Magnetism and Electromagnetism

Magnetism and Electromagnetism

1. MAGNETS AND MAGNETIC FIELDS

   • Magnets Produce Magnetic Fields
     • MAGNETS HAVE TWO POLES- NORTH AND SOUTH
     • MAGNETIC FIELD= A REGION WHERE MAGNETIC MATERIALS EXPERIENCE A FORCE
     • FIELD LINE ARE USED TO SHOW THE SIZE AND DIRECTION OF MAGNETIC FIELDS. NORTH TO SOUTH
     • PLACING TWO MAGNETS CLOSE TOGETHER= UNIFORM FILED BETWEEN THEM
   • Magnetic Field Patterns can be Seen Using Compasses
     • COMPASSES AND IRON FILLINGS ALIGN WITH MAGNETIC FIELDS
   • Magnetism can be Induced
     • MAGNETS AFFECTS MAGNETIC MATERIAL AND OTHER MAGNETS
     • LIKE POLES= REPEL OPPOSITE POLES=ATTRACT
     • WHEN MAGNETIC MATERIALS COME CLOSE THEY ACT LIKE MAGNETS. THIS MEANS THAT THE MATERIAL HAS BEEN INDUCED
       • THE CLOSER IT IS TO THE ORIGINAL MAGNET THE STRONGER IT WILL BE

2. ELECTROMAGNETISM

   • A Current-Carrying Wire Creates a Magnetic Field
     • ELECTRIC CURRENT IN A CONDUCTOR PRODUCES A MAGNETIC FIELD AROUND IT
     • LARGER THE ELECTRIC CURRENT, STRONGER THE M.F.
     • THE DIRECTION OF THE M.F. DEPENDS ON THE DIRECTION OF THE CURRENT
   • The Magnetic Field around a Straight Wire
     • THERE IS A M.F. AROUND A STRAIGHT, CURRENT CARRYING WIRE
       • MADE UP OF CONCENTRIC CIRCLES
   • The Magnetic Field Around a Flat Circular Coil
     • M.F. IN THE CENTRE OF A FLAT CIRCULAR COIL OF WIRE IS SIMILAR TO THAT OF A BAR MAGNET
     • THERE ARE CONCENTRATED ELLIPSES OF M.F. LINES AROUND THE COIL
   • The Magnetic Field Around a Solenoid
     • M.F. IN A CURRENT CARRYING SOLENOID IS STRON AND UNIFORM. OUTSIDE THE COIL, THE FIELD IS JUST LIKE THE ONE AROUND A BAR MAGNET
     • THE ENDS OF A SOLENOID ACT LIKE THE POLES. THIS IS CALLED AN ELECTROMAGNET
   • Magnetic Materials can be 'Soft' or 'Hard'
     • SOFT= IF IT LOSES ITS INDUCED MAGNETISM QUICKLY. HARD= KEEPS IT PERMANENTLY
     • IRON= SOFT. STEEL= HARD
     • IRON IS USED IN TRANSFORMERS FOR THIS PURPOSE. NEEDS TO MAGNETISE AND DEMAGNETISE 50 TIMES A SECOND
     • INCREASE STRENGTH OF M.F. AROUND SOLENOID BY ADDING IRON CORE [SOFT]

3. THE MOTOR EFFECT

   • A Current in a Magnetic Field Experiences a Force
     • CURRENT- CARRYING WIRE IS PUT BETWEEN MAGNETIC POLES, THE TWO M.F. AFFECT ONE ANOTHER= FORCE ON THE WIRE. THIS CAN CAUSE WIRE TO MOVE= MOTTOR EFFECT
     • THIS IS BECAUSE CHARGED PARTICLES MOVING THROUGH A MAGNETIC FIELD WILL EXPERIENCE A FORCE, AS LONG AS THEY'RE NOT MOVING PARALLEL TO THE FIELD LINES
     • FULL FORCE= WIRE IS AT 90 DEGREES TO M.F. IF THE WIRE RUNS ALONG WITH THE M.F. IT WON'T EXPERIENCE ANY FORCE
       • FORCE ALWAYS ACTS IN THE SAME DIRECTION RELATIVE TO THE M.F. OF THE MAGNETS AND THE DIRECTION OF THE CURRENT.
         • THE MAGNITUDE OF THE FORCE INCREASES WITH THE STRENGTH OF THE MAGNETIC FIELD
           • THE FORCE ALSO INCREASES WITH THE AMOUNT OF CURRENT PASSING THROUGH THE CONDUCTOR
             • REVERSING CURRENT OF M.F. ALSO REVERSES THE DIRECTION OF THE FORCE
   • Fleming's Left-Hand Rule Tells You Which Way the Force Acts
     • LEFT-HAND POINT THE FIRST FINGER IN THE DIRECTION OF THE MAGNETIC FIELD THE SECOND FINGER IN THE DIRECTION OF THE CURRENT AND THE THUMB SHOWS YOU THE MOTION

