1. Magnetism

2. All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the south pole. All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the south pole.  A North seeking pole is called the North Pole.  A South seeking pole is called the South Pole. Like magnetic poles repel and unlike magnetic poles attract. Like magnetic poles repel and unlike magnetic poles attract. A naturally occurring magnet is a mineral called Magnetite (looks like a stone)

3. Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to the poles, revealing their location.

4.  Magnetic Fields:  A magnet that is moved in space near a second magnet experiences a magnetic field.  A magnetic field can be represented by field lines.  The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.

5. These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.

6.  The Source of Magnetic Fields:  Permanent Magnets:  Moving electrons produce magnetic fields.  In most materials these magnetic fields cancel one another and neutralize the overall magnetic effect.  In other materials such as iron, cobalt, and nickel, the atoms behave as tiny magnets because of certain orientations of the electrons inside the atom.  These atoms are grouped in a tiny region called the magnetic domain.

7. Our Earth is a big magnet.  The Earth’s magnetic field is thought to originate with moving charges.  The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.  The earth’s magnetic field is called the magnetosphere.

8. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract.

9. A bar magnet cut into halves always makes new, complete magnets with both a north and a south pole. The poles always come in pairs. You can not separate a pair into single poles.

10. Electric Currents andMagnetism

11. Oersted discovered that a compass needle below a wire (A) pointed north when there was not a current, (B) moved at right angles when a current flowed one way, and (C) moved at right angles in the opposite direction when the current was reversed.

12. (A) In a piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect (not magnetic). (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron becomes magnetized.

13. A magnetic compass shows the presence and direction of the magnetic field around a straight length of current- carrying wire.

14. When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet.

15.  Applications of Electromagnets:  Electric Meters:  The strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet.  A galvanometer measures electrical current by measuring the magnetic field.  A galvanometer can measure current, potential difference, and resistance.

16. A galvanometer measures the direction and relative strength of an electric current from the magnetic field it produces. A coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.

17.  Electric Motors:  An electrical motor is an electromagnetic device that converts electrical energy into mechanical energy.  A motor has two working parts - a stationary magnet called a field magnet and a cylindrical, movable electromagnet called an armature.  The armature is on an axle and rotates in the magnetic field of the field magnet.  The axle is used to do work.

18. Electromagnetic Induction

19.  Induced Current:  If a loop of wire is moved in a magnetic field a voltage is induced in the wire.  The voltage is called an induced voltage and the resulting current is called an induced current.  The induction is called electromagnetic induction. A current is induced in a coil of wire moved coil of wire moved through a magnetic field. The direction of the The direction of the current depends on the direction of motion.

20. The magnitude of the induced voltage is proportional to:  The number of wire loops cutting across the magnetic field lines.  The strength of the magnetic field.  The rate at which magnetic field lines are cut by the wire.  Applications:  DC and AC Generators,  Transformers (step-up and step-down).

21.  Electromagnet – a magnet made from a current bearing coil of wire wrapped around an iron or steel core. What is an electromagnet?

22.  Generator – a machine that changes mechanical energy to electrical energy  Usually use moving magnets to create currents in coils of wire. What is a generator?

23.  Motor – a device that changes electrical energy to mechanical energy that can do work. What is a motor?