P.Sci. Unit 6 cont. Ch. 21 Magnetism

Magnets Permanent magnets – are magnetic all the time (lodestones) Other substances can be made into permanent magnets by placing a strong permanent magnet near them or by stroking them with a permanent magnet.

Materials are classified as either magnetically hard or soft. Hard – easily magnetized but looses magnetic properties easily. Soft – difficult to magnetize but does not loose its magnetic properties.

Magnetic Force the invisible force exerted by magnets. It depends on: the strength of the magnet the distance an object is from the magnet

Magnetic Poles Magnets have a pair of poles, a north pole and a south pole. Like poles repel (S/S or N/N) Unlike poles attract (N/S) Just like with electrical charges North Pole South Pole

Magnetic Fields The region where a magnetic force can be detected. Magnetic Field lines are used to show the magnetic field. A compass needle will always point along a field line. The lines are close together where the magnetic force is strong, and spread out where it is weak

Earth as a Magnet Earth acts like a giant bar magnet. It has a North and South pole. These magnetic poles are not the same as the geographic poles. The pole that is above Canada is actually a magnetic South pole and the one in Antarctica is a North pole – to reduce confusion they are named for the geographic pole

. Earth’s magnetic field has changed direction more than 20 times in the last 5 million years.

Magnetism and Electric Currents In 1820, Hans Oersted found that magnetism is produced by moving electric charges by experimenting with the effects of an electric current on the needle of a compass.

A current-carrying wire creates a magnetic field that forms in concentric circles around the wire. Right Hand Rule – if you hold a wire in your right hand with your thumb pointing in the direction of the + current, your fingers would curl in the direction of the magnetic field.

The magnetic field created by the current-carrying wire is very weak The magnetic field created by the current-carrying wire is very weak. To create a stronger magnetic field: Increase the current in the wire Wrap the wire in a coil (solenoid)

Solenoids Each loop of wire adds to the strength of the magnetic field of the loop next to it. The strength of the magnetic field depends on the number of loops of wire and the amount of current in the wire.

Electromagnet If you add an iron rod in the center of the coil (in a solenoid) it will create an electromagnet. This adds to the magnetism of the coil creating a stronger magnet. Electromagnet

Electromagnetic Devices Galvanometers – used to measure current in ammeters and voltage in voltmeters. Electric motors – convert electrical energy to mechanical energy to cause motion. Stereo speakers – magnetic forces cause motion that produce sound waves

Faraday’s Law An electric current can be produced in a circuit by a changing magnetic field. Electromagnetic induction -moving a magnet in and out of a coil of wire causes charges in the wire to move.

Electric Generator converts mechanical energy to electric energy by electromagnetic induction

Alternating Current (AC) For each ½ rotation of the wire loop, the current produced by the generator reverses direction or alternate. See table on page 477

Electricity in Your Home The electricity you use in your home is created by large generators at the power plants. The mechanical energy used to power the generators can come from: Running water (dams), burning coal, nuclear fission, wind, hot water (geothermal)

The electricity coming from a power plant is too strong for you to use in your home.

Transformers increase or decrease the voltage so that it can be used. Step up – increases voltage primary coil < secondary coil Step down – decreases voltage primary coil > secondary coil