Magnetism and Electricity
Chapter 13
Reminders 1
• Lab this week due by Friday at 4:00 pm: B3-CLE:
Coulomb’s Law of Electrostatics
• Weekly Reflections #11 (due 11/04) and #12 (due
11/11) to be sent out later today.
• Next week’s lab B1-WA: Wave Addition
• Reading quiz due prior to the start of class on
Tuesday (Chap 29, Sec 1-3) and Thursday (Chap
29, Sec 4) next week – now available.
Reminders 2
• In-class Quiz #5 (Chapters 11-13) on Tuesday,
November 4th (12 MC 1-pt questions)
• Test #3 (Chapters 8, 9, 11-13) on Tues, November
11th (29?@1pt, 1?@2pst) Max score = 120%; Test
starts at 8:15 AM. Mostly qualitative.
• No class on Thursday, November 13th
• In-class Quiz #6 moved to Thurs, November 20th
• When in doubt, consult the syllabus.
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 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!
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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.