Transcript Magnetism and its uses

Magnetism and its uses
Physical Science Chapter 7
Section 1: Magnetics
• Discovered over 2000 years ago
• Magnetite-Naturally magnetic material
• Magnetism-Properties and interactions of magnetics.
Magnetic Force
• Magnets exert a force on one another and on certain objects (mostly metals)
• The strength depends on distance and the strength of the magnetic field
• As magnets move closer, the force increases.
• Magnets range in strength from cheap refrigerator magnets to ones designed
to pick up cars.
Magnetic Fields
• Region of space that surrounds a magnet and exerts a force on other
magnets and objects made up of magnetic materials
• Stronger field=stronger force
• MRI is about 200x stronger than fridge magnet
Magnetic Field Lines
• The closer the lines, the stronger the force of the magnet.
• Shows that the magnetic field is related to the magnetic force.
• Lines point from the N to the S pole
• If they were strong enough, all lines would reach the south pole
• Earth has a magnetic field
• The geographical North pole is actually the magnetic South pole
• The magnetic poles reverse every few 1000 years
Magnetic Poles
• Magnetic fields are strongest near the ends of a magnet
• These are called the magnetic poles
• North and South
• Like repel; opposites attract
Compasses
• The needle on a compass acts as a little bar magnet
• The north pole of the magnet points “North”
• Actually, the north pole is attracted to the Earth’s magnetic South pole
• Remember, this is the geological North Pole
Source of Earth’s Magnetic Field
• Not certain
• Probably has something to do with Earth’s core, which contains iron and nickel
• The core is surrounded by liquid, and the interaction potentially creates a magnetic field
Magnetic Materials
• Electrons have magnetic properties
• In most elements, the electron’s magnetic properties cancel one another out.
• In magnetic materials, they do not
Magnetic Domains
• Groups of atoms with aligned magnetic poles
• In non-magnetized materials, the domains point in random directions.
• This allows the domains to cancel out
• When the object becomes magnetized, the domains line up and point in
the same direction
• This causes a magnetic force
• These domains act as tiny magnets themselves
• Breaking a magnet into tiny pieces will result in pieces that each have a
north and south pole.
Permanent magnets
• If you have a non-magnetic object and bring it near a magnet, it causes the
domains to line up by attracting the opposite pole. This causes the magnet to
be attracted to the object.
• This is reversible because the field is not that strong.
• Naturally, over time, the random movement of atoms will cause the domains to revert to
random alignments and cancel one another out
• Magnets can be made by stroking a magnet to a magnetic material
Electric Current and Magnetism
• Hans Christian Oersted
• 1820 Danish physics teacher
• Did a demonstration of current, and happened to have a compass nearby
• Noticed that the current affected the compass needle
• Determined that current through a wire creates a magnetic field
• The magnetic fields are circular around the wire
• The strength of the magnetic field increases as the current increases
Electromagnetism
• The interactions between electric charges and magnets
• Electromagnetic force=attractive or repulsive force between electric charges
and magnets
Electromagnets
• A temporary magnet created when there is charge in a wire coil
•
A single wire wrapped into a cylindrical wire coil is called a solenoid
•
Leads to many interacting magnetic fields, and strengthens the total
• Electromagnets can me made very simply
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Wrapping wire around a metal object, and connecting to a battery
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Makes the metal object become temporarily magnetic
• We’ll make some of these in lab
• Electromagnets act like any other magnet when there is a current through it
• There will be a north and south pole
• There will be a magnetic force on objects
• They are useful because the magnetic field can be controlled
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Adding more current or more turns increases the strength
You can also cut the power, and it stops being magnetic
Using electromagnets
• Electromagnetic energy can be transferred into different forms
• Speakers:
• Electromagnetic energy is changed to mechanical energy that vibrates the speakers
• Devices send current to the electromagnet in the speaker
• The electromagnet becomes charged, and so has a magnetic field
• This field interacts with a permanent magnet, and causes the electromagnet to move back and forth,
causing the speaker cone to vibrate and emit sound waves
• https://www.youtube.com/watch?v=AP2Nu4MZJRs
Galvanometers
• Devices that uses an electromagnet to measure electric current
• Used in vehicles to determine amount of gas remaining
Electric Motors
• A device that changes electrical energy into mechanical energy
• Used in many everyday devices
• Simple electric motors
• Magnetic repulsion and attraction between electromagnet and permanent magnet
causes electromagnet to spin
• https://www.youtube.com/watch?v=ziWUmIUcR2k
Electromagnetic Induction
• Generation of an electric current by changing magnetic field
• Generators
• Use electromagnetic induction to transform mechanical energy into electrical energy
• Faraday Flashlight
• https://www.youtube.com/watch?v=gfJG4M4wi1o
Types of Current
• Alternating Current (AC)
• Directions reverse
• Also causes magnetic field to reverse
• Direct Current (DC)
• Direction does not reverse
• Magnetic field is always in one directions
Transformers
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Device that increases or decreases the voltage of AC
Made up of a primary coil and a secondary coil wrapped around the same iron core
If the transformer increases voltage, it is called a step-up transformer
If the transformer decreases voltage, it is called a step-down transformer
Calculation output voltage
• The ratio of the output voltage to the input voltage is equal to the ratio of the number of turns
in the secondary and primary coils
𝑉𝑜𝑢𝑡
•𝑉
𝑖𝑛
=
𝑁2
𝑁1
• Where N2 is the number of turns in the secondary coil and N1 is the number of turns in the primary coil.
• To solve for output voltage,
• 𝑉𝑜𝑢𝑡 =
𝑉𝑖𝑛 𝑁2
𝑁1
Example Problem
• A transformer has 150 turns in its primary coil and 50 turns in its secondary
coil. If the input voltage is 12 V, what is the output voltage?
• 4V
Guided Practice
• The input voltage into a device is 12 V. If there are 2x as many turns in the
secondary coil as there are in the first, what will be the output voltage?
• 24 V
You Try it
• A device converts 12 V to 48 V. Its secondary coil has 100 turns. How many
turns does its primary coil have?
• 25