CHAPTER 18 NOTES FOR EIGHTH GRADE PHYSICAL
SCIENCE
ANY MATERIAL THAT ATTRACTS IRON OR MATERIALS CONTAINING IRON
IS CALLED A MAGNET.
ALL MAGNETS HAVE TWO POLES, EXERT FORCES, AND ARE
SURROUNDED BY A MAGNETIC FIELD.
THE MAGNETIC EFFECTS ARE STRONGEST NEAR THE ENDS OF THE BAR
MAGNET. THE PARTS OF A MAGNET WHERE THE MAGNETIC EFFECTS
ARE STRONGEST ARE CALLED POLES.
A NATURAL MAGNET IS CALLED A LODESTONE.
MAGNETIC P0LES ALWAYS OCCUR IN PAIRS; YOU WILL NEVER FIND A MAGNET WITH ONLY A NORTH
POLE OR ONLY A SOUTH POLE.
THE FORCE OF REPULSION OR ATTRACTION BETWEEN THE POLES OF MAGNETS IS CALLED THE
MAGNETIC FORCE.
MAGNETIC FIELD LINES AROUND A MAGNET ARE CLOSEST TOGETHER AT THE POLES, SHOWING
THE MAGNETIC FORCE IS STRONGEST AT THESE TWO PLACES.
MOVING ELECTRONS PRODUCE MAGNETIC FIELDS THAT CAN GIVE AN ATOM A NORTH AND A SOUTH
POLE. IN MOST MATERIALS, SUCH AS COPPER AND ALUMINUM, THE MAGNETIC FIELDS OF THE
INDIVIDUAL ATOMS CANCEL EACH OTHER OUT, SO THE MATERIALS AREN'T MAGNETIC. HOWEVER IN
MATERIALS LIKE IRON, NICKEL, AND COBALT, THE ATOMS GROUP TOGETHER IN TINY REGIONS
CALLED DOMAINS.
DOMAINS ARE LIKE TINY MAGNETS WITHIN AN OBJECT.
DROPPING A MAGNET OR STRIKING IT TOO HARD CAN JOSTLE THE DOMAINS OUT OF ALIGNMENT,
CAUSING THE MAGNET TO LOSE IT MAGNETIC PROPERTIES. INCREASING THE TEMPERATURE OF A
MAGNET CAN ALSO DEMAGNETIZE IT.
WHEN YOU CUT A MAGNET IN HALF, YOU END UP WITH TWO MAGNETS, EACH WITH ITS OWN NORTH
POLE AND SOUTH POLE.
MAGNETS MADE OF IRON, NICKEL, COBALT, OR ALLOYS OF THESE METALS ARE CALLED
FERROMAGNETS.
AN ELECTROMAGNET IS A MAGNET, USUALLY WITH AN IRON CORE, PRODUCED BY AN ELECTRIC
CURRENT.
TEMPORARY MAGNETS ARE MADE FROM MATERIALS THAT ARE EASY TO MAGNETIZE BUT TEND TO
LOSE THEIR MAGNETIZATION EASILY.
PERMANENT MAGNETS ARE DIFFICULT TO MAGNETIZE BUT TEND TO RETAIN THEIR MAGNETIC
PROPERTIES BETTER.. STRONG PERMANENT MAGNETS ARE MADE WITH ALNICO - AN ALLOY OF
ALUMINUM, NICKEL, AND COBALT.
THE PROCESS BY WHICH A MATERIAL IS MADE INTO A MAGNET BY STROKING IT IS CALLED
MAGNETIC INDUCTION.
THE EARTH'S GEOGRAPHIC POLES ARE ON THE AXIS ON WHICH THE EARTH ROTATES.
EARTH'S MAGNETIC POLES ARE THE POINTS ON EARTH'S SURFACE WHERE ITS MAGNETIC FORCES
ARE THE STRONGEST.
THE MAGNETIC NORTH POLE IS LOCATED IN NORTHEASTERN CANADA, ABOUT 1600 KILOMETERS
FROM THE GEOGRAPHIC NORTH POLE. THE MAGNETIC SOUTH POLE IS LOCATED NEAR THE
ANTARCTIC CIRCLE.
THE ERROR IN A COMPASS CAUSED BY THE DIFFERENCE IN LOCATION OF THE EARTH'S MAGNETIC
AND GEOGRAPHIC POLES IS CALLED MAGNETIC VARIATION.
FROM TIME TO TIME THE NORTH AND SOUTH MAGNETIC POLES CHANGE PLACES. AT LEAST NINE
OF THESE REVERSALS HAVE OCCURED IN THE LAST 3.5 MILLION YEARS.
THE MAGNETIC POLE OF EARTH THAT IS CLOSEST TO THE GEOGRAPHIC NORTH POLE IS ACTUALLY
A MAGNETIC SOUTH POLE! SO A COMPASS NEEDLE POINTS TO THE NORTH BECAUSE ITS NORTH
POLE IS ATTRACTED TO A VERY LARGE MAGNETIC SOUTH POLE.
ONE OF THE MOST SPECTACULAR EFFECTS CAUSED BY THE EARTH'S MAGNETIC FIELD IS A
CURTAIN OF LIGHT CALLED AN AURORA.
