Download Electricity

Electricity
Can you get a light bulb to
light?
Using the equipment given to you, try to find as many
ways as possible to get the light bulb to light.
 Diagram four ways in which you were able to get the
light bulb to light. Label the battery, the wire, and the
bulb.
 Diagram at least three ways in which you were not able
to get the bulb to light.
Questions:
1. From your observations, what conditions seem to be
necessary in order for the bulb to light?

Successful Attempts
Producing Electric Current
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The flow of charged particles is an electric current.
Charges flow from a conductor with a higher potential
difference through a wire to a conductor with a lower
potential difference.
Conventional current is defined as positive charges
flowing from the positive (high potential difference) plate
to the negative (low potential difference) plate.
The flow stops when the electric potential difference
between the two plates is zero.
If the charge particles are pumped back to the original
conductor (plate) by an outside energy source (battery),
the flow of charges will continue.
Electric Circuits
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Any closed loop or conducting path allowing electric
charges to flow is called an electric circuit.
A circuit includes a charge pump and a device that
reduces the potential energy of the charges as they flow
from one plate to another.
The potential energy lost by the charges as they move
through the connected device is usually converted to
some other form of energy such as kinetic (motor), light
(lamp), or thermal (heater).
Requirements that must be met
to establish an electric current.

There must be a closed conducting path
that extends from the positive terminal to
the negative terminal. It is not enough
that there is simply a closed conducting
loop; the loop itself must extend from the
positive terminal to the negative terminal
of the electrochemical cell.
Requirements (continued)

The second requirement of an electric
circuit that is common in each of the
successful attempts is that there must be
an electric potential difference across the
two ends of the circuit. This is most
commonly established by the use of an
electrochemical cell, a pack of cells (i.e., a
battery) or some other energy source.
In Summary
In conclusion, there are two requirements that must be
met in order to establish an electric circuit. The
requirements are
1)There must be an energy supply capable of doing work
on charge to move it from a low energy location to a
high energy location and thus establish an electric
potential difference across the two ends of the external
circuit.
2)There must be a closed conducting loop in the external
circuit that stretches from the high potential, positive
terminal to the low potential, negative terminal.

