Download Circuits and Magnets

Group Work
Which has the greater resistance: a 75-W
light bulb or a 100-W light bulb?
Bear in mind the formula P = VI.
• At a given voltage, which dissipates more
power?
• Which must draw more current?
Bear in mind Ohm’s law I = V/R.
• At a given voltage, which presents the
greater resistance?
DC Circuits
Current and energy
Objective
• Analyze the flow of current in simple,
series, and parallel circuits.
Series Circuit
R1
R2
R3
Same I through each:
V = IR1 + IR2 + IR3 = I (R1 + R2 + R3)
Circuit resistance = R1 + R2 + R3
Series Summary
• The same current flows through all
resistors in series.
• The series resistance is the sum of the
resistors’ resistances.
• The resistors’ voltages add up to the
series total.
• The voltage across a resistor is
proportional to its resistance.
Group Poll Question
A 100-W bulb and a 75-W bulb are wired in
series. Which dissipates the greater power?
A.
B.
C.
D.
The 100-W bulb.
The 75-W bulb.
Both dissipate the same power.
It is impossible to determine from what we
have learned so far.
Parallel Circuit Diagram
Potential Differences
Currents
Total current = I = I1 + I2 + I3 = V/R1 + V/R2 + V/R3
1
Circuit resistance =V/I = 1/R + 1/R + 1/R
1
2
3
Parallel Summary
• The same voltage drops across all
resistors in parallel.
• The total current is the sum of the
resistors’ currents.
• The effective circuit resistance is less than
for any individual resistor.
• Formula is
1/R = Si 1/Ri
Group Poll Question
A 100-W bulb and a 75-W bulb are wired in
parallel. Which dissipates the greater
power?
A.
B.
C.
D.
The 100-W bulb.
The 75-W bulb.
Both dissipate the same power.
It is impossible to determine from what we
have learned so far.
Series + Parallel Circuit
1
Circuit resistance = R1 +
1/R2 + 1/R3
Group Work
Determine current direction, current, voltage,
and power for each resistor in each circuit.
12 W
9W
18 W
6W
8W
9W
10 W
Magnetism
The shadow of electricity
Objectives
• Identify the direction and distance
dependence of the force between
magnetic poles.
Magnetic fields
Magnets Attract and Repel
Magnets apply forces to each other.
Opposite poles attract, like poles repel.
Poll Question
The magnitude of the force between
magnetic poles
A. is the same at all distances.
B. becomes stronger as the poles come closer.
C. becomes weaker as the poles come closer.
Magnetic Field Lines
Conventions as with electric field lines.
direction is
direction of force
on a north pole
strong field
where lines are
close together
Field lines point from North to South pole.
Board Work
Sketch the pattern of magnetic field lines for
two dipole magnets positioned as
N
S
S
N
Naming the poles
N = northseeking
Earth’s north
pole is a south
pole!
Poll Question
What happens if you break a magnet
between its north and south poles?
A. You make two smaller magnets, each with a
north and south pole.
B. One piece will be a north pole and one
piece will be a south pole.
C. One piece is a magnet and the other is not.
D. None of these.
Do Magnetic Poles Exist?
North or south poles have never been
isolated!
Source: Hewitt, Fig. 24.7
Magnetic “monopoles” probably do not exist.
Reading for Next Time
• Magnets and charges
• Main ideas
– How magnetic fields act on electric charges