# Circuits and Magnets

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```					               Group Work
1. 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.
Circuit Diagrams
Potential source

resistor
Kirchoff’s Laws
1. Current into any node equals current out
of the node
I2
I1
I3
I1 = I2 + I3
Kirchoff’s Laws
2. Potential drop around any closed loop is
zero
V1

e – V1 – V2 = 0
e
V1 + V2 = e

V2
CPS: Series Circuit

R1
R2
R3

Circuit resistance is
A. More than with one bulb.
B. Less than with one bulb.
C. The same as with one bulb.
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’ voltage drops add up to the
series total.
• The voltage drop across a resistor is
proportional to its resistance.
Group CPS Question
A 100-W bulb and a 75-W bulb are wired in
series. Which dissipates the greater power?

A.   The 100-W bulb.
B.   The 75-W bulb.
C.   Both dissipate the same power.
D.   It is impossible to determine from what we
have learned so far.
CPS: Parallel Circuit

Circuit resistance is
A. More than with one bulb.
B. Less than with one bulb.
C. The same as with one bulb.
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 CPS Question
A 100-W bulb and a 75-W bulb are wired in
parallel. Which dissipates the greater
power?

A.   The 100-W bulb.
B.   The 75-W bulb.
C.   Both dissipate the same power.
D.   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
2. Determine current direction, current,
voltage drop, and power for each resistor
in each circuit.
12 W

9W          18 W
6W

8W

9W
10 W
Magnetism

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.
CPS 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                      strong field
direction of force                where lines are
on a north pole                   close together

Field lines point from North to South pole.
Naming the poles
N = north-                  Earth’s north
seeking                     pole is a south
pole!
CPS 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.
• Magnets and charges
• Main ideas
– How magnetic fields act on electric charges

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