PEP Conceptual PhysicsClass Notes

Unit 6 – Electricity and MagnetismChapter 15 – Magnetism

Section 15.1• Properties of Magnets

• Magnets and Magnetic Materials

• Magnetic Forces

• Magnetic Fields

Magnets and Magnetic Materials• Magnetic – the ability to exert forces of magnets or

other magnetic materials

• Magnet – a material that produces magnetism by itself

• Magnetic Material – a material that is affected by magnets but does not produce magnetism

• Iron and steel are magnetic materials that can also be made into magnets

Magnets and Magnetic Materials• Permanent Magnet – a material that retains its

magnetic properties even when no external energy is supplied

• Magnetic Pole – one of two opposite places on a magnet where the magnetic field is the strongest• All magnets must have at least one north pole and one

south pole

• It is impossible to have only a north pole or south pole without the other – it would be like having a coin with only one side

Magnetic Forces• When one magnet gets near another magnet, it will

either repel or attract the other magnet• Two north poles will repel

• Two south poles will repel

• One north pole and one south pole will attract

• Most materials are non-magnetic• They are transparent to magnetic forces and will not

affect or respond to a magnetic force

• Most insulators – wood, plastic, rubber, glass

• Other materials are magnetic• Most metals – especially iron, cobalt, and nickel

Magnetic Fields• Magnetic Field – the magnetic forces that surround

an object at all points in space• It represents the magnetic force around a magnet

• It is like Newton’s gravitational field or Coulomb’s electrical field

• Describing the field• The field will map out the lines that an imaginary lone

north pole (remember, a real north pole cannot exist alone) will move in the force

• It will move away from the magnet’s north pole (repel)

• It will move towards the magnet’s south pole (attract)

Magnetic Fields“Magnetic field lines always point away from a

magnet’s north pole and towards its south pole”Page 367

• Field lines cannot just start or stop in the middle of nowhere – they must start on a north pole and stop on a south pole

• The strength of the magnetic field is shown by the density of the field lines • Closer together means a stronger field

• Farther apart means a weaker field

Bar Magnet Field

Source: Just Science (India)

Horseshoe Magnet Field

Source: Quora

Section 15.2• The Source of Magnetism

• Electromagnets

• Right-Hand Rule

• Building an Electromagnet

• Magnetic Materials

• Permanent Magnets

Electromagnets• Magnetism is the result of an electric current

• There is always a magnetic field around a current carrying wire

• Straight wires have magnetic fields around them

• Coiled wires have magnetic fields through them

• You can determine the direction of the magnetic field using the “right-hand rule”• For a straight wire, point your thumb in the direction of

the current, and the magnetic field curls in the direction of your fingers

• For a coil, curl your fingers in the direction of the current, and the magnetic field points in the direction of your thumb

Right-Hand Rule• You can determine the direction of the magnetic

field using the “right-hand rule”• For a straight wire, point your thumb in the direction of

the current, and the magnetic field curls in the direction of your fingers

• For a coil, curl your fingers in the direction of the current, and the magnetic field points in the direction of your thumb

Source: Wikipedia

Building an Electromagnet• Get a nail or other iron or

steel core (like a nail)

• Wrap multiple turns of insulated wire around the core

• Attach the ends of the wire to the terminals of a battery

Source: Google Sites

Building an Electromagnet• Three factors affect the strength of electromagnets

1. Amount of current in the wire

2. Number of coils of wire around the core

3. The type of metal used in the core

• There are four ways to increase the strength1. Wrap more turns of insulated wire around the core –

this adds resistance and reduces current

2. Use a larger diameter wire – this reduces resistance and increases current

3. Add another battery – this increases current

4. Change the core material – you’ll need to experiment to find the best material

Magnetic Materials• As mentioned, any current (moving charge) will

create a magnetic field, and that includes electrons orbiting inside atoms• In many materials, the motion of individual electrons in

atoms are random and cancel one another’s fields and leaves the atom with a net field of zero

• Magnetic behavior is very different among the different elements• Diamagnetic materials

