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
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physics ch 29 4
Resistors in parallel
There is a split current path. The voltage is the same across all
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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
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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%