Magnetism

Magnets  Poles of a magnet are the ends where objects are most strongly attracted Two poles, called north and south  Like poles repel each other and unlike poles attract each other Similar to electric charges  Magnetic poles cannot be isolated If a permanent magnet is cut in half repeatedly, you will still have a north and a south pole This differs from electric charges There is some theoretical basis for monopoles, but none have been detected

More about Magnetism  An unmagnetized piece of iron can be magnetized by stroking it with a magnet Somewhat like stroking an object to charge an object  Magnetism can be induced If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized

Magnetic Domains  A magnetic domain is a microscopic magnetic region composed of a group of atoms whose magnetic fields are aligned in a common direction When an object is magnetized, the domains are aligned When an object is nonmagnetized, the domains are randomly oriented.

Types of Magnetic Materials  Soft magnetic materials, such as iron, are easily magnetized They also tend to lose their magnetism easily  Hard magnetic materials, such as cobalt and nickel, are difficult to magnetize They tend to retain their magnetism

Sources of Magnetic Fields  The region of space surrounding a moving charge includes a magnetic field The charge will also be surrounded by an electric field  A magnetic field surrounds a properly magnetized magnetic material

Magnetic Fields  A vector quantity  Symbolized by  Direction is given by the direction a north pole of a compass needle points in that location  Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look

Magnetic Field Lines  A compass can be used to show the direction of the magnetic field lines (a)  A sketch of the magnetic field lines (b)

Magnetic Field Lines  Magnetic Field Lines originate in the North Pole and terminate in the South Pole Out of the North Into the South (ONIS)  They generally come out of the North Pole bend around the entire magnet and terminate in the south pole at the opposite end of the magnet

Force & Magnetic Field  When a charge is placed into a B field (magnetic field) it experiences a magnetic force Middle Finger – direction of the B field Pointer Finger – direction of the Current (velocity) Thumb – direction of the magnetic force

Current Carrying Wire  A current carrying wire also experiences a force when placed into a magnetic field.  You can also use the right hand rule to find the direction of the magnetic force in a current carrying wire. Curl your fingers in a half circle around the wire this is the direction of the magnetic field Your thumb now points in the direction of the magnetic force

Current & Solenoids  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.

Electromagnets 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.

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. Basically, Changing the magnetic field strength near a conductor induces an emf or voltage within the conductor

DC Current  Current that is produced from a battery (positive and negative ends of the battery are set in stone – current always flows from the positive to the negative)

Electromagnetic Induction 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).

AC Current  AC current is used to constantly change the direction that current flows within a circuit

Using AC and DC Current  What type of electromagnet will result from AC and DC Current?  DC Current: Has current that is circulating in one direction at all times. Follows the right hand rule – creates a permanent magnet.  AC Current: Has current that is constantly changing direction, which causes a constant change in the direction of the magnetic field. Causes a loss of energy which is transferred and dissipated as heat Used in motors, transformers, inductors

Transformers  Utilized in electricity in order to change one AC potential difference into a different AC potential difference  Consists of two coils of wire around a core of soft iron. Utilizes the alternating current in the primary circuit to induce a current in the secondary circuit by running a magnetic field throughout the iron core

Primary vs. Secondary Coils  If the primary and secondary coils have equal number of wire loops (usually called loops) then the input and output voltages would be equal.

Step Up Transformers  If the secondary coil has more turns than the primary coil, then the alternating voltage produced in the secondary coil will be greater than that produced in the primary.  This is called a step up transformer.

Step Down Transformer  If the secondary coil has fewer turns than the primary, the alternating voltage produced in the secondary will be lower than that produced in the primary.  This is called a step down transformer.

Primary/Secondary Voltage Relationship  Remember: a transformer will step up or step down voltage without an overall change in energy so the amount of power input is always going to be equal to the power output.