Physics 10 - Magnetism - Ch 14 (COMPLETE)

Is it possible for a bar magnet to have just one pole?

No

Application

Computer hard drives contain information stored magnetically. - Magnetic image of computer hard disk, showing bits of information.

Intro to Auroras

Earth is constantly bombarded by energetic charged particles from the Sun & outside the solar system.

Permanent magnets attract unmagnetized ferromagnetic metals.

[Elements 26 (Fe/Iron), 27 (Co/Cobalt), 28 (Nickel) on periodic table.] U.S. coinscontain nickel, but not enough be magnetic.

How does a piece of iron get magnetized?

- A very miniature compass would show that unmagnetized iron has very small regions (called "domains") which are magnetized in random directions. - Each domain is a tiny magnet with N and S poles. - Size is up to a few hundred microns.

Direction of the Earth's Magnetic Field

- At mid-latitudes, a compass points north. - Near the geographic North Pole, a compass points into the ground!

How is the force between two magnetic poles similar to the force between two electric charges?

- Electric and magnetic forces both vary with the "inverse square" of the distance. - Like charges and like poles repel. Opposite charges and opposite poles attract

Super magnets are alloys of ferromagnetic metal and a lanthanide element.

- Elements 58 to 71, next-to-last row in periodic table. - Samarium-cobalt, SmCo5. - Neodymium-iron-boron, Nd2Fe14B.

Speaker

- Our next topic is waves. - Solenoid is glued to back of paper cone. - A permanent magnet is mounted behind cone. - Electrical signals cause solenoid to be pushed in and out of magnet. - Speaker generates sound waves

Doorbell

- Push doorbell. - Current flows. - Solenoid becomes electromagnet. - Iron core strikes bell.

Permanent Magnets

- Some materials (certain alloys) retain a strong degree of magnetization and are used as "permanent" magnets. - Other materials (e.g., pure iron) are only magnetized temporarily

Likes repel; opposites attract

- Two kinds of magnetic poles, designated north & south; - Like poles repel; opposite poles attract. Obvious parallels between electricity and magnetism makes us ask, what are the connections?

Earth's Magnetic Field

A compass is a small magnet, which rotates freely, & aligns with Earth's magnetic field. - North pole, or North-seeking pole, points northward; - South pole,or South-seeking pole, points southward.

Magnetic Domains

A) Unmagnetized iron with domains of magnetization. In a magnetic field: - Domains with magnetization in direction of magnetic field grow larger; - With other domains, direction of magnetization rotates slightly to align with magnetic field.

Magnetic poles always occur in pairs

Breaking a magnet apart creates two magnets, each with two poles. - Magnets are always dipoles; - No magnetic mono poles have ever been observed, although no physical law prohibits this. - In electricity, single charges can be isolated.

How is geographic north defined?

By the rotation of the Earth

Electricity and Magnetism

Difference: Magnetic field are created by moving charges. - Charged particles move in circles in magnetic fields. - Changing magnetic fields through a surface cause circulating currents

Oersted's Experiment (1820)

L: No current in wire. Compass needle points north. R: Close switch; current flows. Compass needle points in direction of magnetic field of Earth + wire. - At location of compass, magnetic field due to wire points west. - Oersted's Experiment shows that moving charges create magnetic fields.

Strength of Magnetic Forces

Magnetic forces are strongest at the poles. [They obey a Coulomb-type law.]

How is magnetic north defined?

Magnetic north is the direction that the north pole (or "north seeking" pole) of a compass points

Auroras

Occur when these charged particles, moving around magnetic field lines, ionize the upper atmosphere near the poles. E.g. colors: Green: Ionized oxygen. Pink: Nitrogen.

Earth's Magnetic Field

Opposite poles attract, so the north pole of a compass is attracted to a south magnetic pole. - Earth's north geographic pole contains a south magnetic pole. - Earth's south geographic pole contains a north magnetic pole.

Electricity and Magnetism

Similarities: (See image)

Earth's Magnetic Field

Strongest magnetic field near poles, where field lines are closest together.

You hold a bar magnet in your hand, with the north pole facing outward. When you bring the north pole of this magnet near a second bar magnet on the table, its north pole will be repelled, and its south pole will be attracted. So the second magnet will spin around and be attracted to the magnet in your hand.

TRUE

Direction of Magnetic Fields

The direction of a magnetic field is the direction that the north pole of a magnet points.

A magnetic compass points towards the north geographic pole of the Earth because

there is a south magnetic pole near the north geographic pole.

Magnets are made of

Magnets are made of ferromagnetic metals: iron (Fe), cobalt (Co), & nickel (Ni);

Navigation

Man, birds, fish, and insects navigate via the Earth's magnetic field.

The magnetic field of the Earth is caused by

Movement of electrical charges as the molten core convects, and the Earth rotates.

Origins of Permanent Magnetism

Moving charges create magnetic fields. Magnetism in iron, cobalt, and nickel originates from spin. Spin is a relatively advanced topic, the quantum mechanical equivalent of moving charge.

Origins of Earth's Magnetic Field

Moving charges in molten core cause magnetic field of Earth. Charges arise due to friction when molten material convects. Magnetic field reverses direction every ten thousand years or more. Magnetic north is currently several degrees from the geographic north pole.