When a battery no longer works, which
requirement is not met?
33%
1.
33%
2.
33%
3.
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An external energy supply to pump
the charge
The external circuit must make up a
closed conducting loop
Neither requirement is met.
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When a light bulb is burned out and no
longer works, which requirement is not met?
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3%
1.
3%
2.
3%
3.
An external energy supply to pump
the charge
The external circuit must make up a
closed conducting loop
Neither requirement is met.
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Electric Current
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Current is the rate at which charge flows past a point
on a circuit.
Current flows from the positive terminal to the negative
terminal.
Current (I) = q/t = Coulombs/sec=amperes (A)
Example: A 2 mm long cross section of wire is isolated
and 20 C of charge is determined to pass through it in
40 s. I = _____amperes
Example: A 1 mm long cross section of wire is isolated
and 2 C of charge is determined to pass through it in 0.5
s. I = ________amperes
Current is measured in units of ______.
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25%
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25%
2.
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3.
25%
4.
Amperes
Coulombs
Volts
Ohms
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The direction that conventional current flows in the
external circuit is from the ___ terminal to the ____
terminal.
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negative, positive
positive, negative
negative, negative
positive, positive
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Electrons move very quickly through an
electric circuit.
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50%
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True
False
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If the current is measured as 6 A, then ____ C of
charge flows past any point in the circuit in 3 s.10
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Power
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Power (measured in watts, W) measures the
rate at which energy is transferred. (1 W = 1J/s)
The energy carried by an electric current
depends on the charge transferred and the
potential difference across which it moves.
Current is the rate of charge flow (I=q/t). A flow
of 1 C/s is called an ampere (A).
Power is equal to the current times the potential
difference.
P=IV
Total energy transferred = P x t (W.s or J)
Practice Problems
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A 6.0 V battery delivers a 0.50 A current
to an electric motor connected across its
terminals.
What power is delivered to the motor?
If the motor runs for 5.0 min, how much
electric energy is delivered?
The Kilowatt- Hour
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The electric company (power company) provides energy
rather than power.
When a bill is paid, the customer is paying for the
electric energy used.
Because of the large amounts of energy consumed,
electric companies measure sales in a units of kilowatthours.
One kilowatt hour is equal to 1000 W being delivered for
3600 s (1 hr) or 3.6 x 106 J.
How many 100 W light bulbs would have to be used for
1 full hour to consume 1 KWh of energy?
Practice Problem
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An electric heater draws 15.0 A from a
120 V source. It is operated, on the
average, for 5.0 h each day.
How much power does the heater use?
How much energy in KWh does it
consume in 30 days?
At $0.12 per KWh, how much does it cost
to operate the heater for 30 days?
Problem
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A TV has a power rating of 103 W. How
much does it cost per month to operate it
for 6 hours per weekday and 15 hours/day
on the weekend? (Assume a 4 week
month)
How much current does it draw?
Power is measured in units called
0%
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2.
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4.
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amperes
joules
kilowatt-hours
watts
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Current is measured in units called
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amperes
kilowatt-hours
potential
watts
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If a 9.0 V battery delivers 0.50 A of
current to a heater, the power
consumed by the heater is
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0.056 W
4.5 W
9.0 W
18 W
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If a light bulb is rated at 50 W, the
amount of energy consumed in 30
minutes is
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2 kWh
30 kWh
30 J
90,000 J
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The kilowatt-hour is a unit of
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current
energy
potential
power
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A 60 W light bulb illuminated day
and night for 30 days consumes
___ of energy,
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43.2 J
43.2 kWh
1800 kWh
1.5 x 1012 kWh
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If a family’s electric bill is $74.00 per month
and the cost of electricity is $0.12 per kWh,
how much electricity does the family use per
month?
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0%
1. 8.9 kWh
0%
2. 270 kWh
3. 620 kWh
0%
4. 62000 kWh
0%
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Resistance
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Resistance (R) is a property that
determines how much current will flow.
Resistance is measured in units called
ohms (Ω)
Factors Impacting Resistance
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Length
Resistance increases as length of wire increases.
Cross-sectional area
Resistance increases as cross-sectional area decreases.
Temperature
Resistance increases as temperature increases.
Material
Resistance varies with the material used.
silver<copper<gold<aluminum<iron<platinum
(in order of increasing resistance)
Ohm’s Law
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Resistance is equal to potential voltage
divided by current.
V=IR
According to the law, 1 ohm is the
resistance permitting an electric charge of
1 A to flow when a potential difference of
1 V is applied across the resistance.
For a wire that obeys Ohm’s Law, the
resistance of a wire depends on the
___ the wire.
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current in
length of
power delivered by
voltage across
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A battery with a voltage of 9.0 V is connected to a
lamp. The current flowing in the circuit is 0.30 A.
The resistance of the lamp is
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0.030 Ω
2.7 Ω
27 Ω
30 Ω
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What factor impacting resistance explains why
light bulbs tend to burn out more frequently when
they are first switched on rather than while they
are operating?
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Length
Cross-sectional area
Temperature
Material
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Which wire conducts electricity with
the least resistance?
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One with a large cross-sectional area
One with a small cross-sectional area
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World’s Longest Burning Light
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Livermore's Centennial Light
Resistors
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Wires used to complete circuits have low resistance.
A wire 1 m in length that is typically used in a physics
lab has a resistance of 0.03 Ω.
Wires used in home wiring offer as little as 0.004 Ω of
resistance for each 1 m of length.
Resistors are devices with specific resistances that are
used to control current in circuits or parts of circuits.
A variable resistor (potentiometer) is used when a
smooth, continuous variation of the circuit is desired
(example: volume, brightness, contrast, hue, and tone
controls on a TV)
The human body acts as a variable resistor. (Dry skin
has high resistance-wet skin has lower resistance)
A device that can be used to
change the current in a circuits in a
continuous way is a
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potentiometer
battery
motor
lamp
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If a 200 Ω resistor is connected to
a 5 V battery, the current in the
circuit will be
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0.025 A
2.5 A
40 A
1000 A
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Types of Circuits
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There are three types of circuits:
Series circuits-all current travels
through each device.
Parallel circuits-a circuit in which there
are several different current paths
Combination series-parallel circuitcircuits which includes both series and
parallel branches.
Series Circuits
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In a series circuit, the current is the same throughout.
Itotal = IA =IB = IC = …
The equivalent resistance (total resistance) of the circuit
equals the sum of the individual resistances of the
resistors.
Rtotal = RA + RB + …
The total voltage is the sum of the individual voltages
across each device.
Vsource = VA + VB + …
If the voltage source does not change, adding more
devices in series will decrease the current.
Because the current flows through one pathway, if one
light goes out, they all go out.
The current is ___ in a series
circuit.
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higher at the beginning of
the same everywhere
lower at the beginning
variable in
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In a series circuit, the equivalent
resistance is ___ any single resistance.
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larger than
determined by
equal to
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If the battery voltage does not change,
adding more devices in the series ___
the current.
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sometimes decreases
always decreases
sometimes increases
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To find the current through a series
circuit, first calculate the ____.
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voltage
equivalent resistance
power
equivalent voltage
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Parallel Circuits
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A parallel circuit is one in which there are several paths.
The total current is the sum of the currents through each
path.
Itotal = IA + IB + Ic + …
Placing two or more resistors in parallel always
decreases the equivalent resistance of a circuit.
1=1 + 1 + 1 + …
R RA
RB
RC
The potential difference across each path is the same.
Vtotal = VA = VB = VC = …
Safety Devices
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A short circuit occurs when a circuit with very low resistance
is formed.
Parallel circuits are susceptible to short circuits.
Safety devices are incorporated into the circuit to prevent a
circuit overload.
1. Fuse-a piece of metal melts when too much current flows
and breaks the circuit.
2. Circuit breaker-an automatic switch that opens when the
current reaches a value greater than the rated value in the
circuit.
3. Ground-fault interrupter-an outlet that prevents injuries
that are caused when an electric appliance falls into water. It
contains an electric circuit that detects an extra current path
and opens the circuit.
A 16.0 Ω and a 20.0 Ω are connected in
parallel. 40.0 V is applied to the
combination.
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What is the equivalent resistance?
What is the total current in the circuit?
What is the current in the 16.0 Ω resistor?
What is the power dissipated by the
16.0 Ω resistor?
A heating coil has a resistance of
4.0 Ω and operates on 120 V.
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What is the current in the coil?
What is the power rating of the coil?
What energy is supplied to the coil if it is
operated for 30 min per day for 30 days?
At $.10 per kWh, how much does it cost
to operate the coil for 1 month?
A 20.0 Ω, 50.0 Ω, and 10.0 Ω resistor are
connected in parallel to each other. 5.5 A of
current flows through the 20.0 Ω. Resistor.
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What is the
resistor?
What is the
resistor?
What is the
resistor?
What is the
circuit?
voltage across the 20.0 Ω
current through the 50.0 Ω
current through the 10.0 Ω
total current through the
A person has a $5 and a 200 W stereo.
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At $0.15 per kWh, how many kWh can the
person use before the money runs out?
How many hours can the person operate
the stereo at this rate?
If the stereo is plugged into a 120 V
outlet, what is the current flowing through
the stereo when it is operating?
What is the resistance of the stereo?
A 22.0 Ω lamp and a 4.5 Ω lamp are
connected in series to a 45 V battery.
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What is
circuit?
What is
What is
lamp?
What is
lamp?
the equivalent resistance of the
the current in the circuit?
the voltage drop across the 22 Ω
the power dissipated in the 22.0 Ω
An ammeter is connected in series to a
10.0 V battery and a 5 Ω and 15 Ω resistor.
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What should the ammeter read?
What is the power dissipated by the
circuit?
How much energy is supplied by the
battery each minute?
What is the voltage across each resistor?
The load across a battery consists of two
resistors, 15 Ω and 47 Ω, connected in
series.
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What is the equivalent resistance?
What is the voltage in the 15 Ω resistor if
the current in the circuit is .097 A?
What is the voltage in the 47 Ω resistor?
What is the voltage of the battery?
A string of 18 identical holiday lights is
connected in series to a 120 V source. The
string dissipates 64 W.
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What is the current that flows through the
string?
What is the equivalent resistance of the
light string?
What is the resistance of a single light?
What power is dissipated by each light?
A 30.0 Ω resistor is connected in series
to two other 30.0 Ω resistors connected
in parallel.
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What is the equivalent resistance?
If the circuit is connected to a 1.5 V
battery, what is the total current that
flows through the circuit?
How much current flows through the
30.0 Ω that are connected in parallel?
Practice Problem #2
A 30.0 Ω is wired in series with a 20.0 Ω
lamp. These two resistors are wired in
parallel with 10.0 Ω and 40.0 Ω resistors
that are wired in series. All 4 of these
resistors are wired in series with a 25.0 Ω
resistor and a 25 V battery.
1) Find the total resistance.
2) Find the current through each bulb.
3) Find the voltage drop across each bulb.

