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Transcript
Electric Circuits The three basic components of an electric circuit: Electric Current Electric Resistance Potential Difference (Voltage) Electric Current Electric Current: The flow of electric charge The loosely bound outer electrons of conductors carry the charge through circuits Protons tightly bound to the nuclei of atoms Electric Current current = charge / time or I = q/t Units: Amps (A) An amp is the flow of 1 C of charge per second NOTE: 1 C = the charge of 6,240,000,000,000,000,000 electrons Electric Current Usually the number of electrons entering a wire is the same as the number leaving This gives the wire a net charge of zero Voltage Sources Voltage Source: A device which provides a potential difference in order to keep current flowing Dry/Wet Cells: Convert chemical energy to electrical energy Generators: Convert mechanical energy to electrical energy The voltage available to electrons moving between terminals is called electromotive force, or emf. Voltage Sources Note: Voltage flows across a circuit Current circuit flows through a Current vs. Voltage Current – Flow rate Measured in Amperes Amount of flowing water Voltage – Potential Measured in Volts Water Pressure Electric Resistance Electric Resistance: The ability of a material to resist the flow of charge Units: Ohms (W) The amount of charge that flows through a circuit depends on two things: Voltage provided by source Electric resistance of the conductor Resistors What is the resistance of this resistor? Review What is the resistance of this resistor? Resistance in a Wire Electric Resistance Factors Thick wires have less resistance than thin wires Short wires have less resistance than long wires Higher temperatures usually cause more resistance The resistance in some materials becomes almost zero at very low temperatures Ohm’s Law The current in a circuit is Directly proportional to the voltage across the circuit Inversely proportional to the resistance of the circuit Therefore: Current = Voltage/Resistance or I = V/R Ohm’s Law: Sample Problems How much current flows in a 1000-ohm resistor when 1.5-volts are impressed across it? V = 1.5V R = 1000Ω Ohm’s Law: Sample Problems How much current flows in a 1000-ohm resistor when 1.5-volts are impressed across it? V = 1.5V R = 1000Ω Ohm’s Law: Sample Problems How much resistance allows an impressed voltage of 6-Volts to produce a current of 0.006-Amps? V = 6V R=? Ohm’s Law: Sample Problems How much resistance allows an impressed voltage of 6-Volts to produce a current of 0.006-Amps? V = 6V Ohm’s Law: Sample Problems V=? Ohm’s Law: Sample Problems V = 45V Electric Power Electric Power How much power is being dissipated across a 10Ω resistor when it draws 3A of current form a 30V battery? V = 30V P = IV = (3A)(30V) I = 3A = 90W R = 10Ω , P = 90W Electric Power Electric Power What is the power when a voltage of 60V drives a load with a resistance of 20Ω? Electric Power How much current does a100W lamp draw when connected to 120 V? Electric Power Kilowatt-Hour (kWh): The amount of energy consumed in 1 hour at a rate of 1 kW Example: In a place where energy is 5¢ per kWh, a 100 W light bulb can be lit for 10 hours for 5¢ Jacksonville Electric Authority JEA charges 12¢ per kWh if you use less than 2,750 kWh a month and 13¢ if you use more than 2,750 kWh a month. Electrical Tools Voltmeter Ammeter Measures potential difference Measures the flow of charge Ohmmeter Measures the resistance to the flow of charge Ohm’s Law and Electric Shock The current flowing through the human body depends on two factors: Voltage applied to the body Resistance of the body The resistance of the human body could range from 100W (soaked in salt water) to 500,000W (very dry) The lower the resistance, the greater the shock Ohm’s Law and Electric Shock Current Effect 0.001 A (1 mA) Can be felt 0.005 A (5 mA) 0.010 A (10 mA) Painful Involuntary muscle contractions (spasms) 0.015 A (15 mA) Loss of muscle control 0.070 A (70 mA) If through the heart, serious disruption. More than 1 second, probably fatal End