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Circuits
Chapter 18: Electric Circuits
Circuit Symbols:
Battery:
Resistor:
Light-bulb:
Switch:
Wire:
Three general types of circuits:
1. _____________________:There is a complete loop with wires going from one side of the battery through a
resistor(s) to the other side of the battery.
2. ______________________ - There is not a complete loop.
3. ______________________ - There is a complete loop, but it does not contain any resistors.
Examples:
Current:
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Most modern day appliances require electricity to operate.
The electricity generates a ________________ which runs through the appliance.
The current is made out of charged particles
Current is defined as the ____________________________________________
____________________________ to the direction of flow.
o For example: if the charges are flowing through a wire, the area is the cross-sectional area of the
wire
Equation for Current:
Where: I = current (amps, A)
Q = change in charge or how much charge flows (Coulombs, C)
t = time (seconds)
Example: The amount of charge that passes through the filament of a certain lightbulb in 2.00 sec is _______.
Find (a) the current in the bulb and (b) the number of electrons that pass through the filament in
______________ sec. (remember: Qelectron = 1.6x10-19C)
Electric Current Flow
1. In almost all real circuits it is __________ (- charges) that are flowing. We call this kind of current
“___________________”.
2. However, for simplicity we assume that all charge carriers are _____________. We draw current
arrows in the direction that such charges would move….that is, ______________.
3. Historically this is because ____________________ guessed wrong about the charge carriers in a metal
being positive.
Two conditions for an electric current to flow between two points:
1. There must be a conducting path between the points along which the charges can move.
WHY?
2. There must be a difference of electric potential (volts) between the two points.
WHY?
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________________ is the measure of _________________ moving within a conductor.
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_________________, or amps, measures the amount of current that flows. Amps is named after a 19thcentury French physicist, Andre-Marie Ampere, and abbreviated ___.
Voltage
Voltage, or _________________, is measured in volts, named after a physicist named Volta, and abbreviated V.
Examples: AAA, AA, C, D are all 1.5 V a car battery is 12 V. Power lines are several thousand volts.
Common Misconception:
MYTH:
TRUTH:
Electrons come from the power utility company.
People thing the electrical outlets in their homes
are the source of electrons. The electrons flow
from the power utility company through the power
long and into the wall outlets.
Electrical Resistance
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A ___________________ is anything that electricity cannot travel through ____________.
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When electricity is forced through a ________________, often the energy is changed into another
______________________, such as ____________________.
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The reason a light bulb glows is that the electricity is forced through tungsten, which is a resistor. The
____________________________________________.
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Resistance is sort of like the __________ associated with __________________ or electrical current.
Types of Conductors
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Good conductors, like most _____________, allow current to flow _________
__________________________
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Poor conductors, like most _________________, ________________ the flow of current to a
______________ extent.
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______________________ like very cold niobium-tin, are special substances that allow current to flow
with _________________________;
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_________________________, like silicon, are either good or poor conductors
_______________________________.
Resistance
The resistance of a conductor depends upon four factors:
1. The _______________ of which it is composed; the ability to carry an electric current varies more than
almost any other physical property of matter.
2. Its ______________ : The longer the conductor, the greater its resistance.
3. Its ___________________: Resistance generally increases as temperature increases.
4. Its ___________________ (A). The thicker the conductor the less its resistance.
Example: The filament in a 60 W light bulb has a larger resistance than the filament in a 100 W light bulb. If
both filaments are made out of tungsten, how do their length’s & cross-sectional areas compare?
Example: You are wiring up a circuit and want the lowest resistance possible. What type of wire should you
choose?
Rubber or Copper
Lots of extra wire or just enough to get the job done.
Thick or Thin
Run the circuit at a high temperature or a low temperature
Ohm’s Law
Resistance quantifies how much current you get across something per volt applied.
Resistance has units of Volts/Amp, which get another name, ohms, represented by the Greek letter omega Ω.
Example: How much voltage difference does it take for _________ amps to flow through a resistance of 8.0 Ω?
Power

or
or
Power = volts x amps = Joules/second = Watt or W
P = power (W)
V = voltage (V)
I = Current (A)
R = resistance ()
Example: What is the current flow to a ______ hairdryer plugged into a 120V socket?
Example: Lets say you want to protect your new speakers by putting a fuse in line with the speakers. Assume
you have _____ speakers and that they are _____. What size (amps) fuse should you use so that it just blows at
100 Watts?
Example: Two light bulbs operate from _____, but one has a power rating of _____, and the other has a power
rating of 100.W. Which bulb has the higher resistance?
Thermal Energy:
The sole purpose of some appliances is to produce heat (electric stove, toaster, hair dryer). These appliances
have a resistor in them which turns electrical energy into thermal energy (or heat)
E = Thermal Energy (J)
P = Power (W)
t = time (sec)
I = Current (A)
R = Resistance (W)
V = Voltage (V)
There are two ways to put a circuit together:
1. Series - Resistors are considered to be in series if the current must go through all of the resistors in
order.
Current:
(wait for the equations)
Equivalent/Total resistance:
Voltage:
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In a series circuit, the current is determined by which resistor? _____________________
Example 1: A circuit has three ________________________________ resistors in series along with a 24 V
battery.
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Draw the Circuit:
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Calculate the total resistance of the circuit.
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Calculate the total current through the circuit.
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What is the current through each resistor?
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Calculate the voltage across each resistor.
2. Parallel - Resistors are considered to be in parallel if the current is shared between multiple resistors.
Current:
(wait for the equations)
Equivalent/Total resistance:
Voltage:

In a parallel circuit, will a resistor with a large resistance have more or less current through it then a
resistor with a small resistance?
Example 2: A circuit has three resistors: ____________________________ resistors in parallel along with a 24
V battery.
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Draw the circuit
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Calculate the total resistance of the circuit.
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Calculate the total current through the circuit.
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What is the voltage across each resistor?
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Calculate the current across each resistor.
Electrical Outlets & Light Bulbs:
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Electrical outlets provide electric potential (or the voltage) for any appliance plugged in to it.
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In the United States ALL outlets provide ________ (in Europe it is 240 V)
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Light bulbs are made to be the only appliance plugged into a socket.
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The power rating of a light bulb (25 W or 100 W…) is as if that bulb was the ___________ plugged in to
_________ power source.
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The ____________ of a light bulb is calculated with the power rating and the voltage (120 V)
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Current and actual voltage used by a light bulb depends on the circuit.
Example 3: What will be power output be if an American-made 45 W light bulb is plugged in to a 310 V power
source?
Using 120 V, calculate the resistance of the light bulb.
Using the resistance and the voltage of the new source, calculate the new power
Example: A 25W bulb and a 100W bulb are connected in series. Which bulb will glow brighter?
Note: Since there are two bulbs in the circuit, the power ratting isn’t very helpful.
(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.