Download - Discuss the measurement of power in circuits

TODAY’S OUTCOMES:
ELECTRICITY
- Discuss the measurement of power in
circuits
- Learn how to construct parallel circuits
and how they behave differently from
series circuits
GRADING OF
VOLTAGE, CURRENT AND POWER
5 points:
2 points:
1 - Complete expression of ideas
1 - Correct circuit chosen
3 points:
1 - Method used to get answer shown
2 - Correct currents given
13. In the circuit of Q12, there are more light bulbs lit when the switch is open, but the light bulbs
are brighter when it is closed. For which case is the power supplied by the battery greater?
(Write out an explanation of your answer).
In both cases, the voltage supplied is the same,
(it is the 4 V supplied by the battery).
When the light bulbs are brighter, there is more
current, so the current is greater when the
switch is closed.
Because power = voltage ✕ current, there is
3 light bulbs in series is dimmer
more power supplied when the switch is closed.
than 2 because the voltage must
be shared among all devices in the circuit.
CONCEPTUAL EXAMPLE
3 volts
3 volts
What happens to
brightness? It increases
What happens to
current? It increases (brighter bulb = more current)
Which number of
3 volts
3 volts
light bulbs drains
the battery fastest?
Fewer bulbs in a
series circuit uses
more power;
What
if
we
have
battery
drains
What happens now?
zero bulbs?
faster
Brightness, current,
short
circuit;
power go up again
VERY HIGH power
REVISIT LED + BULB EXAMPLE
+
LED
–
+
–
light bulb
In this case, the LED was
brighter than the bulb, but both
have the same current....
The LED requires less current
(and less power)
than the bulb to glow!
CURRENT AT ALL POINTS IN A GIVEN SERIES CIRCUIT
IS THE SAME.
CURRENT AND POWER DECREASE IN A SERIES
CIRCUIT WHEN MORE DEVICES ARE ADDED, IF THE
VOLTAGE IS KEPT THE SAME.
14. When all the generators are running at Hoover Dam, it produces 2,000,000,000 Watts. This power is then
sent elsewhere by power lines. The diagram at right is a simplified version of the situation, where the battery
represents the generators and the light bulb represents the city that uses the power. If the “battery” is driving the
circuit at 120 V (household voltage), what is the current in the power line?
power = voltage ✕ current
current = power / voltage
current = 2,000,000,000 W / 120 V
= 16666666.6666.... A
≈ 16.7 million A
They use transformers to change the voltage, so that the Hoover Dam power station acts like a
100,000 Volt battery. Now what is the current in the power lines?
current = 2,000,000,000 W / 100000 V
= 20,000 A
Which of these 2 currents is easier to transport along a wire?
TRANSMISSION OF ELECTRIC POWER
power
generation
16.7 million A in a power line
would not work
homes,
business
TRANSMISSION OF ELECTRIC POWER
transformer raises
the voltage
power
generation
high voltage
transmission
lines
substation
(transformers)
100000 V
120 V
homes,
business
distribution
lines
transformer lowers
the voltage
What voltages do most household
appliances require?
Most require ~120 V (some exceptions, like clothes
dryers, have special outlets for higher voltages).
What typical currents do most household
appliances operate at?
Current varies but not TOO high - more
than A total will trip most circuit breakers.
Some devices, like cell phone chargers and electronics
(especially computers) can be damaged by more than
about 10 Volts. How can these be plugged into 120 V
outlets?
Such devices have special plugs that
transform the voltage yet again to a lower
value.
WHAT YOU ARE EXPECTED TO KNOW:
- Power = current × voltage; how to solve for any
of these 3 quantities
- What happens to the current and power in a
series circuit when elements are added and
removed
- The purpose of transformers between power
generation and household electrical usage
TODAY’S OUTCOMES:
ELECTRICITY
- Discuss the measurement of power in
circuits✓
- Learn how to construct parallel circuits
and how they behave differently from
series circuits