Chapter 20 Magnetism. 20.1 Magnets and Magnetic Fields Magnets have two ends – poles – called...

Chapter 20

Magnetism

20.1 Magnets and Magnetic Fields

Magnets have two ends – poles – called north and south.

Like poles repel; unlike poles attract.

However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

Magnetic field intensity (B) is the region around which the effects of magnetism can be experienced by a current-carrying wire

Unit of magnetic field is Tesla (T)

Magnetic fields can be visualized using magnetic field lines, which are always closed loops.

Notice that the magnetic field goes from North to South.

The Earth’s magnetic field is similar to that of a bar magnet.

Note that the Earth’s “North Pole” is really a south magnetic pole, as the north ends of magnets are attracted to it.

A uniform magnetic field is constant in magnitude and direction.

The field between these two wide poles is nearly uniform.

20.2 Electric Currents Produce Magnetic Fields

Experiment shows that an electric current produces a magnetic field. (1st Right Hand Rule)

"if you point your thumb of your right hand in direction of conventional current (+ to - ), the curl of your fingers will indicate direction of magnetic lines of force around wire"

20.2 Electric Currents Produce Magnetic Fields

-symbol for magnetic field going into page

-symbol for magnetic field coming out of page

Magnetic field lines due to a circular loop of wire

Magnetic Field of a Current-Carrying Coil (Solenoid)

Right-hand Rule #2“if you grasp the coil with the right hand and curl your fingers around the loops in the direction of the conventional current, your thumb points towards the North pole of the electromagnet”

Magnetic Field of a Current-Carrying Coil (Solenoid)

Solenoid- long, straight wire bent into several closely-spaced loops (aka with an iron core its an "electromagnet“)

-acts like a magnet only when it carries a charge

-B inside solenoid is nearly uniform and strong

-outside, B is non-uniform

-strength of solenoid can be increased by adding iron core

-one end acts like North Pole; other acts like South Pole

•strength of magnetic field of an electromagnet is proportional to current flowing in coil•# of loops in coil proportional to strength of magnetic field

20.3 Force on an Electric Current in a Magnetic Field; Definition of B

Right-hand Rule #3:"if you point the thumb of your right hand in the direction of the current velocity (v) or conventional current (I), then your fingers point in the direction of the magnetic field (B) and the magnetic force (Fm) on a +ve charge is directed out of the palm of your hand"

Current-carrying wire in a magnetic field. The force on the wire is directed into the page.

The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation.

(on formula sheet.. F = BIL)

If the direction of I is perpendicular to B, then θ = 90˚ and sin θ = 1. If I is parallel to B, then θ = 0˚ and sin θ = 0. This equation defines the magnetic field B.

Unit of B: the tesla, T.

1 T = 1 N/A·m.

Another unit sometimes used: the gauss (G).

1 G = 10-4 T.

Measuring a magnetic field B

What is the magnetic field if the force is 0.0348 N and the current is 0.245 A? What about the magnetic forces on the two vertical sections of the wire that are in the magnetic field?

Measuring a magnetic field B

T 1.42

m) N)(0.100 (0.245

N 103.48 2

20.4 Force on Electric Charge Moving in a Magnetic Field

The force on a moving charge is related to the force on a current:

(on formula sheet…F = qvB)

Once again, the direction is given by 3rd Right Hand Rule. The rule is for positive particles. Notice the difference between positive and negative particles.

The force is greatest when the particle moves perpendicular to B (θ = 90°)

If a charged particle (in this case an electron) is moving perpendicular to a uniform magnetic field, its path will be a circle.

An electron travels in a circular path at 2.0 x 107 m/s in a plane perpendicular to a 0.010-T magnetic field. What is the radius of its circular path?

m 101.1

C)(0.010T)10(1.6

m/s)10kg)(2.010(9.109

FF magneticc

What is the path of a charged particle if its velocity is not perpendicular to the magnetic field?

The parallel component of v experiences no force, so it remains constant. The perpendicular component of v results in circular motion. Together this produces a spiral motion.

Charged ions approach the Earth from the sun (solar wind) and are drawn toward the poles. This causes the Aurora Borealis

or Northern lights. Use the Bohr Model to explain how these lights are produced….

