Direct Current CircuitsSections 1, 2, 5

Chapter 29

physics ch 29 2

Complex circuits

Real circuits often contain several sources, resistors, capacitors, transformers, and motors that are interconnected.

When several resistors are connected in series or parallel they can always be represented as a single equivalent resistor.

physics ch 29 3

Resistors in series

There is only a single current path. The current is the same in all

resistors.ybxyaxab VVVV

321 IRIRIRIR

321 RRRIIR

321 RRRR

physics ch 29 4

Resistors in parallel

There is a split current path. The voltage is the same across all

resistors.321 IIII

321 R

V

R

V

R

VI

321

111

RRRV

I

321

1111

RRRR

physics ch 29 5

example

Exercise 4

physics ch 29 6

Kirchoff’s rules

Point rule: The sum of the currents into any branch point is zero.

Loop rule: The sum of the potential differences in any loop must equal zero.

physics ch 29 7

Problem-Solving strategy

On page 652 – read together. Examples 2 – 4 – go over together

on board

physics ch 29 8

Household power distribution

Cars use DC power. Most homes use AC power. Most of the same basic wiring

concepts apply to both. In the US, most homes are wired at

120 V.

physics ch 29 9

Heating and resistance

The resistance of most electrical devices goes up with use because the circuit elements get hot.

Some power is lost to the wires in a circuit.

physics ch 29 10

Overheating In extreme cases, if wires get too

hot they can cause a fire or melt. Most homes use 12-gauge wire

which can withstand 20A without overheating.

High-current appliances such as stoves and dryers use thicker 8- or 6-gauge wire and run at 240 V to reduce the risk of fire.

physics ch 29 11

Fuses and circuit breakers Are designed to avoid overheating

due to large currents. Fuses contain links with low

melting points. At high currents the link melts, breaking the circuit. Then the fuse must be replaced.

Circuit breakers do the same thing but contain an electromagnet and can be reset instead of replaced.

physics ch 29 12

Grounding wires The third prong Normally carries no current. Connects to the metal housing of a

device. If the “hot” line of the circuit

breaks inside the device and hits the housing, it is grounded through the prong instead of through your body.

physics ch 29 13

GFI or GFCI

Ground-fault interrupter Used in outlets near water or

outside Like an extra circuit breaker

located right in the outlet Trips when there is current in the

grounding wire.

physics ch 29 14

Kilowatt-hour Unit of energy – what you buy from the

power company 1 kWh = (1000 W)(3600 s)

= 3.6 x 106 W ∙ s = 3.6 x 106 J

If you know the power rating of an appliance in kW and multiply that by the number of hours it is used, you get the kWh used. 1 kWh usually costs between 5 and 15 cents.

physics ch 29 15

Power

R

VRIIVP

22

physics ch 29 16

Resistors For the first two bands, black means 0,

brown means 1, and red means 2. For the third band, black means x 1,

brown means x 10, and red means x 100.

For the fourth band, gold means ± 5% and silver means ± 10%. Brown – red – red – silver

1200 ohms ± 10% Red – black – black – gold

20 ohms ± 5%