• Magnets and the magnetic field

• Electric currents create magnetic fields

• Magnetic fields of wires, loops, and solenoids

• Magnetic forces on charges and currents

• Magnets and magnetic materials

Chapter 24Magnetic Fields and Forces

Topics:

Sample question:This image of a patient’s knee was made with magnetic fields, not x rays. How can we use magnetic fields to visualize the inside of the body?

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Magnetic Fields Around Us

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Key Points• Three types of magnetic interactions

1. no interaction with either pole of a magnet => object is non-magnetic

2. attracted to both poles of a magnet => object is magnetic

3. Attracted to one pole and repelled by the other pole=> object is a magnet

• Magnetic field vector from a bar magnet is a super position of the magnetic field vectors from the N and S poles:

• Vector from N pole points away from N pole

• Vector from S pole points towards S pole

• Field lines form complete loops inside and outside of magnet• Field lines outside magnet go from N to S poles

• Field lines inside magnet go from S to N poles

• Magnetic Field vectors at a point are tangential to Magnetic Field Lines

Magnetic Fields from Two MagnetsBar Magnets A and B are placed at right angles. Two compasses, X and Y are placed so that they are equidistant from the two magnets as shown

A.) The arrow in compass X indicates the direction in which the North pole of the compass is pointing. Indicate the North and South ends of both magnets in the diagram

B.) Draw an arrow in compass Y to show the direction in which the North pole of the compass needle would point.

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Electric Currents Also Create Magnetic Fields

A long, straight wire

A current loop A solenoid

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Drawing Field Vectors and Field Lines of a Current-Carrying Wire

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The Magnitude of the Field due to a Long, Straight, Current-Carrying Wire

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Checking Understanding

Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P?

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Answer

Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P?

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The Magnetic Field of a Current Loop

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The Magnetic Field of a Solenoid

A short solenoid A long solenoid

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Checking Understanding

A. 10 A to the right.

B. 5 A to the right.

C. 2.5 A to the right.

D. 10 A to the left.

E. 5 A to the left.

F. 2.5 A to the left.

The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire?

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E. 5 A to the left.

The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire?

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Answer

The Magnetic Field of a Current Loop

Magnetic field at the center of a current loop of radius R

Magnetic field at the center of a current loop with N turns

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Example

What is the direction and magnitude of the magnetic field at point P, at the center of the loop?

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The Magnetic Field Inside a Solenoid

Magnetic field inside a solenoid of length L with N turns.

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The Force on a Charged Particle Moving in a Magnetic Field

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The Right-Hand Rule for Forces

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Paths of Charged Particles in Magnetic Fields

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Magnetic Fields Exert Forces on Currents

Magnitude of the force on a current segment of length L perpendicular to a magnetic field

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