1. Physical Science Chapter 7
Magnetism and its uses
Physical Science Chapter 7

2. Section 1: Magnetics Discovered over 2000 years ago
Magnetite-Naturally magnetic material
Magnetism-Properties and interactions of magnetics.

3. 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.

4. 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

5. 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

6. 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

7. 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

8. 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

9. Magnetic Materials Electrons have magnetic properties
In most elements, the electron’s magnetic properties cancel one another out.
In magnetic materials, they do not

10. 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.

11. 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

12. 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

13. Electromagnetism The interactions between electric charges and magnets
Electromagnetic force=attractive or repulsive force between electric charges and magnets

14. 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

15. 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

16. Galvanometers
Devices that uses an electromagnet to measure electric current
Used in vehicles to determine amount of gas remaining

17. 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

18. Electromagnetic Induction
Generation of an electric current by changing magnetic field
Generators
Use electromagnetic induction to transform mechanical energy into electrical energy
Faraday Flashlight

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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