Chapter 6: Electric...
Transcript of Chapter 6: Electric...
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Chapter 6: Electric Circuits
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Electric Circuits
• Electrons possess electric potential energy that can be transformed into heat, light, and motion.
• For such transformations to occur, a source of electric potential energy needs to connect to one or more components by means of an electric circuit (path for electric current)
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• Any device in a circuit that converts electric potential energy into some other form of energy (causing an electric potential drop) is called a resistor
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• In the adjacent circuit, the charges pass from the positiveterminal of the battery, through the light bulb, and then back to the negative terminal of the battery.
• Electric potential energy acquired in the battery is carried by electric charges as they pass through the circuit.
• The electric potential energy is transferred to the light bulb and converted to light and heat.
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• Electric current can only flow through a circuit if there is a continuous conducting path.
• Any break in the circuit will stop the flow.
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Circuits
• Any circuit can be represented with a schematic diagram using a set of common symbols:
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Series Circuits
• Simple way of joining several loads together
• Charges have only oneconducting path
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Parallel Circuits
• Charges can move along several paths through the circuit
• Charge could pass through only one of the several loadsbefore returning to the energy source.
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Cells vs. Batteries
• Chemical Cell – electrochemical device that converts chemical energy into electrical energy
• Battery – collection of cells that work together to provide electrical energy to a circuit
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Cells in Series
• Positive terminal is connected to negative terminal of another cell
• Voltage is cumulative
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Cells in Parallel
• Positive terminals are connected together
• Voltage is constant, but increases the current that flows
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Resistance
• When charges pass through a material or device, they experience a resistance to their flow
• This results in a loss of electrical potential energy
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Ohm’s Law
• German scientist Georg Ohm (1787-1854) found that, for any conductor, the ratio of voltage to current is constant
• The potential difference between any two points in a conductor varies directly as the current between the two points (if the temperature remains constant).
R = V/I
R = resistance (ohms - )
V = potential difference (volts - V);
I = current (amperes - A)
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Ex.1: What is the resistance in a toaster, connected to a 120V power supply, if the current through it is 8.7 A?
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Ex.2: Calculate the maximum rating (in volts) of a battery used to operate a toy electric motor that has a resistance of 2.4 and runs at top speed with a current of 2.5 A.
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Ex.3: How much current is supplied by a 6 V battery if it is connected to a light bulb with a resistance of 20 ?
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Power in Electric Circuits (Joule’s Law)
• To predict the amount of energy used by an electrical device, we first need to know the amount of time the device will be used.
E = VIt
P = E/t
Therefore, P = VIt/t or P = VI
Since P = VI and V = IR Since P=VI and I = V/R
Then P = (IR) I Then P = V (V/R)
P = I2R P = V2/R
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Ex.1: Calculate the resistance of a 7.5 W light bulb plugged into a 120 V household outlet.
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Ex.2: What is the power rating on a light bulb with a resistance of 240 if a 0.50A current runs through it?
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Ex.3: A 110 V household circuit contains a 1800 W microwave and an 800 W coffee maker, which are connected to a 20 A fuse. Will the fuse melt if both the microwave and coffeemaker are on?
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Kirchhoff’s Laws for Electric Currents
Law of Conservation of Energy
• As electrons move through an electric circuit, they gain energy in sources and lose energy in loads
• The total energy gained in one trip through a circuit is equal to the total energy lost.
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Law of Conservation of Charge
• Electric charge is neither created nor lost in an electric circuit, nor does it accumulate at any point in the circuit.
Kirchhoff’s Voltage Law
• Around any complete path through an electric circuit, the sum of the increases in electric potential is equal to the sum of the decreases in electric potential
Kirchhoff’s Current Law:
• At any junction point in an electric circuit, the total electric current into the junction is equal to the total electric current out.
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Resistance
Remember…
• When charges pass through a material or device, they experience a resistance to their flow
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Resistance in Series
Vs = V1 + V2 + V3
IsRs = I1R1 + I2R2 + I3R3
Since Is = I1 = I2 = I3
Then Rs = R1 + R2 + R3
• Equivalent Resistor: Resistor that has the same current and potential difference as the resistors it replaces.
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Ex.1: What is the equivalent resistor in a series circuit containing a 16 light bulb, a 27 heater, and a 12 motor?
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Ex.2: A 22 , and 18 and an unknown resistor are connected in series to give an equivalent resistance of 64 . What is the resistance of the unknown resistor?
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Resistance in Parallel
IP = I1 + I2 + I3
I1 = V1/R1 I2 = V2/R2 I3 = V3/R3
VP/RP = V1/R1 + V2/R2 + V3/R3
Since VP = V1 = V2 = V3
1/RP = 1/R1 + 1/R2 + 1/R3
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Example #1: Find the equivalent resistor when a 4.0 bulb and a 8.0 bulb are connected in parallel.
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Example #2: Calculate the equivalent resistance of two, three, four and five 60 bulbs in parallel. What is the simple relationship for the equivalent resistance of in equal resistances in parallel?
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1. 2.
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3. 4.
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Resolve the following circuits:
CircuitPosition
Voltage (V)
Current (A)
Resistance(ohms)
1 10.0
2 20.0
3 30.0
Total 6.0
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Resolve the following circuits:
CircuitPosition
Voltage (V)
Current (A)
Resistance(ohms)
1 10.0
2 20.0
3 30.0
Total 6.0
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Resolve the following circuits: CircuitPosition
Voltage (V)
Current (A)
Resistance
(ohms)
1 10.0
2 20.0
3 30.0
Total 6.0
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Resolve the following circuits: CircuitPosition
Voltage (V)
Current (A)
Resistance
(ohms)
1 80.0
2 20.0
3 20.0
4 30.0
Total 120