A 25W bulb and a 100W bulb are connected in
series. Which bulb will glow brighter?
25W
100W
120V
The Light Bulbs are really Resistors
A) Calculate the resistance for each resistor shown.
B) Calculate the total resistance of the circuit.
C) Calculate the current through each resistor.
D) Calculate the power used by each resistor.
E) Calculate the voltage across each resistor.
25W
100W
25W Bulb
Part A.
100W Bulb
2
P=
V
R
R=
V2
P
120 2
100
120 2
R=
25
R=
R = 576!
R =144!
120V
144 Ω
576 Ω
720 Ω
= .167 amps
= .167 amps
120V
B) Calculate the total circuit resistance Rtotal
Rtotal = R1 + R2
= 576 + 144
Series Resistors
120V
C) Calculate the total circuit current (I)
I=
V 120volts
=
= .167 amps
720!
R
= 720 Ω
1
144 Ω
576 Ω
96 volts
144 Ω
576 Ω
= .167 amps
= .176 amps
120V
120V
D) Calculate the Power used by each resistor.
25 W Bulb
E) Calculate the Voltage across each resistor.
= .1672 x 576
= 16 watts
25W Bulb
P = I2R = .1672 x 144
= 4 watts
100W Bulb
P = I2 R
100 W Bulb
24 volts
V = IR
V = IR
= .167 x 576
= 96 volts
= .167 x 144
= 24 volts
120 volts
E) Consider the Percent Power Needed to
Light Each Bulb
The circuit below consists of two identical light bulbs
burning with equal brightness and a single 12V battery.
When the switch is closed, the brightness of bulb A…
A
1. Increases
100 W Bulb
2. Decreases
4 watts
! 100 = 4 percent
100 watts
3. Remains unchanged
25 W Bulb
16 watts
! 100 = 64 percent
25 watts
Q
The circuit below consists of two identical light bulbs
burning with equal brightness and a single 12V battery.
When the switch is closed, the brightness of bulb A…
1. Increases
2. decreases 3. remains unchanged
Which bird is in trouble when the switch is closed?
1) Bird 1
1
2
2) Bird 2
3) Neither
When the switch is closed,
bulb B goes out because all of
the current goes through the
wire parallel to the bulb. Thus,
the total resistance of the circuit
decreases, the current through
bulb increases, and it burns
brighter.
4) Both
A
Q
2
Which bird is in trouble when the switch is closed?
1) Bird 1 2) bird 2 3) neither
1
4) both
2
Charge flows through a light bulb. Suppose a
wire is connected across the bulb as shown. When
the wire is connected…
1. All the charge continues to flow through the
bulb, and the bulb stays lit.
2. Half the charge flows through the wire, the
other half continues through the bulb.
3. Essentially all the charge flows through the
wire and the bulb goes out.
4. None of these.
Charge flows through a light bulb. Suppose a
wire is connected across the bulb as shown. When
the wire is connected…
Charge flows through a light bulb. Suppose a
wire is connected across the bulb as shown. When
the wire is connected…
1. All the charge continues to flow through the
bulb, and the bulb stays lit.
1. All the charge continues to flow through the
bulb, and the bulb stays lit.
2. Half the charge flows through the wire, the
other half continues through the bulb.
2. Half the charge flows through the wire, the
other half continues through the bulb.
3. Essentially all the charge flows through the
wire and the bulb goes out.
3. Essentially all the charge flows through the
wire and the bulb goes out.
4. None of these.
4. None of these.
Q
3