20.5 Magnetic Field Due to a Long Straight Wire

The field is inversely proportional to the distance from the wire:

The constant μ0 is called the permeability of free space (or vacuum permeability), and has the value:

20.6 Force between Two Parallel Wires

The magnetic field produced at the position of wire 2 due to the current in wire 1 is:

The force this field exerts on a length l2 of wire 2 is:

(20-6)

20.6 Force between Two Parallel Wires

Parallel currents attract; anti-parallel currents repel.

20.7 Definition of Ampere and Coulomb

One ampere is defined as the current flowing in each of two long parallel conductors 1 m apart, which results in a force of exactly 2 x 10-7 N/m of length of each conductor

One coulomb is one ampere-second:

1 C = 1 A·s

20.14 Solenoids and Electromagnets

If a piece of iron is inserted in the solenoid (i.e. a wire with many loops), the magnetic field greatly increases. Such electromagnets have many practical applications.

20.10 Applications: Motors & Loudspeakers

An electric motor takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.

20.10 Applications: Motors & Loudspeakers

Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.

Back to…

WHY???

20.13 Ferromagnetism & Domains

Ferromagnetic materials, such as iron, can be permanently magnetized

Each atom has a net magnetic effect and the atoms tend to align their magnetic fields in arrangements known as domains. Each domain contributes to the overall magnetic field of the piece of iron.

In an ordinary piece of iron or other ferromagnetic material, the magnetic fields produced by the individual domains cancel out so that the object is not a magnet.

In a magnet, the domains are larger in one direction than in any other and a net magnetic effect is produced.

If you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

20.13 Ferromagnetism & Domains

Chapter 20 Magnetism. 20.1 Magnets and Magnetic Fields Magnets have two ends – poles – called...

Documents

Magnetism. Property of Magnets We say that unlike poles attract, and like poles repel

Chapter 20 & 21. Like Poles of a magnet repel; unlike poles attract

What do we already know ? Understand that some materials are attracted to magnets What the characteristics of magnets are That magnets can repel and

Chapter 19 Magnetism. Magnets Poles of a magnet are the ends where objects are most strongly attracted Two poles, called north and south Like poles repel

Top Ten List 1. There are North Poles and South Poles. 2. Like poles repel, unlike poles attract. 3. Magnetic forces attract only magnetic materials

MAGNETISM SPH3U. Permanent Magnets A permanent magnet has two poles: North and South. Like poles repel. Unlike poles attract. These repulsive or attractive

The law of magnets Like poles repel unlike poles attract TRIPLE ONLY

Force, Motion, Energy, Matter. Magnets Magnets have an effect on some materials. Magnets can make things move. Magnets can push/pull or attract/repel

PRE-AP Physics. Magnets have 2 poles (north and south) Like poles repel Unlike poles attract Magnets create a MAGNETIC FIELD around them

Lecture 14 Magnetism. Magnets... two poles: N and S Like poles repel Unlike poles attract

Concept Summary. Magnetic Poles Magnetic forces are produced by magnetic poles. Every magnet has both a North and South pole. Like poles repel,

Refresher Basic concept Like poles will repel Unlike poles will attract

Magnetic Fields and Forces - · PDF fileMagnetic Fields and Forces AP Physics B. Facts about Magnetism Magnets have 2 poles (north and south) Like poles repel Unlike poles attract

1 Magnets Magnet; an object that Attract certain types of material, like iron. Attract or repel another magnet. All magnets have two poles, named

Lecture 7: Magnets and Magnetism - Montana State · PDF fileMagnets • Laws of magnetic attraction and repulsion – Like magnetic poles repel each other – Unlike magnetic poles

Chapter 20 Magnetism. Magnets Poles of a magnet are the ends where objects are most strongly attracted –Two poles, called north and south Like poles repel

Chapter 22 Magnetism. Permanent bar magnets have opposite poles on each end, called north and south. Like poles repel; opposites attract. If a magnet

Magnetism. Magnetism Magnets have two poles that attract opposite poles and repel similar Magnets have two poles that attract opposite poles and repel

blogsaverroes.juntadeandalucia.es · Use the stickers. The magnets attract. The magnets repel. We use magnets to hold things together. There are magnets on the fridge door to keep

Magnetism and its applications. Laws of Magnetism - 1) Like magnetic poles repel, and 2) unlike poles attract