Download Circuit Elements: capacitor, resistor, and Ohm`s law

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 email for special appointments: [email protected]
• PH gone Monday, Feb. 8; in-class review by Dr. Andy Linscheid
• Remote numbers 666908, 676699,701738 register or reregister!
• Test 10 Feb. 12 problems chs 19-21 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,..
I2
I1 = I2 + I3
I1
I3
I
D
(2) Loop rule, V-law,..
B
Sum of the voltage drops around
any loop is zero!
V- IR1 – IR2 =0
C
R1
V
A
R2
F
E
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
A
V
V measures
potential
difference
across
resistor
Ammeter
A
Should have ideally zero internal resistance.
Voltmeter
V
Should have ideally infinite internal resistance.
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.
6 V
4 V
A
D
E
7 W
3 W
I1
I2
8 W
I3
12 V
C
5 W
The loop equation for ABCDA is
1) I1 +I3 = I2
2) 6 +7I2 + 8I1=0
3) 10 -8I1 +10 I3 =0
4) I1 +I2 = I3
5) 16 +10I3+7I2=0
B
F
2 W
24 V
1.
2.
3.
4.
5.
Both go out
the intensity of light bulb A increases
the intensity of light bulb B increases
the intensity of light bulb A decreases
nothing changes
Example 21.3
40
3 V
50
4 V
I1
10
5 V
I2
20
I3
30
I2 = I1 + I3
4 – 40I1 – 3 – 10I1 – 20I2 = 0
4 – 50I3 – 5 – 30I3 – 20I2 = 0
1 – 50I1 – 20I2 = 0
-1 – 80I3 – 20I2 = 0
Outlet in US
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.
Household safety: avoid large currents!
0.1A flowing through your body will kill you!
220V
110V
Typical kitchen appliance or lamp consumes small amt. of power
e.g. lightbulb: 100W
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
More info: hyperphysics!
AC vs. DC power
•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
Thomas Edison
Nikolai Tesla
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.