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16.3 Ohm’s Law / Energy and Power / Electric Meters Voltage Within a battery, a chemical reaction occurs that transfers electrons from one terminal to another terminal. This potential difference across the terminals is called the voltage. Ohm’s Law Voltage produces a flow of charge, or current, within a conductor. The flow is restrained by the resistance it encounters. The rate at which energy is transferred by electric current is power. Ohm’s Law The relationship among voltage, current, and resistance is called Ohm’s Law. Ohm’s Law states: Current in a circuit is directly proportional to the voltage across the circuit, and is inversely proportional to the resistance of the circuit. Ohm’s Law Voltage (Volts) (Joules / Coulomb) Current (Amperes) (Coulomb / Sec) Drop across a resistive device Current passing through a resistive device Resistance (Ohms Ω) Ohm’s Law Ohm’s Law Ohm’s Law Example #1 The current in a wire is 24 amperes when connected to a 1.5 volt battery. Find the resistance of the wire. R = 0.0625 Ω Ohm’s Law Example #2 In a simple electric circuit, a 24 Ω resistor is connected across a 6 volt battery. What is the current in the circuit? I = 0.25 A Ohm’s Law Example #3 A high-beam filament of an automobile headlight carries a current of 4.5 amps. The voltage difference between its terminals is 12 V. Calculate the resistance of the filament. R = 2.667 Ω Ohm’s Law Example #4 Calculate the current when a 12-V battery is connected across a 4 Ω resistor. I=3A Ohm’s Law I=2A V = 12 V R = 6Ω I=4A V = 12 V R=3Ω Electric Circuits Electric Circuits In an electric circuit, an energy source and an energy consuming device are connected by conducting wires through which electric charges move. Electric Circuits Electric Circuits are typically represented using diagrams know as schematics. Schematics are simplified, standard representation in which common circuit elements are represented with specific symbols, and wires connecting the elements in the circuits are represented by lines. Circuit Symbols Electric Circuits In order for current to flow through a circuit, you must have a source of potential difference. Typical sources of potential difference are batteries (which are just two or more cells connected together), and power supplies (electron pumps). In drawing a cell or battery on a circuit schematic, remember that the longer side of the symbol is the positive terminal. Electric Circuits Electric circuits must form a complete conducting path (closed loop) in order for current to flow. In the example circuit shown below left, the circuit is incomplete because the switch is open, therefore no current will flow and the lamp will not light. Electric Circuits In the circuit below, the switch is closed, creating a closed loop path. Current will flow and the lamp will light up. Energy and Power Just like mechanical power is the rate at which mechanical energy is expended, electrical power is the rate at which electrical energy is expended. Formula(s): P IV 2 P I IR I R V V P V R R 2 SI Unit of Power: Watts (W) Energy and Power Example #1 A 110 volt toaster over draws a current of 6 amps on its highest setting as it converts electrical energy into thermal energy. What is the toaster’s power rating? P = 660 W Energy and Power Example #2 A potential drop of 50 volts is measured across a 250 Ω resistor. What is the power in the resistor? P = 10 W Energy and Power Example #3 How much electrical energy is required to move 4µC charge through a potential difference of 6 volts? W = 2.4 x 10-5 J Energy and Power Example #4 What is the resistance of a 60 watt light bulb operated at 120 volts? R = 240 Ω Electric Meters - Voltmeters Voltmeters are tools used to measure the potential difference (voltage) between two points in a circuit. The voltmeter is connected in parallel with the element to be measured, meaning an alternate current path around the element to be measured and through the voltmeter is created. Voltmeters have very high resistance so as to minimize the current flow through the voltmeter and the voltmeter's impact on the circuit. Electric Meters - Voltmeters In the diagram below, a voltmeter is connected to correctly measure the potential difference (voltage) across the lamp. Electric Meters - Ammeters Ammeters are tools used to measure the current in a circuit. The ammeter is connected in series with the circuit, so that the current to be measured flows directly through the ammeter. Ammeters have very low resistance to minimize the potential drop (voltage) through the ammeter and the ammeter's impact on the circuit, so inserting an ammeter into a circuit in parallel can result in extremely high currents and may destroy the ammeter. Electric Meters - Ammeters In the diagram below, a ammeter is connected to correctly measure the current flowing through the circuit. Electric Meters Electric Meters - Voltage Electric Meters - Series Electric Meters Electric Meters Electric Meters Example #1 In the electric circuit diagram, possible locations of an ammeter and voltmeter are indicated by circles 1, 2, 3, and 4. Where should the ammeter be located and where should a voltmeter be located to correctly measure the total current and voltage? Electric Meters Example #2 Which circuit diagrams below correctly shows the connection of ammeter A and voltmeter V to measure the current through and potential difference across resistor R? Electric Meters Example #3 A student uses a voltmeter to measure the potential difference across a resistor. To obtain a correct reading, the student must connect the voltmeter: a. b. c. d. In series with the resistor In parallel with the resistor Before connecting the other circuit components After connecting the other circuit components