Direct Current Circuits Sections 1, 2, 5 Chapter 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. physics ch 29

4 Resistors in parallel There is a split current path. The voltage is the same across all resistors. 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 10 6 W ∙ s = 3.6 x 10 6 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 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% physics ch 29