• Paramagnetic materials

• Ferromagnetic materials

Magnetic Materials• Diamagnetic materials react very weakly to external

magnetic fields• They align opposite to the applied field and are slightly

repelled

• Examples: lead, diamond, copper, silver, gold, marble, water, and glass

• Paramagnetic materials react weakly to external magnetic fields• They align in the same direction as the applied field and

are slightly attracted

• Examples: aluminum, titanium, molybdenum, platinum

Magnetic Materials• Ferromagnetic materials react strongly to external

magnetic fields• They align in the same direction as the applied field and

are strongly attracted

• Atoms align with neighboring atoms in groups

• Examples: iron, cobalt, nickel

• Magnetic Domain – a region of a material in which atoms align in the same direction, increasing the applied magnetic field strength• Domains are small – hundreds in a paper clip

• Domains aligned in the direction of the field grow and domains aligned opposite shrink

Permanent Magnets• Permanent Magnet – created when the magnetic

domains become so well aligned that they stay aligned even when the external field is removed• Usually, removing the external field will allow the

domains to return to random

• Soft Magnet – a magnetic material that is relatively easily magnetized and demagnetized

• Hard Magnet – a magnetic material in which the magnetic domains remain aligned after being magnetized, making them harder tio demagnetize

Permanent Magnets• Strongest permanent magnets are ceramics made

of cobalt or nickel• Liquids or gases cannot be permanent magnets

• Three ways to “break” a permanent magnet• Heat – a few hundred degrees Celsius

• Strong shock – beating, hammering, or dropping

• Stronger magnetic fields – especially alternating fields• This is a common way to erase magnetic computer disks

Permanent Magnets• You can hang magnetic material (paper clips, nails)

from a magnet and hang other magnetic materials from them

Section 15.3• Earth’s Magnetic Field

• History of Magnetism

• Earth’s Magnetic Field

• The Compass

• Planetary Magnetic Fields

History of Magnetism• Natural lodestone

• Discovered by the Greeks around 500 BC

• Magnetite, a form of iron oxide

• It naturally aligned north-south

• Chinese “south pointer”• Discovered by the Chinese around 220 BC

• Lodestone shaped like a spoon that pointed south

• First iron magnetic compass by Chinese in 1088

• Compasses were in common usage by 1200s

Early-Style Lodestone Compass

Source: Appalachian State University

Chinese South-Pointing Compass

Source: ComputerSmiths

Earth’s Magnetic Field• Electric currents within the Earth’s core create the

Earth’s magnetic field (old-earth dynamo theory)

• The magnetic field’s south pole is near the Earth’s geographic north pole• That’s why the north pole of a compass points north

• Gauss – a unit used to measure the strength of a magnetic field

• The Earth’s magnetic field strength is weak• Earth’s magnetic field is about 0.5 𝐺 at the surface

• Small ceramic permanent magnets have a magnetic field strength from about 300 𝑡𝑜 1,000 𝐺

Earth’s Magnetic Field

Source: Britannica

Compass• Compass – a device containing a magnet that

interacts with Earth’s magnetic field to indicate direction• The needle is free to spin

• The north pole of a compass always points to the south pole of a permanent magnet along the field lines

• The magnetic “north” is offset from the geographic north pole and is moving several kilometers per year

Compass• Magnetic Declination – the difference between the

direction a compass points and the direction of true (geographic) north• If the declination is 6∘47′ 𝑊 (Jacksonville, FL), and you

want to head true northeast (45∘), then adjust your compass to read 38∘13′ (left of your destination)

Source: Princeton University

Planetary Magnetic Fields• Some planets and moons have magnetic fields

• Jupiter has a strong magnetic field

• Earth’s moon have a very weak magnetic field but does not have a liquid core to create currents, so it may just be remaining from its original formation

• The Sun and other stars have magnetic fields• Since the equator of the Sun rotates faster (25 Earth

days) than the polar regions (35 Earth days), the field twists and varies greatly

• Sudden “snaps” to realign the field result in solar storms and flares

Solar Wind• The Earth’s magnetic

fiend deflects the solar wind (high energy ionized gases) from the Sun

• That protects life on Earth from damage

• It also creates the Aurora Borealis (northern lights) and Aurora Australis (southern lights)

Source: Antarctic/Arctic

Aurora Borealis (Northern Lights)

Source: Pinterest