Download 16 Magnetism

GSCI 101 Chapter 24 Magne3sm Magne3sm 1.  Magnets a7ract some materials 2.  Magnets do not a7ract others materials 3.  Magnets a7ract or repel other magnets (depending on orienta3on) Repulsion S N F F A7rac3on S N N F GSCI 101. Prof. M. Nikolic N S S F Like poles repel Unlike poles a7ract Sound familiar? 2 Conceptual ques3on: Magne3sm Magnet A has twice the field strength of Magnet B. The two magnets are brought close together. Which magnet pulls harder on the other? Magnet A. Magnet B. Both magnets pull equally hard. ✓ It depends on their separa3on. Prof. M. Nikolic, GSCI 101 3 Magne3c Poles Magnets are never only N or only S always have at least two poles. N S N S Bar magnet S N S N S N S N S N Refrigerator magnet Horseshoe magnet North poles point north (on Earth) South poles point south If you find a magnet with only one pole, you will win a Nobel Prize! Break a dipole magnet, you get two smaller dipole magnets: N S N S N S S3ll two poles each! GSCI 101. Prof. M. Nikolic A compass needle is a bar magnet 4 Magne3c Field You can visualize the magne3c field using •  a compass needle which points in the direc3on of the field •  Iron filings which line up along the field lines Magne3c field lines point from north pole to south pole Lines show the direc3on of the magne3c field at each point Lines closer together è stronger field GSCI 101. Prof. M. Nikolic 5 Magne3c Force Make a magne3c field Put a compass needle in the magne3c field •  Force on the north pole points parallel to the magne3c field lines •  Force on the south pole points an3-­‐parallel to the field lines F N Two offset equal forces è •  Zero net force •  Non-­‐zero net torque Needle twists F Magne3c field lines F S GSCI 101. Prof. M. Nikolic N F Two aligned equal forces è •  Zero net force •  Zero net torque Needle points to Earth’s North Pole 6 Earth’s Magne3c Field The Earth has a magne3c field. We use it to make compasses point north. 1.  Compasses do not work well near the poles 1.  At the north and south magne3c poles, compasses point down 2.  Magne3c north pole is at about la3tude 80o north 1.  It’s moving about 35 miles per year 3.  The Earth’s North pole a7racts the north poles of compasses so it is really a south magne3c pole GSCI 101. Prof. M. Nikolic 7 Source of Magne3sm Caused by moving electric charges -­‐ explained by Einstein’s Theory of Rela3vity But a bar magnet does not move! The electrons in the magnet move: 1.  The electrons spin (like a top) 2.  The electrons orbit the nucleus If all the electrons spin in the same direc3on, the magnet is stronger. If a pair of electrons spin in opposite direc3ons, their magne3c fields cancel. Most atoms have paired electrons. Iron has four unpaired electrons èeach iron atom is a 3ny magnet GSCI 101. Prof. M. Nikolic 8 Magne3c Domains The magne3c field of an iron atom makes nearby atoms line up Ø  Clusters of aligned atoms Ø  Magne3c domains (microscopic) Bring a magnet close to a piece of iron or put the iron in an external magne3c field Unmagne3zed iron Domains not aligned Domains start to align with the external field The iron becomes magne3zed. The iron is now a7racted to the magnet. GSCI 101. Prof. M. Nikolic 9 Source of Magne3sm Observa3ons: 1.  A current in a wire can a7ract a magnet 2.  A magnet can exert a force on a current carrying wire 3.  Two current carrying wires can a7ract each other Conclusion: A current in a wire produces a magne3c field. GSCI 101. Prof. M. Nikolic 10 Direc3on of the Field Right hand rule: point your thumb along the direc3on of the current and your fingers will curl in the direc3on of the magne3c field GSCI 101. Prof. M. Nikolic 11 Magne3c Forces An electric field exerts a force on all charged par3cles. the electric force is parallel to the electric field A magne3c field exerts a force only on moving charged par3cles. the magne3c force is perpendicular to × × × × × × –  the magne3c field × × × × × × Velocity –  The velocity of the charged par3cle Force × × × × × × (right) (down) × × × × × × The Earth’s magne3c field deflects many × × × × × × of the high speed charged par3cles that Magne3c field (Poin3ng into the page) make up cosmic radia3on. Old fashioned televisions and computer monitors used electron beams to make spots on the screen glow. The electron beams were steered with magnets. GSCI 101. Prof. M. Nikolic 12 Conceptual ques3on: Magne3c Force on Electron A magne0c field cannot exert a force on an electron when the electron is at rest. moves parallel to magne3c field lines. Both of these. ✓ A magne3c field will always exert a force on an electron Prof. M. Nikolic, GSCI 101 13 Conceptual ques3on: Electric Force on Electron An electric field cannot exert a force on an electron when the electron is at rest. moves parallel to electric field lines. Both of these. An electric field will always exert a force on an electron ✓ Prof. M. Nikolic, GSCI 101 14 Conceptual ques3on: Magne3c Force A charged par3cle moving with velocity v enters a region with uniform magne3c field B. The force on the par3cle is Parallel to the velocity (its speed increases) An3 parallel to the velocity (its speed decreases). Perpendicular to the velocity. ✓ × × × × × × × × × × × × Velocity × × × × × × × × × × × × × × × × × × Magne3c field Poin3ng into the page Prof. M. Nikolic, GSCI 101 15 Conceptual ques3on: Magne3c Force A charged par3cle moving with velocity v enters a region with uniform magne3c field B. The par3cle will follow a path that is a Parabola. Circle. ✓ Spiral. × × × × × × × × × × × × Velocity × × × × × × × × × × × × × × × × × × Magne3c field Poin3ng into the page Prof. M. Nikolic, GSCI 101 16 Magne3c Levita3on (Maglev) Trains Use electromagnets on the train and underneath the train to repel each other and lij the train. This eliminates fric3on between the train and the track. Use electromagnets on the side of the train and on the sides of the track to provide the force to make the train go. Shanghai Maglev: 19 miles, 220 mph (max 311 mph) Next phase: 99 miles (15 min ride) GSCI 101. Prof. M. Nikolic 17 Electric Meter A current in the wire makes a magne3c field makes the compass needle twist (deflect). This also applies a torque to the coil of wire (by Newton’s 3rd law). The amount of deflec3on is propor3onal to the current through the wire. Measuring the deflec3on measures the amount of current è electric meter! GSCI 101. Prof. M. Nikolic 18 Generator A coil of wire turns in a
magnetic field. The flux in
the coil is constantly
changing, generating an emf
in the coil.
ε t 19 Motor: a generator run backwards Mechanical output AC A coil of wire turns in a
magnetic field. The flux in
the coil is constantly
changing, generating an emf
in the coil.
Run an alternating current
through a coil of wire in a
magnetic field. This
generates a torque on the
coil.
20 Simple Motor Current in wire loop makes the loop rotate to line up with the magnet. The current reverses direc3on. The loop rotates another 180o to line up. Repeat. GSCI 101. Prof. M. Nikolic 21 Step-­‐down transformers 240,000 V in the power lines
2,400 V local substations
120 V in houses
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