Transcript Slide 1

Questions of the Day:
a/ Draw a parallel circuit with a 6.0 V source and
R1 = 40 W and R2 = 20 W resistors.
b/ Now add an ammeter and voltmeter to show how
you would measure total I and V.
c/ Redraw the circuit with ammeter and voltmeter
to show how you would measure I1 and V1.
d/ Redraw the circuit with ammeter and voltmeter
to show how you would measure I2 and V2.
rate at which electrical
Electrical Power P is the _________
energy is converted to
_____________________________
light, heat, mechanical, etc,
energy or vice versa.
scalar
P is a _____________
P
= VI = I2R = V2/R
The units of P are watts, W
(_____________)
derived
1 watt = 1 W = V·A
= A2·W
= V2/W
Since W =
Pt
Electrical work:
, you can write:
W = VIt
= I2Rt = V2t/R
coal power
solar power
flower power
wind power
An energy meter.
“Watt” does this meter read?
Hint: Read left to right and round down.
8 4 1 6
kilowatt-hours
an energy unit,
just like
watt-seconds
Power
Time
energy = power x time
Ex. At what rate is electrical energy converted to heat
and light in a 75 W bulb?
75 W = 75 J/s
Ex: A 55-W toaster oven is used for 25 seconds.
How much electrical energy is converted to heat?
W = Pt = (55 W)(25 s) = 1400 W·s
= 1400 J
Ex. A 12-W resistor carries a current of 3.0 A for 5.0 s.
a/ At what rate does the resistor convert energy?
rate  power P = I2R
= (3.0A)2(12W)
=110 A2 W = 110 W
b/ How much energy does the resistor convert in the
5.0 s?
W = I2Rt = Pt = (110 W) (5.0 s) = 550 W·s = 550 J
Moses Niagara - 2200 MW = 2.2 GW
RAVENSWOOD
2300 MW = 2.3 GW
Nine Mile points 1 and 2
620 MW and 1200 MW
Ex: How much power
is developed in the
circuit at right?
Given: V = 9.0 V
50 W
9.0 V
and R = 50 W
P = V2/R = (9.0 V)2/ 50 W
= 1.6 V2/ W
= 1.6 W
At what rate is energy converted to heat?
P = 1.6 W
How much energy is converted in one minute?
W = Pt
= (1.6 W)(60 s) = 96 W·s
Ex. What quantity does P “·” represent?
= 96 J
Powerpoint
The Flux Capacitor
1.21 GW = 1210 MW
= 1210 x 106 W
Ex.
V
series
parallel
R1
V
R2
V1I1 = I12R1 =
Power of resistor R1:
P1 =
Power of resistor R2:
P2 = V2I2 = I22R2 =
Total power:
R2
R1
V12/R1
V22/R2
P = VI = I2Req = V2/Req
or sum up the powers: P =
P1 + P2 +
···
for both types of circuits .
All of these equations work ___________________________
Ex: Find the power in each resistor and the total power.
20 W
5.0 V
P1 =V1I1
10 W
= V12/R1
= I12R1
V1 = 3.3 V
I1 = 0.17 A
R1 = 20 W
P1 = 0.57 W
V2 = 1.7 V
I2=
0.17 A
R2 = 10 W
P2 = 0.29 W
V = 5.0 V
I=
R = 30 W
P = 0.83 W
P = VI = …
or P = P1 + P2 +
···
5/30 A
0.17
Ex: Find the power in each resistor and the total power.
20 W
5.0 V
10 W
P1 =V1I1
= V12/R1
= I12R1
V1 = 5.0 V
I1 = 0.25 A
R1 = 20 W
P1 = 1.25 W
V2 = 5.0 V
I2=
0.50 A
R2 = 10 W
P2 = 2.5 W
V = 5.0 V
I=
0.75 A
6.7 W
R = 5/.75
P = 3.8 W
P = VI = …
or P = P1 + P2 +
···
Notice both circuits above have ____________V,
R1 and R2.
the same
Which of the two circuits would drain a battery faster?
parallel because P is greater
smaller
parallel
The ____________
circuit has a ______________
Req.
greater
This means it will have ____________
I. Because
V , the
P = ______
and both circuits have the _____
VI
_____________
circuit will develop more power.
parallel
Which resistor develops more power in the series circuit?
I but it has more ____________
R1 b/c both have same ___
voltage
___
Which develops more power in the parallel circuit?
R2 b/c both have same ___
current
V but it has more ____________
___
Prescott Joule
current through a wire that
__________________
passed ___________
water
was submerged in __________
and found that the amount
I2 and _____.
R
heat produced was proportional to ____
of ________
joule or _____
I2R heating. Less ___
I means
This is called_________
heat
much less
______________
electrical energy is converted to _______.
VI , the current can be reduced if ____
V
Because P = ____
is
increased. For this reason, power is transmitted at high
voltages .
_____________
At your
house:
240-120
_________V
Power
plant:
30 kV
~_____
The power is
transmitted at
155-765 kV
_____________
A substation cuts
7200 V
the V to __________
Why does Europe use 440-220 V
instead of 220-110 V?
Higher V  can transmit longer distances with
less energy loss
One transformer can supply an entire block.