The current is the same
everywhere throughout the circuit.
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The total resistance is equal to the
sum of the individual resistances.
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Series
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The voltage drop across each
resistor in the circuit is the same.
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The voltage drop is the circuit is
proportional to the resistance.
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Parallel
Series
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Adding a resistor to the circuit
decreases the total resistance.
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Parallel
Series
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Adding a resistor to the circuit
increases the total resistance.
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Parallel
Series
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5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
If the current through one resistor goes
to zero, there is no current in the entire
circuit.
0%
1.
0%
2.
10
Parallel
Series
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
If the current through one resistor in the
circuit goes to zero, the current through all
of the other resistors remains the same.
0%
1.
0%
2.
10
Parallel
Series
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
This form is suitable for household
wiring.
0%
1.
0%
2.
10
Parallel
Series
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
An ammeter is connected in ____
in a circuit.
0%
1.
0%
2.
10
Parallel
Series
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
A voltmeter is connected in ___ in
a circuit.
0%
1.
0%
2.
10
Parallel
Series
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
The basic unit of resistance is the
10
0%
1.
0%
2.
0%
3.
0%
4.
volt
ohm
amp
coulomb
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
Electrical force of push is measured
in
0%
1.
0%
2.
0%
3.
0%
4.
10
coulombs
ohms
amps
volts
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
A short thick wire has more
resistance than a long thin wire.
0%
1.
0%
2.
10
True
False
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20
In a DC circuit, current flows from the ___
electrode to the ___ electrode.
0%
1.
0%
2.
0%
3.
0%
4.
10
negative, positive
positive, negative
positive, positive
negative, negative
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
27
28
29
30
11
12
13
14
15
16
17
18
19
20