Download Outline - University of British Columbia

More on circuits:
power and short circuits
Combinations of resistors
• Resistors in parallel
• I = I1+I2+I3 = V/ R1 + V/ R2+ V/R3
• I = V/Req
• 1/Req = 1/ R1+ 1/ R2+ 1/R3
R1
R2
R3
V
2
Q8
The equivalent resistance for a group of
parallel resistors is
1. less than any resistor in the group.
2. equal to the smallest resistance in the group.
3. equal to the average resistance of the group.
4. equal to the largest resistance in the group.
5. larger than any resistor in the group.
1
1
1
1
 
  ...
Req R1 R2 R3
3
Electrical Power
• P = DV I
• Unit: Watt (W) = V A
• kW, MW, GW
• In circuits, charges do work on light bulbs,
appliances, etc. and electric energy is
transformed into other forms of energy
(food blender example).
Q1
When you connect a light bulb to a battery and a
current flows through the circuit, which is true?
A) Electric energy is conserved.
B) Charges are absorbed in the thin wire inside the
bulb causing the wire to glow.
C) All of the electric energy is transformed into light.
D) All statements above are true.
E) All statements are above false.
Q2
• If the four light bulbs in the figure are
identical, which circuit emits more light
(more power)?
1) Circuit I 2) Circuit II
3) Both emit
the same amount of light.
+ I
II
+ -
Q3
Rank in order, from brightest to dimmest,
the identical bulbs A to D.
1. A = B = C = D
2. A > B > C = D
3. A > C > B > D
4. A > C = D > B
5. C = D > B > A
Q4
Consider two light bulbs designed for 120 V.
Is the filament resistance lower or higher in
a 500 W light bulb than in a 100 W light
bulb?
1. Higher in the 500 W light bulb.
2. Lower in the 500 W light bulb.
3. Same in both bulbs.
4. Answer depends on the current.
8
Q5
Consider two light bulbs in series. The
rated power for each bulb is indicated.
Which bulb emits more light in this circuit?
“120V,60 W” “120V, 40 W”
1.
2.
3.
4.
The 40 W bulb.
+ The 60 W bulb.
Both emit the same amount of light.
I am so confused….
9
Power Bars
Good power bars have a build-in breaker as a safety
feature limiting the current to a maximum of ~ 15 A.
You are working at your computer, which consumes ~ 200
W of power. You are cold and you also want to connect
your space heater that consumes 500 W of electric power.
Will your power bar be able to handle both appliances?
A) yes;
B) No;
C) I don’t know.
10
Short Circuits
• Sometimes faulty appliances can lead to
short circuits (often due to overheating,
moisture buildup on circuit boards, etc.)
• Short circuit: Positive and negative terminal
connected by very small resistance leading
to large currents.
• Household wiring and can only handle
current of fixed amount ~ 15A. Larger
currents can damage the wire.
Q7
12
Q8
The circuit below consists of two identical
light bulbs burning with equal brightness
and a single 12 V battery. When the switch
S is closed, bulb a
S
1. burns more brightly.
a
b
2. burns as brightly.
3. burns more dimly.
4. goes out.
+ 13