Phy2005 Applied Physics II Spring 2016 Announcements: Hirschfeld ofc. hour T 4th cancelled, extra hour Wed. 11am-1230pm Dr. Andy Linscheid will take T 4th office hour next week Linscheid for special appointments: PH gone Monday, Feb. 8; in-class review by Dr. Andy Linscheid Remote numbers , , register or reregister! Test 10 Feb. 12 problems chs handwritten formula sheet 1 side Bring: UFID, calculator, scratch paper, pencil

Science news page UNIVERSITY OF FLORIDA LIGHTNING RESEARCH GROUP Department of Electrical and Computer Engineering

Last time: Kirchoff’s Laws (1) Node rule, current law,.. I1I1 I2I2 I3I3 I 1 = I 2 + I 3 (2) Loop rule, V-law,.. R1R1 R2R2 A BC D E F V Sum of the voltage drops around any loop is zero! V- IR 1 – IR 2 =0 I

A V A V Should have ideally zero internal resistance. Should have ideally infinite internal resistance. Ammeter Voltmeter Devices to measure current and voltage have some resistance too, but we don’t want them to affect the circuit we are trying to measure A measures current through wire V measures potential difference across resistor Resistivity of metal increases with temperature! Today: more Kirchoff, electrical safety

ACADEMIC HONESTY Each student is expected to hold himself/herself to a high standard of academic honesty. Under the UF academic honesty policy. Violations of this policy will be dealt with severely. There will be no warnings or exceptions.UF academic honesty policy

3  4 V 6 V 8  2  5  7  12 V A BC D E F I1I1 I2I2 I3I3 The loop equation for ABCDA is 1)I 1 +I 3 = I 2 2) 6 +7I 2 + 8I 1 =0 3) 10 -8I I 3 =0 4) I 1 +I 2 = I 3 5) I 3 +7I 2 =0

24 V 1. Both go out 2. the intensity of light bulb A increases 3. the intensity of light bulb B increases 4. the intensity of light bulb A decreases 5. nothing changes

V4 V5 V I1 I2 I3 I 2 = I 1 + I 3 4 – 40I 1 – 3 – 10I 1 – 20I 2 = 01 – 50I 1 – 20I 2 = 0 4 – 50I 3 – 5 – 30I 3 – 20I 2 = 0-1 – 80I 3 – 20I 2 = 0 Example 21.3

When you look at a normal 120-volt outlet in the United States, there are two vertical slots and then a round hole centered below them. The left slot is slightly larger than the right. The left slot is called "neutral," the right slot is called "hot" and the hole below them is called "ground." The prongs on a plug fit into these slots in the outlet. Outlet in US

Household safety: avoid large currents! 0.1A flowing through your body will kill you! Q: What household devices consume lots of power? A: appliances: fridge, washer, dryer – about 1000W Since P=IV, if we use 120V we need a large I too! So: avoid this by using higher voltage: 240V Typical kitchen appliance or lamp consumes small amt. of power e.g. lightbulb: 100W 110V 220V

More info: hyperphysics!hyperphysics!

The big advantage of AC current is that it is relatively easy to change the voltage, using a device called a transformer. Power companies use very high voltages to transmit power over long distances. Say that you have a power plant that can produce 1 million watts of power. One way to transmit that power would be to send 1 million amps at 1 volt. Another way to transmit it would be to send 1 amp at 1 million volts. Sending 1 amp requires only a thin wire, and not much of the power is lost to heat during transmission. Sending 1 million amps would require a huge wire. So power companies use very high voltages for transmission (such as 1 million volts), then drop it back down to lower voltages for distribution (such as 1,000 volts), and finally down to 120 volts inside the house for safety. AC vs. DC power

Tesla championed alternating current, while Edison insisted that it was too dangerous. The only casualties in this "war of currents" were the animals Edison publicly electrocuted with Tesla's high voltage system to prove his point. The early victims were dogs and cats, but Edison eventually electrocuted an elephant named Topsy. Nikolai Tesla Thomas Edison