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

Same I through each:

V = IR1 + IR2 + IR3 = I (R1 + R2 + R3)

Circuit resistance = R1 + R2 + R3

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. 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.

Parallel Circuit Diagram

Potential Differences

Currents

Total current = I = I1 + I2 + I3 = V/R1 + V/R2 + V/R3

Circuit resistance =V/I = 1/R1 + 1/R2 + 1/R3

1

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 is1/R = i 1/Ri

Group Poll 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

Circuit resistance = R1 + 1/R2 + 1/R3

1

Group Work

Determine current direction, current, voltage, and power for each resistor in each circuit.

12

6 9 18

9

8

10

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

Field lines point from North to South pole.

Conventions as with electric field lines.

strong field where lines are close together

direction is direction of force on a north pole

Board Work

Sketch the pattern of magnetic field lines for two dipole magnets positioned as

N S

NS

Naming the polesN = north-seeking

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