Download Week6Mon

Monday, 25 July 2016
The outline
Today:
• Sources of magnetic fields
• Capacitor review
• Charging Capacitor
• Discharging Capacitor
• Tomorrow: Midterm Review
Magnetism and magnetic fields
• Lab this week!
• Today: some magnetic facts…
• Source of magnetic field: moving charges
Magnetic field (B)
Magnetic Force on a Compass
 The figure shows a compass
needle in a magnetic field.
 A magnetic force is exerted
on each of the two poles of
the compass, parallel to for
the north pole and opposite
for the south pole.
 This pair of opposite forces
exerts a torque on the
needle, rotating the needle
until it is parallel to the
magnetic field at that point.
How did we know if there is a field?
– be careful that there are no other
fields present
•
• Electric field?
• Put a positive point charge
into the region.
• No gravity!
Magnetic field?
• Place a magnetic dipole
into the region.
• Earth’s magnetic field!
Magnetic Fields Around Us
Slide 24-21
Earth’s North pole is a magnetic
south pole
Earth’s North pole is a magnetic
south pole
Magnetic Fields from More Than One Source
 The total magnetic field at any point is the vector sum of
the individual fields at that point. This is the principle of
superposition.
© 2015 Pearson
Electric Current Causes a Magnetic Field
Sources of magnetic field: current (moving
charge)
The magnetic field is revealed by the pattern of iron
filings around a current-carrying wire.
Slide 32-35
Electric Current Causes a Magnetic Field
Sources of magnetic field: current (moving
charge)
The right-hand rule determines the orientation of
the compass needles to the direction of the current.
Slide 32-37
Slide 24-24
Drawing Field Vectors and Field Lines of a
Current-Carrying Wire
Slide 24-28
QuickCheck 32.4
A long, straight wire extends into and out of the screen. The current in
the wire is
A. Into the screen.
B. Out of the screen.
C. There is no current in the
wire.
D. Not enough info to tell the
direction.
Slide 32-38
QuickCheck 32.4
A long, straight wire extends into and out of the screen. The current in
the wire is
A. Into the screen.
B. Out of the screen.
C. There is no current in the
wire.
D. Not enough info to tell the
direction.
Right-hand rule
Slide 32-39
QuickCheck 24.10
 The following diagram shows a current loop perpendicular
to the page; the view is a “slice” through the loop. The
direction of the current in the wire at the top and at the
bottom is shown. What is the direction of the magnetic
field at a point in the center of the loop?
1. To the left
2. Up
3. To the right
4. Down
© 2015 Pearson
QuickCheck 24.10
 The following diagram shows a current loop perpendicular
to the page; the view is a “slice” through the loop. The
direction of the current in the wire at the top and at the
bottom is shown. What is the direction of the magnetic
field at a point in the center of the loop?
1. To the left
2. Up
3. To the right
4. Down
© 2015 Pearson
On to circuits...
A circuit
 Rank the brightness of each
resistor assuming each resistor
represents a light bulb. Pick your
own labeling system
 Find the current through each
resistor and the voltage drop
across the resistor using
Kirchoff's Rules and Ohm's Law
A circuit
 Rank the brightness of each
resistor assuming each resistor
represents a light bulb. Pick your
own labeling system
B12Ω>B3Ω>B4Ω
 Find the current through each
resistor and the voltage drop
across the resistor using
Kirchoff's Rules and Ohm's Law
I12Ω=0.22A, I3Ω=0.12A, I4Ω=0.10A
V12Ω=2.6V, V3Ω=V4Ω=0.4V
A circuit
 What is the potential
difference between
point B and C?
 What is the potential at
A, B and C?
B
C
A
A circuit
 What is the potential
difference between
point B and C? 0.4V
 What is the potential at
A, B and C? (0V, -0.4V,
2.6V)
B
C
A
A circuit – Can you do this one?
What if the 4Ω resistor is
replaced with a capacitor
(the capacitor is initially
uncharged?
 Draw the voltage with
respect to time across the
capacitor and each
resistor.
To be done on Tuesday!
C
Capacitor Review
• The charge on the capacitor plates is directly
proportional to the potential difference between
the plates. C=capacitance
The SI unit of capacitance is the farad:
• Energy stored in a capacitor:
• Dielectric in a capacitor:
Q
q
W   dq
C
0
RC circuits: Discharging
• See book page 909-911 (by integration)
where Q0 is the charge at t = 0, and  = RC is
the time constant.
The capacitor voltage is directly
proportional to the charge, so:
RC Circuits
 The current and the
capacitor voltage decay to
zero after the switch
closes, but not linearly.
© 2015 Pearson
Which capacitor discharges more
quickly after the switch is closed?
A. Capacitor A.
B. Capacitor B.
C. They discharge at
the same rate.
D. Can’t say without
knowing the initial
amount of charge.
QuickCheck 31.18
Which capacitor discharges
more quickly after the switch
is closed?
Smaller time constant= RC
A. Capacitor A.
B. Capacitor B.
C. They discharge at
the same rate.
D. Can’t say without
knowing the initial
amount of charge.
Slide 31-109