Foundations of Physical Science

Unit 3: Electricity and Magnetism

Chapter 10: Magnets and Motors

• 10.1 Permanent Magnets

• 10.2 Electromagnets

• 10.3 Electric Motors and Generators

Learning Goals• Describe the properties of a

permanent magnet.

• Describe the forces that magnets exert on other.

• Explain why materials like iron and steel are attracted to magnets.

• Explain why a compass points north.

• Build an electromagnet.

• Analyze how electric current affects the strength of the magnetic field in an electromagnet.

• List three ways that the strength of an electromagnet can be increased.

• Compare permanent magnets and electromagnets.

• List several applications of electromagnets.

• Explain electromagnetic induction.

• Describe how electric motors and generators work.

Vocabulary

• electromagnet

• electromagnetic induction

• generator

• magnet

• magnetic

• magnetic field intensity

• magnetic force

• permanent magnet

• south pole

Permanent Magnet

• A material that keeps its magnetic properties, even when it is not close to other magnets

– alnico– ferrite– lodestone

– Bar magnets– Refrigerator magnets– Horseshoe magnets

Ferromagnets• Materials that are attracted to

nearby magnets but do not show magnetism otherwise

• “Domains” become aligned in the presence of a permanent magnet

– iron – steel– nickel– cobalt

Properties of Magnets

• Have two opposite poles.

– north– south

• Magnets exert forces on each other.

• Forces depend on the alignment of the poles.

Magnetic Forces & Electrical Forces

Differences

• Electric charges produce electrical forces

• Magnetic poles produce magnetic forces

• Magnetic forces cannot be isolated

Similarities

• Objects attract and repel

• Act between objects that are not touching

• Strength of force depends on distance between the objects

Magnetic Forces

• A north magnetic pole never exists without the company of a south pole, and vice versa

• The north and south poles of a magnet are like the head and tail of the same coin

Break a Magnet

• Each half still behaves as a complete magnet

• Even a piece 1 atom thick has two poles

• Atoms are magnets!

Discovery and Use of Magnetism

• Lodestone: magnetic properties – Greeks created the compass– Chinese created the “south pointer”

• By 1200 explorers from Italy were using the compass to guide ocean voyages beyond the site of land

• By the 1400 the Chinese were using compasses to travel to Africa

• The compass led to the interactions amongst cultures!

How does a compass work?

• The origins of the terms “north pole” and “south pole” of a magnet come from the direction that a magnetized compass needle points

• Pointing north north pole

• Pointing south south pole

• The north pole of the compass needle must point north because it is attracted by the south pole of another magnet. Where is this other magnet???

How does a compass work?

It is EARTH!!!!

• Compass needle swings toward the geographical north pole

• …or the magnetic south pole!

The Magnetic Field

• The force between two magnets

• Every magnet creates an energy field, called the magnetic field, in the space around it

• The field exerts forces on any other magnet that is within its range, the magnetic force

Michael Faraday

• Born in 1791

• London book-binder turned scientist

• Invented early motors using electromagnets

Magnetic Field Around a Current-Carrying Wire

• A magnetic field is produced by:

– A single moving charge

– A current of charges

– Demonstrated with compasses around the wire

Magnetic Field Around a Current-Carrying Wire

• More loops means more magnetic field intensity

• Current-carrying coil of many loops = strong magnetic field intensity

Electromagnet

• A magnet created when electric current flows in a wire

• The simplest electromagnet uses a coil of wire, wrapped around iron

• Because iron is magnetic, the magnetic field is concentrated in the current in the coil

Electromagnets

• Can switch north and south pole by reversing the direction of the current

• Advantage over permanent magnets

• Right Hand Rule

Electromagnet

• A common sight in junk yards

• Strength is limited by overheating of the current carrying-coils

• The most powerful ones omit the iron core and use superconducting coils

Increase Strength: Electromagnet

• Increase the current

• Increase the voltage (add a battery)

• Add more turns of wire around the nail (but this increases resistance & generates more heat)

• Amount of electric current in the wire

• Amount of iron or steel in the core

• The number of turns in the coil

Forces in an Electromagnet Depend on…

Electric Motors

• A device that uses a current-carrying coil forced to rotate in a magnetic field

• Electrical Energy Mechanical Energy

Using Magnets to Spin a Disk

• Use a single magnet to attract and repel magnets in a rotor by flipping its poles.

Using Electricity to Reverse the Magnet

• Commutator: the switch that reverses the poles

Electromagnetic Induction

• If you move a magnet through a coil of wire, then electric current is created

• The moving magnet induces electric current to flow

• The current stops if the magnet stops moving

Electromagnetic Induction• Voltage is caused, or induced, by the relative motion between a

wire and a magnetic field

• No battery or other voltage source is needed

• The greater the number of loops of wire moving in a magnetic field the greater the induced voltage

• Current is proportional to the number of loops

Faraday’s Law

The induced voltage in a coil is proportional to the number of loops multiplied by the rate at which the

magnetic field changes within those loops

Generating Electricity

• Generator: a combination of mechanical and electrical systems that converts kinetic energy into electrical energy

• Power plans use generators

• The magnetic field alternates north to south as the disk spins-thus, generators produce AC

Generating Electricity

• A power plant generator contains a turbine that turns magnets inside loops of wire to generate electricity.

Motors and Generators• Motor:

– Electrical energy: input – Mechanical energy: output

• Generator:– Mechanical energy: input – Electrical energy: output

• Both devices transform energy from one form to another