetic Fields

Magnetic fields are produced by permanent

magnets and by electric currents

Magnetic Field from Coil

Magnetic Field from Solenoid

Magnetic Field from a Loop of Wire

Magnetic Field from a Loop of Wire

Magnetic Field from a Straight Wire

Electric Field, Magnetic Field

Magnets and electric currents (moving charges) produce magnetic fields around them

When another magnet or electric current enters the magnetic field, it will experience a force on it

Force on a Current

Force on a Current

Force on a Current

Using the right hand place the thumb in the direction of the current and the fingers in the direction of the magnetic field. The direction away from the palm is the direction of the resulting force.

Force on a Current

Current

Field

Force

Thumb

Fingers

Palm

Force on a Length of Wire

Θ is the angle between the current and the direction of the magnetic field

sinBILF

Force on a Length of Wire

Θ is the angle between the current and the direction of the magnetic field

sinBILF

I

B

θ

Magnetic Force on a Moving Charge

sinBILF

t

qI

tvL

sin

sin

qvBF

tvt

qBF

Magnetic Force on a Moving Charge

sinqvBF

Magnetic Force on a Moving Charge

Work Done by Magnetic Force

None

Since the magnetic force is always perpendicular to the velocity, it cannot do work

Magnets in particle accelerators merely deflect or direct particles

Electric fields are used to increase the particle’s kinetic energy

Orsted’s Discovery

The magnitude of the magnetic field B created by the current in a wire varies linearly with the current in the wire and inversely with the perpendicular distance from the wire.

r

IB

2

0

What does that remind you of?

What does that remind you of?

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2

0r

QE

04

1

Orsted’s Discovery

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IB

2

0

Orsted’s Discovery

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0

µ0 is the magnetic permeability of a vacuum

(ε0 is the electric permittivity of a vacuum)

27

0 104 ANx

Orsted’s Discovery

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0

Unit for the magnetic field is the tesla (T)

Magnetic field of the earth at the surface is 10-4 T

A high voltage transmission line carrying a 2000 A current produces a magnetic field of 8x10-5 T

Direction of the Magnetic Field

Direction of the Magnetic Field

Strength of the Magnetic Field

Field Strength from Single Wire Loop

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IB

2

0

Field Strength from a Solenoid

L

NIB 0

Field Strength Between Two Current-Carrying Wires

Field Strength Between Two Current-Carrying Wires

Field Strength Between Two Current-Carrying Wires

r

IB

2

0sinBILF

LIr

IF

LIBF

120

1

121

2

LIr

IF

LIBF

210

2

212

2

Field Strength Between Two Current-Carrying Wires

The force on each wire will be the same, even though the magnetic fields produced by each wire individually is different

r

IILF

LIr

IF

1201

120

1

2

2

r

qqkF 21

r

mmGF 21

Fun Facts:

The Ampere is defined through the magnetic force between two parallel wires. If the force on a 1m length of two wires that are 1m apart and carrying equal currents is 2x10-7 N, then the current in each wire is defined to be 1 A.

Fun Facts:

The coulomb is defined in terms of the ampere as the amount of charge that flows past a certain point in a wire when a current of 1 A flows for 1 second.

Summary: Are you able to

Understand the meaning of magnetic field and find its magnitude and direction in simple situations involving straightline conductors

and solenoids

using the right-hand rule

where appropriate

r

IB

2

0

L

NIB 0

Summary: Are you able to

Find the force on moving charges

and currents,

in magnetic fields and appreciate the definition of the ampere as a fundamental SI unit, using the right-hand rule for forces where appropriate.

sinqvBF

sinBILF

QUESTIONS?

Homework

SL: #1-20, 23, 25-27, 30-32

HL: #1-32, skip 21, 22, 24, 28, 29