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Magnetism and Electricity

Magnetism and Electricity. Chapter 13. Reminders. Lab this week due by Friday at 4:00 pm: B3-CLE: Coulomb’s Law of Electrostatics Reading quiz due prior to the start of class on Thursday. Test #3 (Chapters 8, 9, 11-13) on Thursday, April 10 th

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Magnetism and Electricity

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  1. Magnetism and Electricity Chapter 13

  2. Reminders • Lab this week due by Friday at 4:00 pm: B3-CLE: Coulomb’s Law of Electrostatics • Reading quiz due prior to the start of class on Thursday. • Test #3 (Chapters 8, 9, 11-13) on Thursday, April 10th • Deadline to Car Crash Reconstruction optional extra credit problem due Thursday, May 1st

  3. Compasses and Lodestones • Magnetic compass known from ancient times • Chinese probably the first (Marco Polo reported) • Compass needle prepared from lodestone • Needles float on water • Compasses have “north” and “south” ends • Like poles repel – opposites attract. • Magnetic monopoles do not exist.

  4. Earth’s Magnetic Field • Earth acts as though there is a large bar magnet at its center – with its south magnetic pole ~4o from Earth’s north geographic pole.

  5. On the Move and Weakening! • The “north” end of the compass points to the south magnetic pole of the Earth • The magnetic south pole is located north of Canada and is on the move! • Strength is currently decreasing at a rate of about 6.3% per century.

  6. Fields in Physics 1 • A field is a hypothetical construct designed to explain action at a distance. • Two types of forces: • Contact forces • Pushing and pulling • Action-at-a-distance forces • Gravitational force • Electrical force • Magnetic force • Certain nuclear forces (weak and strong)

  7. Fields in Physics 2 • Gravitational: • Arrows indicate direction of motion of any matter. • Always attractive on a “small” scale • Electrical: • Arrows indicate direction of motion of a positive test particle, p+ (a proton) • Opposites attract; likes repel • Magnetic: • Arrows indicate direction of motion of a north “monopole” which DNE. • Opposites attract; likes repel

  8. Fields in Physics 3 • The concentration of field lines indicate field strength (measured in N/kg, N/C, or Tesla) • The closer lines are, the more intense the field. • The more intense the field, the stronger the attractive or repulsive force. • When field lines point in the same direction, this indicates attraction and visa versa.

  9. Magnetism from Electricity • Öersted shows that direct (one way) flowing electricity produces a constant magnetic field. • Ampère shows that parallel current carrying wires have an attractive force between them. • Ampère argues that currents flowing within the Earth are responsible for the planet’s magnetic field.

  10. Faraday’s Lines of Force • Field lines trace out magnetic fields. • Magnetic fields are directed from N to S. • A long, current-carrying wire produces a circular field consistent with the right hand rule #1. • Magnetic fields of currents – the solenoid. Current is defined to flow in the same direction as a positive (+) test particle.

  11. F on a Charge as a Result of B, v • Charged particles (+ and –) moving in a magnetic field experiences a force, F, but only so long as v is not parallel to B. • The direction of the force is given by right hand rule #2. Current is defined to flow in the same direction as a positive (+) test particle.

  12. Electricity from ChangingB Field • Faraday discovers electric induction. • Today we have the electric generator. • Charges in the presence of a changing magnetic field are forced to flow in conducting wires. • Faraday’s law – The strength of an electromotive force induced in a loop of wire is proportional to the rate at which the magnetic flux through the area of the loop changes. • AC is used in preference to DC current.

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