4. ELECTRIC MOTORS AND LOUDSPEAKERS

   • 4 Factors which Speed Up an Electric Motor
     • 1. MORE CURRENT, 2. MORE TURNS OF THE COIL, 3. STRONGER M.F., 4. A SOFT IRON CORE IN THE COIL
     • SPILT-RING COMMUTATOR SWAPPS THE CONTRACTS EVERY HALF TURN FOR IT TO KEEP ON SPINNING
     • DIRECTION OF THE MOTOR CAN BE REVERSED EITHER BY SWAPPING THE POLARITY OF THE D.C. SUPPLY OR SWAPPING THE MAGNETIC POLES OVER.
     • SPEED CAN BE INCREASED BY ADDING MORE TURNS TO THE COIL, INCREASING THE CURRENT, INCREASING THE STRENGTH OF THE MAGNETIC FIELD, OR BY ADDING SOFT IRON CORE
   • Loudspeakers Work Because of the Motor Effect
     • A.C ELECTRICAL SIGNALS FROM AN AMPLIFIER ARE FED TO A COIL OF WIRE IN THE SPEAKER, WHICH IS WRAPPED AROUND THE BASE OF A CONE
       • COIL IS SURROUNDED BY A PERMANENT MAGNET, SO THE A.C. SIGNALS CAUSE A FORCE ON THE COIL AND MAKE IT MOVE BACK AND FOURTH
         • THESE MAKES THE CONE VIBRATE AND THIS CREATES A SOUND

5. ELECTROMAGNETIC INDUCTION

   • ELECTROMAGNETIC INDUCTION IS THE CREATION OF A VOLTAGE IN A WIRE WHICH IS EXPERIENCING A CHANGE IN MAGNETIC FIELD
   • The Dynamo Effect- Move the Wire of the Magnet
     • USING ELECTROMAGNETIC INDUCTION TO GENERATE ELECTRICITY USING ENERGY FROM KINETIC ENERGY STORES IS CALLED THE DYNAMO EFFECT.
     • TWO SITUATIONS: ELECTRICAL CONDUCTOR MOVES THROUGH A M.F.., M.F. THROUGH AN ELECTRICAL CONDUCTOR CHANGES
     • TEST BY CONNECTING AN AMMETER TO A CONDUCTOR AND MOVING THE CONDUCTOR THROUGH A M.F. THE AMMETER WILL SHOW THE MAGNITUDE AND DIRECTION OF THE INDUCED CURRENT
     • DIRECTION IS REVERSED THEN THE INDUCED VOLTAGE/ CURRENT WILL BE REVERSED TOO
     • BIGGER VOLTAGE= INCREASE THE STRENGTH OF THE MAGNET, THE NUMBER OF TURNS ON THE COIL, THE SPEED OF MOVEMENT
   • A.C Generators- Just Turn the Coil and There's a Current
     • GENERATORS ROTATE A COIL IN A M.F. THEIR CONSTRUCTION IS PRETTY MUCH LIKE A MOTOR
       • AS THE COIL SPINS, A CURRENT IS INDUCED IN THE COIL. THIS CURRENT CHANGES DIRECTION EVERY HALF TURN
         • INSTEAD OF THE SPLIT- RING COMMUTATOR, A.C. GENERATORS HAVE SLIP RINGS AND BRUSHES SO THE CONTACTS DON'T SWAP EVERY HALF TURN
           • THIS MEANS THEY PRODUCE A.C. VOLTAGE, AS SHOWN BY THESE CRO DISPLAYS. NOTE THAT FASTER REVOLUTIONS PRODUCE NOT ONLY MORE PEAKS BYT HIGHER VOLTAGE TOO
             • POWER STATIONS USE A.C. GENERATORS TO PRODUCE ELECTRICITY THEY JUST GET THE ENERGY NEEDED TO TURN THE COIL OF M.F. IN DIFFERENT WAYS

6. TRANSFORMERS

   • Transformers Change the Voltage- but Only Alternating Voltages
     • TRANSFORMERS CHANGE THE SIZE OF THE VOLTAGE OF AN ALTERNATING CURRENT
     • THEY ALL HAVE 2 COILS, PRIMARY AND THE SECOND JOINED WITH AN IRON
     • WHEN AN ALTERNATING VOLTAGE IS APPLIED ACROSS THE PRIMARY COIL, THE MAGNETICALLY SOFT IRON CORE.
     • RATIO BETWEEN THE PRIMARY AND SECONDARY VOLTAGES IS THE SAME AS THE RATIO BETWEEN THE NUMBER OF TURNS ON THE PRIMARY AND SECONDARY COILS
     • STEP-UP TRANSFORMS- INCREASE THE VOLTAGE. MORE TURNS ON SECONDARY THAN PRIMARY
     • STEP-DOWN TRANSFORMERS DECREASE THE VOLTAGE. MORE COILS ON THE PRIMARY THAN ON THE SECONDARY
   • The Transformer Equation
     • INPUT VOLTAGE (P) / OUT VOLTAGE (S) = NUMBER OF TURNS ON P / NUMBER OF TURNS ON THE S
   • Transformers are Nearly 100% efficient So "Power In =Power Out"
     • POWER= VOLTAGE * CURRENT
     • VOLTAGE [P] CURRENT [P]= VOLTAGE [S] CURRENT [S]
   • Transformers Make Transmitting Mains Electricity More Effiecient
     • THE VOLTAGE PRODUCED BY POWER STATIONS IS TOO LOW TO BE TRANSMITTED EFFICIENTLY. THE LOWER THE VOLTAGE THE HIGHER THE CURRENT FOR A GIVEN AMOUNT OF POWER, AND CURRENT CAUSES WIRES TO HEAT UP
       • STEP-UP USED TO BOOST VOLTAGE BEFORE TRANSMITION
         • STEP-DOWN USED END OF JOURNEY TO REDUCE THE VOLTAGE SO IT'S MORE USEFUL AND SAFER TO USE