Magnets and Magnetic Fields

Magnets Magnets can be used along with circuits to create an alarm – A magnet can pull a switch closed to complete the circuit, but when the magnet is moved out of place, then it doesn’t pull the switch closed and instead the switch is open and thus the circuit is now interrupted and an alarm goes off.

How Magnets work for Alarms

Magnets get their name from a stone found 3000 years ago in Magnesia, which is now modern day Greece. This stone is called Iodestone and is composed of magnetite. Some material can be made into permanent magnets – You can change any piece of iron, such as a nail, into a permanent magnet by stroking it several times with a permanent magnet. – A slower method is to place the piece of iron near a strong magnet and will eventually become magnetic because of the magnetic field and will remain magnetic even after the strong magnet is removed

Even though a material can be considered permanent, its magnetism can be weakened or even removed. – Possible ways to do this is to heat or hammer the material. Magnetically Hard: materials that are difficult to magnetize but keep their magnetism (like Nickel or Cobalt) Magnetically Soft: materials that are easily magnetized, however they lose their magnetism easily as well (like Iron)

Magnetic Force A magnetic force can be created that allows the magnetism to be carried through material. – Like when a magnet picks up a nail, the magnetic force carries through the nail and attract yet another nail. – Nail attached to nail through that magnetic field, despite not actually touching the magnet. – There is a limit to this

Magnets have poles – North pole and south pole – The pole of a magnet exert a force on one another Like poles repel and opposite poles attract. The north pole of one magnet will attract the south pole of a different magnet The north pole of one magnet will repel the north pole of a different magnet. Poles are always present and cannot be separated. If you cut a magnet in half, two opposite poles still exist.

Magnetic Fields always travel from north pole towards the south pole

Magnetic forces are dependant on distance just like electrical force and gravitational force Magnetic force is a filed force and is dependant on the strength of the magnetic field. A magnetic field gets weaker the farther away you get. Magnetic fields are strongest near its poles A compass can track magnetic fields

Compasses: a compass is a magnet suspended on top of a pivot so that the magnet can rotate freely. – A compass aligns with Earth’s magnetic field just as iron filing align with the field of a bar magnet. The Earth: is like a big bar magnet – The polarity of the Earth has reversed around 20 times in the last 5 million years. – So the magnetic field has changed directions – The Earths magnetic poles are not the same as its geographical poles

For historical reasons, the poles of magnets are named for the graphical pole that they point towards. The end of a magnet that is labeled North is a north seeking pole, and not actually north The end of a magnet that is labeled South is a south seeking pole, and not actually south

Magnetism from electric currents Magnetism is produced by moving electrical charges. Electrical currents produce magnetic fields – You can use magnetic material to show the magnetic field produced from a current carrying wire.

Right hand rules: determines the direction of the magnetic field.

Magnetic fields exert forces, but the force is weak. One way to increase the force is to increase the current in the wire, but large currents can be fire hazards. A safer way to create a strong magnetic field that will provide a strong force is to wrap the wire into a coil. This creates a solenoid. – The coil of a solenoid allows the magnetic field of one loop to intensify those next to it. – The coil has a north and south pole, just like a magnet

Solenoid: the strength of the magnetic field depends on the number of loops and the amount of the current Electromagnet: A rod made of iron can be inserted into the coil to intensify the magnetic field

Moving charges cause magnetism Negatively charged electrons moving around the nucleus of an atom causes a magnetic field This is because electrons are spinning, producing tiny magnetic fields around each. These magnetic field are balanced by the protons for most atoms, except those that are magnetic (like iron..steel)

Electromagnetic Devices Modern day devices that use the magnetic field produced by coils of current-carrying wire. – Devices like hair dryers, electric motors, and stereo speakers – Electric motors: coil of a wire that spins when a current is running. This spinning coil is attached to a shaft and does work (wheel and axel). This transfers electrical energy into mechanical energy to make a devices like toys run.

Magnetic fields can even cause sound waves- as in stereo speakers – The magnetic field of the coil interacts with the magnetic field of the permanent magnet and causes the speaker cone to vibrate. (essentially it is being pulled in one direction by the coil and pulled in the other direction by the magnet- alternately) – These vibrations produce sound waves – In this way magnetic field is converted into sound waves

Electromagnetic Induction Faraday’s Law: an electric current can be produced in a circuit by a changing magnetic field This is an induced current – creating a current in a circuit by changing magnetic field Someone does work to push an electrically charged loop of wire in between two oppositely charged magnetic. This loop gets caught in the magnetic field and is pushed back out. So mechanical energy is put in but electrical energy is produced.

Generator convert mechanical energy to electrical energy – AC generator- Alternating current : this is the current used in your house and the current changed direction at regular intervals

Electromagnetic energy Moving charge produces a magnetic field Changing magnetic field causes an electric charge to move The energy that comes from these processes is called electromagnetic energy, like that of UV light and X-rays They move through space without the need for matter because the changing electric fields generate a magnetic field and the changing magnetic fields generate an electric field. Each field generates the other, allowing EM waves to travel through empty space