electric circuits. electric circuit is a closed path for the flow of electrons. consists of:...
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Electric Circuits
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Electric Circuit
• Is a closed path for the flow of electrons.
• Consists of:
1. Source of electricity
2. Wires to conduct the flow of electrons (electric current)
3. Objects (resistors or loads) along the path that require electricity to operate (ex. Lamps)
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Current
• Current: the movement of negative charge (flow of electrons). The amount of charge that ‘flows” past a certain point in a conducting wire every second.
• If you were to describe a current of water you might state the amount of litters of water that flows past a point in a pipe in one minute. (watch)
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•Current/Water analogy
Watch
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Current
• One coulomb of charge is equal to the amount of charge in 6.25x1018 electrons (6.2 billion billion electrons).
• The symbol for current is I
• Current is measured in amperes A
• The symbol for charge is Q
• Charge is measured in coulombs C
• Time (t) is usually measured in seconds s
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Current
• The mathematical relationship among these variables is:
Amount of current=amount of charge moving past a point time
t
QI
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Current
• A current of 2.0 A means that 2.0 C of charge is moving past a point in the circuit every second.
I=Q t
2.0 A= 2.0 C1 s
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Calculations
• Example 1
If 350 Coulombs (C) of charge pass a point in a conductor in 20 seconds, what is the electric current (I) through that point?
Formula: I=Q/t
I= ?(unknown)
Q= 350C
t=20 s
I=Q / tI=350C / 20sI=17.5 AThere are 17.5 Amperes of current passing though that point.
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Calculations• Example 2
If 200 C of charge pass a point in 3 minutes, what is the electric current through that point?
• What is your first step?
• Covert minutes to seconds
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• 3 minutes x 60 s = 180s 1 min
I= ? I=Q/t Q= 200 C I= 200/180s t= 180 s I= 1.1A
There are 1.1 amps of current passing though
that point.
Calculations
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• Read p.324-325; 328-329• Questions p.329 (1-5)
Review
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Electricity on the Move
Source of Electricity:
• You can think of a battery as an object that has energy to make electrons move around a circuit.
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Energy
Electrical Potential Energy• The purpose of a battery in an electric
circuit is to provide energy to move the negative charge (electrons) through the conductors in the circuit.
• All Energy is measured in Joules (J)
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Batteries
• Chemical reactions occur in a battery that result in buildup of electrons in the negative terminal
• Electrons in the battery move to the other terminal making it positive.• These “energized” electrons now have the ability to do work on
something.
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Potential Energy
•This electrical energy stored in a battery is called Potential Energy. (This is the driving force responsible for the moving charges in a circuit.)
•In order to actually do work the battery must be connected to an object (load or resistor) and the circuit must be complete. (See page 330, figure 10.3 a & b)
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Potential Energy
The units for potential energy per unit of charge are:
J (Joules)C (Coulomb) or Volt (V)
Another name is given to J/C. It is called a Volt (V)
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Formula
• Voltage can be looked at as how much energy is carried per unit charge
• Voltage (potential energy) = Energy / charge
Q
EV
Potential difference = energy/charge
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Potential Difference
• When describing energy in a circuit we speak of potential difference.
• Potential Difference: the difference in potential energy per coulomb charge at one point in a circuit compared to another. This is measured in Volts (V).
• For example: If one coulomb of charge at one point in a circuit has one more joule, of potential energy, than at another point in the circuit the potential difference is one volt.
1C+ 1J vs. 1C + 2J = Potential Difference of 1 V
• Using a voltmeter, you can measure voltage by looking at the difference in energy between two points on the circuit
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Example
• In a battery, 45J of chemical energy are converted into electrical energy by separating positive and negative charges. This energy places 15C of charge at the negative terminal, leaving a deficit at the positive terminal. What is the potential difference between the two terminals of the battery?
Given• E= 45 Joules• Q= 15 Coulombs• V=?
V =
V = = 3.0 V
A battery that uses 45J of chemical energy to separate 15C of charge generates a potential difference of 3V.
