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REMINDER:
Complete the online attitude survey
IMMEDIATELY if you have not already done so;
it closes TODAY
TODAY’S OUTCOMES:
ELECTRICITY
- Discuss the differences between series
and parallel circuits
- Introduce the concept of resistance
- Conclude our hands-on study of
electricity with construction of a complex
circuit
GRADING OF
PARALLEL CIRCUITS
5 points:
5 points:
10. The circuit for the lights in a house resembles the circuit drawn at right. The bulbs are all in
parallel. What are the advantages of wiring them this way, instead of putting them in series?
1) In a series circuit, if one electrical device (bulb) is turned off,
ALL the devices (bulbs) go off, because the circuit is broken this would not be good in a house!
2) In a series circuit, the brightest of the bulbs depends on the
number of bulbs, but in a parallel circuit the brightness stays
the same .*
* Brightness might change SLIGHTLY in a parallel circuit if
there is a large change in the current drawn from the power
source, but the change is generally small for a good battery or
power source.
11. Terry claims there is another advantage: the additional light bulbs don’t cost anything to operate. He
argues that the light bulbs are independent of each other, and so there is no way for the power company to
know how many are turned on. Paula says there is a way for the power company to tell when a light bulb is
turned on. Please explain who is right.
Paula is correct - the power used is higher if more light
bulbs are turned on. If more bulbs are lit, with the same
brightness, then more current flows from the power
source, and POWER increases with CURRENT.
IMPORTANT: Current is the same at all
points in a SERIES circuit; this is NOT true
in a PARALLEL circuit.
all
CURRENT IN SERIES CIRCUITS bulbs(assume
are identical)
3 volts
D
C
3 volts
J
A
B
E
F
H
G
Currents through A, B, C, and D are all equal
Currents through E, F, G, H and J are all equal
1) Current is the same at all points in a
SERIES circuit.
Currents in the left diagram are GREATER
THAN the currents in the right diagram
2) Adding more devices/bulbs in SERIES
reduces the current.
Bulbs dimmer
in right
diagram
CURRENT IN PARALLEL CIRCUITS
3 volts
3 volts
A
D
(assume all
bulbs are identical)
E
J
B
F
C
G
Bulbs are
(almost)
the same
brightness in
both diagrams
H
Currents through A, B, C, and D are NOT all equal
The current splits at the green points ; so the
current through A or D is the SUM of the
currents through B and C
Current A = Current D = Current B + Current C
Currents through E, F, G, H and J are NOT all equal
The current through E or J is the SUM of the currents
through F, G, H
Current E = Current J = Current F + Current G + Current H
CURRENT IN PARALLEL CIRCUITS
3 volts
3 volts
A
D
(assume all
bulbs are identical)
E
J
B
F
C
G
Bulbs are
(almost)
the same
brightness in
both diagrams
H
1) Current is NOT the same at all points in a
PARALLEL circuit, and is highest in the wires
leading to and from the power source
2) Adding more devices/bulbs in PARALLEL
increases the current at the power source.
IS THERE A LIMIT TO HOW MANY
DEVICES YOU CAN HOOK UP
IN PARALLEL?
No - but remember:
The CURRENT near the power source
keeps going up when you add more!
CONSEQUENCES OF INCREASING
THE CURRENT TOO HIGH:
Power use goes up
Too much power
HEAT
In a house, this blows
a fuse or trips circuit
breaker
SIPHON ANALOGIES
SERIES CIRCUIT
= devices or bulbs
+
–
PARALLEL CIRCUIT
Flow rate highest nearest to source,
before the tube branches off
= devices or bulbs
three outlets
pull more
water from the
source than
one
+
–
9. Susan made an extra bright flashlight. It has two batteries (like most flashlights) but three light bulbs. It
gives off three times as much light, but the batteries don't last very long.
Here is a diagram of Susan's flashlight. Explain how the concepts voltage, current, power, and energy are
involved in this device; by the time you are done, you should have completely explained the sentence, " It
gives off three times as much light, but the batteries don't last very long. "
Adding extra bulbs in parallel to a common voltage
increases the current drawn from the batteries,
and thus the power used, which increases total
brightness. A higher power flashlight increases the
rate at which energy is used, depleting the
batteries faster.
WHAT YOU ARE EXPECTED TO KNOW:
- How series and parallel circuits are different
- What happens in a parallel circuit when more
elements are added
- Where the current is greatest in a parallel
circuit and why
TODAY IS THE LAST ELECTRICAL
CIRCUIT LAB; MONDAY
WE CONTINUE WITH NEXT UNIT
TEST ON ELECTRICITY:
OPEN NOTE
NEXT FRIDAY 9/16/11
Study materials between now and then:
1) Reading assignment on electrical safety at
end of today’s lab (read by Monday)
2) Test practice problems and review sheet (to
be handed out Monday)
TODAY’S OUTCOMES:
ELECTRICITY
- Discuss the differences between series
and parallel circuits✓
- Introduce the concept of resistance
- Conclude our hands-on study of
electricity with construction of a complex
circuit