magnetic fields

Chapter 24: Magnetic Fields

PHYSICS Principles and

Problems

• To prevent hardware disease because there are times that cows swallow metals and sharp objects that can cause harm in the lining of the reticulum.

What is the purpose of orally ingesting magnets to cows?

• By connecting either end of the wire to a source of current like for example a wrapping a nail with copper wire and connecting the ends of the copper wire to a battery.

How is an electro magnet constructed?

• The domains return to a random arrangement because the no longer align with the domains of the field of the permanent magnet

What happens to the domains of a temporary magnet when the temporary magnet is removed from a magnetic field?

• In 1820, Danish physicist Hans Christian Oersted was experimenting with electric currents in wires.

• Oersted laid a wire across the top of a small compass and connected the ends of the wire to complete an electrical circuit, as shown.

Electromagnetism

SECTION24.1

Understanding Magnetism

• He had expected the needle to point toward the wire or in the same direction as the current in the wire.

• Instead, Oersted was amazed to see that the needle rotated until it pointed perpendicular to the wire, as shown in the figure at right.

Electromagnetism (cont.)

SECTION24.1


• The forces on the compass magnet’s poles were perpendicular to the direction of current in the wire.

• Oersted also found that when there was no current in the wire, no magnetic forces existed.

SECTION24.1


Electromagnetism (cont.)

FLEMING’S LEFT AND RIGHT HAND RULE

RIGHT HAND RULE

• When a current-carrying wire is placed in a magnetic field, a force is exerted on the wire that is perpendicular to both the field and the wire.

• The force on a current-carrying wire in a magnetic field is proportional to the current times the length of the wire times the field strength times the sine of the angle between the current and the magnetic field.

F = ILB

SECTION24.2

Applying Magnetic ForcesStudy Guide

Force on a Current-Carrying Wire in a Magnetic Field

F = ILB

• The force on a current-carrying wire in a magnetic field is equal to the product of magnetic field strength, the current, and the length of the wire.

SECTION24.2

Applying Magnetic Forces

Forces on Current-Carrying Wires (cont.)

• The strength of a magnetic field, B, is measured in teslas, T.

• 1 T is equivalent to 1 N/A·m.

• Note that if the wire is not perpendicular to the magnetic field, a factor of sin θ is introduced in the above equation, resulting in F = ILB sin θ.

• As the wire becomes parallel to the magnetic field, the angle θ becomes zero, and the force is reduced to zero. When θ = 90°, the equation is again F = ILB.

SECTION24.2



A straight wire carrying a 5.0-A current is in a uniform magnetic field oriented at right angles to the wire. When 0.10 m of the wire is in the field, the force on the wire is 0.20 N. What is the strength of the magnetic field, B?

Calculate the Strength of a Magnetic Field

SECTION24.2


Identify the known and unknown variables.

Known:

I = 5.0 A

L = 0.10 m

F = 0.20 N

Unknown:

B = ?

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Calculate the Strength of a Magnetic Field (cont.)

B is uniform and because B and I are perpendicular to each other, F = ILB.

F = ILB

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Solve for B.

Substitute F = 0.20 N, I = 5.0 A, L = 0.10 m

B is 0.40 T from left to right and perpendicular to I and F.

SECTION24.2



• A galvanometer is a device used to measure very small currents, and therefore, it can be used as a voltmeter or an ammeter

• can then be used to measure unknown currents. Many galvanometers produce full-scale deflections with as little as 50 µA (50×10−6 A) of current.

SECTION24.2



• All magnets have north poles and south poles and are surrounded by magnetic fields.

• Ferromagnetic materials become magnetic when their domains are in alignment with each other.

• Magnetic fields are vector quantities because they have direction and magnitude. They exist in any region in space where a magnet would experience a force. Magnetic fields can be represented by field lines which exit from a north pole and enter at a south pole, forming closed loops.

SECTION24.1

Understanding MagnetismStudy Guide

• The force that a magnetic field exerts on a charged particle depends on three factors: the charge of the particle, the velocity of the particle, and the strength of the field. The direction of the force is perpendicular to both the field and the particle’s velocity.

F = qvB

SECTION24.2

Applying Magnetic ForcesStudy Guide

Force of a Magnetic Field on a Charged, Moving Particle

F = qvB

• The force on a particle moving in a magnetic field is equal to the product of the field strength, the charge of the particle, and its velocity.

SECTION24.2


Forces on Single Charged Particles (cont.)

• The particle’s charge is measured in coulombs, C, its velocity in meters per second, m/s, and the strength of the magnetic field in teslas, T.

• The direction of the force is perpendicular to both the velocity of the particle and the magnetic field.

• The direction given by the third right-hand rule is for positively charged particles. For electrons, the force is in the opposite direction.

SECTION24.2


Forces on Single Charged Particles (cont.)

Force on a Charged Particle in a Magnetic FieldA beam of electrons travels at 3.0×106 m/s through a uniform magnetic field of 4.0×10−2 T at right angles to the field. How strong is the force acting on each electron?

SECTION24.2


Identify the known and unknown variables.

Known:

v = 3.0×106 m/s

B = 4.0×10−2 T

q = -1.60×10−19 C

Unknown:

F = ?

SECTION24.2


Force on a Charged Particle in a Magnetic Field (cont.)

F = qvB

Substitute q = -1.60×10−19 C, v = 3.0×106 m/s, B = 4.0×10−2 T

F = (-1.60×10-19C)(3.0×106m/s)(4.0×10-2T)

= -1.92×10-14N

SECTION24.2


Force on a Charged Particle in a Magnetic Field (cont.)

magnetic fields

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