Download I 2 R

Electric Currents (R)
General Physics 2
Current & Resistance
1
Resistors (L)
circuit
symbol
what is the resistance of
this resistor?
General Physics 2
Try some others.
Current & Resistance
2
Electric Power (L)
•
Power = Energy / time
Other useful
forms:
General Physics 2
Current & Resistance
3
Think-Pair-Share (L)
•
You buy a 75-W lightbulb in Europe, where
electricity is delivered to homes at 240 V. If you
use the lightbulb in the US at 120 V (assume its
resistance does not change), how bright will it be
relative to 75-W 120-V bulbs? [Hint: assume
that brightness is proportional to power
consumed.]
General Physics 2
Current & Resistance
4
Think-Pair-Share (L)
•
How many kWh of energy does a 550-W toaster
use in the morning if it is in operation for a total of
15 min? At a cost of 9.0 cents/kWh, estimate
how much this would add to your monthly electric
bill if you made toast four mornings per week.
General Physics 2
Current & Resistance
5
Power in Household Circuits (L)
•
•
•
•
Wires heat up at a rate equal to I2R (power)
fire hazard if I is too large
fuses prevent overloads by breaking circuit
short - two wires touching
General Physics 2
Current & Resistance
6
Fuses (L)
fuses that need to
be replaced
circuit breakerbreaks circuit
with a switch
General Physics 2
Current & Resistance
7
Household Circuits (L)
•
General Physics 2
What is the total current draw
by these devices? If the circuit
has a 20-A fuse, will the fuse
blow?
Current & Resistance
8
Alternating Current (L)
•
DC
•
•
•
direct current
current is constant
AC
•
•
•
•
alternating current
current varies like a sine wave
frequency usually 60 Hz (60
cycles per second)
Equations are valid when you
use RMS values
General Physics 2
Current & Resistance
9
Human Nervous System (L)
Messages
transmitted
within
nervous system
are electrical
signals passing
along the
neuron
General Physics 2
Current & Resistance
10
Practice Problems
•
Determine the
equivalent
resistance and
the current
through R1 for
the circuits
shown.
Assume
R1 = 10,
R2 = 20 , and
R1 = 30 , and
the battery is
12 V.
General Physics 2
Circuits
11