1 Lecture series for Conceptual Physics, 8 th Ed.

2 Flow of Charge p398 Recall that heat flows from the hot end to the cold end. Current flows from the high potential (V) end to the low V end of a conductor. Current flows as long as there is a potential difference, ΔV. Potential (V) is like pressure (kPa) A difference in pressure causes water to flow. A pump does the same job that a battery does.

3 Electric Current p399 Water current is made up of water molecules. Electric current is the flow of electric charges. In a wire, conduction electrons flow while the protons stay put. A slight problem: current (I) flows from positive to negative. The electrons are going the other way. The electric field also goes from the positive end of the wire to the negative end of the wire current e-e- Current (I) is measured in Amperes (A).

4 Voltage Sources p400 Batteries (DC) and Generators (AC) Oh! And… The work done by batt.s and gen.s to separate charges creates the potential difference, ΔV, between their terminals. The + and – terminals have a difference in potential energy per charge or voltage. Become familiar with these symbols.

5 Electrical Resistance p401 A hose resists the flow of water. Longer, skinnier hoses have more resistance than short, fat hoses. This wire has more resistance than this wire. Most of the time, hot wires have higher resistance than cold wires. Something to do with the greater jostling of atoms at higher temp.s. Electrical resistance is measured in ohms,

6 Ohm’s Law p401 V = I R Try this: I = V R What is the current in a wire with a potential difference of 10 V across its ends and a resistance of 2 ohms? I = 10 V 2 ohms = 5 A Now, what if you… doubled the voltage? doubled the resistance? Inside electronic devices, current is regulated by resistors.

7 I= V R = 12V 60 ohms = 0.2 A R= V I = 120 V 12 A = 10 ohms Why isn’t the bird shocked? His feet have the same potential. There is no potential difference, V. But, if it were to touch something else…

8 Ohm’s Law and Electric Shock p402 See Table 22.1 p403: Small currents can be dangerous. I = V R = 12 V 100,000 ohms = A To ground. Protects you from any wire touching the inside of the appliance.

9 Direct Current and Alternating Current p404 Batteries produce dc. Electrons go from – to + terminals. Generators produce ac. AC travels better. Electrons just move back and forth in the 1/60 Hz change in electric field polarization. Converting ac to dc p405 A gadget called a diode takes the ac going one way only and passes it along in pulses. A capacitor is used to smooth out the pulses. A capacitor works in a manner similar to the water system at left.

10 Speed and Source of Electrons in a Circuit p405 When you flip the switch on, an electric field is established immediately in the circuit. But, the electrons don’t travel very far or very fast. In fact, they vibrate back and forth in the 1/60 Hz ac current. Fig shows how a wire channels the elec. field. Fig shows how the elec.s travel through a wire. Elec.s are jostled by atoms as they try to accelerate and end up starting over. Basically, e’s just vibrate.

11 Electrons jostle back and forth in the filament to produce heat and light with their “friction”. Do you get the elec.s from the power company? No! They come with the appliance. In a dc current, the elec.s would drift along at 33cm/hour.

12 Electric Power p407 At 120V this bulb would produce 100 W. That’s 100 joules per second. Some of the power is in the form of light and a bit more than half goes as heat. P = I V P = I V = (15 A)(120 V) = 1800 WYes. With 600 W to spare. 1200W x 1 kW 1000 W x 1 hr x 10 cents kWh = 12 cents

13 Electric Circuits p408 e-’s flow along a circuit. The current stops when the switch is open, open or a light burns out. The current is the same everywhere in a series circuit. Resistors in a series act like one resistor according to this formula: R total = R 1 + R 2 + R 3 + …….. If each resistor is 5 ohms, all of them are 15 ohms. AND… The current in the circuit is I = V R = 1.5 V 15 ohms = 0.1 A 1.5 V

14 What current would be indicated on the ammeter? R tot = R 1 + R 2 + R 3 = = 6 ohms I = V / R = 6 V / 6 ohms = 1 A Watt is the power output of the total circuit P = I V = (1 A)(6 V) = 6 W What is the voltage drop (IR) of each resistor? I R 2 = (1 A)(2 ohms) = 2 V I R 3 = (1 A)(3 ohms) = 3 V I R 1 = (1 A)(1 ohm) = 1 V

15 Again What is the power output of each resistor? P 2 = I V 2 = (1A)(2V) = 2 W P 3 = I V 3 = (1A)(3V) = 3 W P 1 = I V 1 = (1A)(1V) = 1 W 6 W total power

16 Parallel Circuits p410 There are 3 pathways for the electrons to follow. If a light fails in one, the others will continue. The voltage through each light equals the voltage of the battery. 1.5V The resistors in parallel act like one resistor according to this formula: 1/R tot = 1/R 1 + 1/R 2 + 1/R 3 + ….. If each resistor is 5 ohms, what is the total resistance? 1/5 + 1/5 + 1/5 = 3/55/3 = 1.7 ohms What is the total current through the circuit? I = V / R = 1.5V / 1.7 ohms = 0.88 A

17 1.5V What is the current through the ammeter? First, find the total resistance. 1/2 + 1/3 + 1/1 = 3/6 + 2/6 + 6/6 = 11/ ohms I = V/R = 1.50V / 0.545A = 2.75 A What current is going through each resistor? I 2 = V/R 2 = 1.50V/2 ohms = A I 3 = V/R 3 = 1.50/3 = A I 1 = V/R = 1.50/1 = 1.50 A What power is going through each resistor? P 2 = I 2 V = (0.750)(1.50) = 1.13 W P 3 = I 3 V = (0.500)(1.50) = W P 1 = I 1 V = (1.50)(1.50) = 2.25 W What is the total power? P=IV=(2.75)(1.50) P = 4.13 W

18 Parallel circuits and Overloading P411 Adding more appliances decreases the resistance AND increases the current through the parallel circuit. Safety Fuses p412 Now, circuit breakers have replaced this type of fuse. The end