AN AURORA IS FORMED WHEN CHARGED PARTICLES FROM THE SUN INTERACT WITH OXYGEN AND
NITROGEN ATOMS IN EARTH'S ATMOSPHERE. WHEN CHARGED PARTICLES FROM THE SUN STRIKE
THESE ATOMS, THE ATOMS EMIT LIGHT OF DIFFERENT COLORS.
AUROAS ARE MOST OFTEN SEEN IN AREAS NEAR THE NORTH AND SOUTH MAGNETIC POLES.
AURORAS SEEN NEAR THE NORTH MAGNETIC POLE ARE CALLED AURORA BOREALIS OR NORTHERN
LIGHTS.
AURORAS SEEN NEAR THE SOUTH MAGNETIC POLE ARE CALLED AURORA AUSTRALIS OR
SOUTHERN LIGHTS.
ENGINEERS HAVE DEVELOPED TRAINS THAT HAVE NO WHEELS AND ACTUALLY FLOAT ABOVE THE
TRACK. THESE TRAINS ARE ABLE TO LEVITATE BECAUSE OF MAGNETIC FORCES BETWEEN THE
TRACK AND THE TRAIN CARS. SUCH TRAINS ARE CALLED MAGLEV TRAINS. THE NAME MAGLEV IS
SHORT FOR MAGNETIC LEVITATION.
FROM HIS EXPERIMENTS, OERSTAD CONCLUDED THAT AN ELECTRIC CURRENT PRODUCES A
MAGNETIC FIELD AND THAT THE DIRECTION OF THE MAGNETIC FIELD DEPENDS ON THE DIRECTION
OF THE CURRENT.
ELECTROMAGNETISM IS THE INTERACTION BETWEEN ELECTRICITY AND MAGNETISM.
A SOLENOID IS A COIL OF WIRE THAT PRODUCES A MAGNETIC FIELD WHEN CARRYING AN ELECTRIC
CURRENT.
THE STRENGTH OF THE MAGNETIC FIELD PRODUCED BY A SOLENOID INCREASES AS MORE LOOPS
ARE ADDED AND AS THE CURRENT IN THE WIRE IS INCREASED.
AN ELECTROMAGNET IS A MAGNET THAT CONSISTES OF A SOLENOID WRAPPED AROUND AN IRON
CORE.
THE STRENGTH OF AN ELECTROMAGNET CAN BE MADE EVEN STRONGER BY INCREASING THE
NUMBER OF LOOPS IN THE SOLENOID, BY INCREASING THE SIZE OF THE IRON CORE, AND BY
INCREASING THE ELECTRIC CURRENT IN THE WIRE.
AN ELECTRIC CURRENT IN THE SOLENOID OF A DOORBELL RPODUCES A MAGNETIC FIELD. THIS
FIELD PULLS THE IRON ROD THROUGH THE SOLENOID, AND THE ROD STRIKES. THE BELL.
AN ELECTRIC MOTOR IS A DEVICE THAT CHANGES ELECTRICAL ENERGY INTO KINETIC ENERGY. ALL
ELECTRIC MOTORS HAVE AN ARMATURE-A LOOP OR COIL OF WIRE THAT CAN ROTATE. THE
ARMATURE IS MOUNTED BETWEEN THE POLES OF A PERMANENT MAGNET OR ELECTROMAGNET.
IN ELECTRIC MOTORS THAT USE DIRECT CURRENT, A DEVICE CALLED A COMMUTATOR IS
ATTACHED TO THE ARMATURE TO REVERSE THE DIRECTION OF THE ELECTRIC CUKRRENT IN THE
WIRE.
A GALVANOMETER IS A DEVICE USED TO MEASURE CURRENT THROUGH THE INTERACTION OF AN
ELECTROMAGNET AND A PERMANENT MAGNET.
THE PROCESS BY WHICH AN ELECTRIC CURRENT IS PRODUCED BY CHANGING MAGNETIC FIELD IS
CALLED ELECTROMAGNETIC INDUCTION.
AN ELECTRIC CURRENT COULD BE INDUCED BY MOVING A MAGNET IN A COIL OF WIRE OR BY
MOVING A WIRE BETWEEN THE POLES OF A MAGNET.
A GENERATOR IS A DEVICE THAT USES ELECTROMAGNETIC INDUCTION TO CONVERT KINETIC
ENERGY INTO ELECTRICAL ENERGY.
HOW A GENERATOR WORKS:
1. AS THE CRANK IS TURNED, THE ROTATING COIL CUTS THROUGH THE MAGNETIC FIELD LINES OF
THE MAGNET, AND ELECTRIC CURRENT IS INDUCED IN THE WIRE.
2. WHEN THE COIL IS NOT CUTTING THROUGH THE MAGNETIC FIELD LINES, NO ELECTRIC CURRENT
IS INDUCED.
3. AS THE COIL CONTINUES TO ROTATE, THE MAGNETIC FIELD LINES ARE CUT FROM A DIFFERENT
DIRECTION, AND AN ELECTRIC CURRENT IS INDUCED IN THE OPPOSITE DIRECTION.
A TRANSFORMER INCREASES OR DECREASES THE VOLTAGE OF AN ALERNATING CURRENT.
IF A TRANSFORMER INCREASES VOLTAGE, IT IS A STEP-UP TRANSFORMER. IF A TRANSFORMER
DECREASES VOLTAGE, IT IS A STEP-DOWN TRANFORMER.