Q
E
C
J
15
45
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Example 2
• Within a battery, 180 J of chemical energy are converted into electrical potential energy. This amount of energy produces 30 C of negative charge (electrons) at the negative terminal, and a deficit of electrons at the positive terminal. What is the potential difference between the negative and positive terminal of the battery?
Given• E= 180 Joules• Q= 30 Coulombs• V=?
V =
V = = 6.0 V
A battery that uses 180J of chemical energy to separate 30C of charge generates a potential difference of 6.0V.
Q
E
C
J
30
180
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Review
• Read pages 330 to 336
• Answer questions 1 to 5 on page 336.
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Resistance
• When electrons move through a conductor the atoms resist the flow of electrons.
• Resistance is the property of a substance which indicates how much that substance will interfere with the flow of electrons
• Example: The resistance of Tungsten (filament in light bulbs) is 400 times greater than copper wire.
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Resistance
• There is a mathematical relationship between resistance potential difference and current.
I
VR
resistance = potential difference / current
•The unit for resistance is V/A or Ohm (Ω)
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Ohm’s Law
• The scientist Ohm did experiments that found resistance was always the same no matter how much voltage (potential difference) was placed on them.
RIV Potential difference = current x resistance
•An ohmic resistor has constant resistance. Many electrical appliances are NOT ohmic
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Example
• What is the resistance of a heating coil if a current of 10.0 A goes through it when connected to a wall outlet providing a potential difference or voltage of 120 V?
I
VR
0.120.120.10
120
A
V
A
V
I
VR
V = 120V
I = 10.0 A
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Example 2
• What is the current running through a light bulb having a resistance of 5.0 Ω (V/A) when the potential difference across the bulb is 12.0 V?
I
VR
AV
R
VI 4.2
0.5
0.12
V = 12.0 V
R = 5.0 Ω (V/A)
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Power
• Power is defined as energy per unit time.• Electrical Power is defined as the amount of
electrical energy that is converted (into light, sound, heat or motion) every second.
• The mathematical equation for power (P) is:
t
EP
Power = energy / time
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Power
• The unit for power, Joules/second, is called a watt W.
• Example: when sixty joules of electrical energy is converted into light and heat by a light bulb every second.
• The bulb has a power of 60 watts (W)• It’s not convenient to talk about power in electric
circuit this way so another equation has been developed. (page 344)
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Power
• The new mathematical equation for power (P) is:
VIP Power = current x potential difference (voltage)
•Power gives us information about how many joules of electrical energy are being converted from electrical energy into another form of energy every second.
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Example
• A current of 0.83 A passes through a light bulb which is connected to a 120 V wall outlet. What is the power of the light bulb??
IVP WVAIVP 10012083.0
V = 120V
I = 0.83 A
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Power Rating
• It’s useful to know how much energy an electrical device would use in a certain amount of time.
• To calculate this we use a Power Rating
• Many electrical devices, like a light bulb have this stamped on them.
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This stamp tells us that the light bulb has a power rating of 60 W, 14 W, or 12.5 W when connected to a 120 V wall
outlet
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Power Rating
• If we want to calculate how much energy an electrical device uses, we would multiply the power (watts) by time (seconds).
PtE Energy (joules) = power (watts) x time (seconds)
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Energy Efficiency
Percent efficiency of electrical device = (useful energy output/total electrical energy input) x 100%
•The useful energy output of a lamp is the amount of energy that a lamp actually converts to light.
•There is no electrical device that converts all of the electrical energy going into it , into the energy the electrical device is producing.
•Not all electrical energy going into a light bulb is converted to light.
•The efficiency of an electrical device can be calculated but using the following equation:
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Example
• An electric kettle has power rating of 1000 W. it takes this kettle 3.5 min to heat up 600ml of water from 22.0 °C to 100.0 °C. This required 196 000 J of energy to heat the water. What is the efficiency of the kettle?
• The energy used by the kettle is
JE
sWE
PtE
210000
2101000
PtE P = 1000 W
t = 3.5 min = 210s
Percent efficiency = (196 000 J/210 000 J) x 100%
Percent efficiency = 93.3 %
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• Read p.337 – 339; 342-346; 358• Questions p.342 (1-6)
»p.348 (1-6)»p.350 (15, 16; 21-22; 24-30)
Review