Magnetism and electricity
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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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Magnetism and electricity

Magnetism and Electricity

Chapter 13


Reminders

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


Compasses and lodestones

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.


Earth s magnetic field

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.


On the move and weakening

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.


Fields in physics 1

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)


Fields in physics 2

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


Fields in physics 3

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.


Magnetism from electricity

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.


Faraday s lines of force

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.


F on a charge as a result of b v

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.


Electricity from changing b field

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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