Physical Science Chapter 7 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 Wrapping wire around a metal object, and connecting to a battery 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 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) Direct Current (DC) Directions reverse Also causes magnetic field to reverse Direct Current (DC) Direction does not reverse Magnetic field is always in one directions

Transformers 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 𝑉 𝑜𝑢𝑡 𝑉 𝑖𝑛 = 𝑁 2 𝑁 1 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, 𝑉 𝑜𝑢𝑡 = 𝑉 𝑖𝑛 𝑁 2 𝑁 1

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? 4 V

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