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Chapter 7 Sound Recording and Reproduction 1Electric Circuits 2AC and DC 3Alternating current and voltage 4Electric Current 5Resistance 6Voltage 7Ohm’s Law 8Series and Parallel Circuits 9Electric Energy and Power

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Electric Circuits Demonstration: AC versus DC with a Capacitor.

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AC adapter INPUT: AC 120 V, 60 Hz, 15 W OUTPUT: DC 9V, 1A

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Direct and Alternating Current Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC).

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Alternating Current

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Alternating Voltage Effective voltage = 115 V. Demo: Measure the outlet voltage.

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How to Get the Bulb to Light?

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Electric Current The electric current, I is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges. The SI unit of electric current is the ampere (A), after the French mathematician André Ampére (1775-1836). 1 A = 1 C/s. Ampere is a large unit for current. In practice milliampere (mA) and microampere (μA) are used.

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Direction of Current Flow Electric current is a flow of electrons. In a circuit, electrons (negatively charged) actually flow through the metal wires. Conventional electric current is defined using the flow of positive charges. It is customary to use a conventional current I in the opposite direction to the electron flow.

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Q: What Limits the Flow of Current? A: Resistance Electrical resistance is an electrical property that limits the current flow when a voltage is applied to it.

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Electric Current Is Analogous to Water Flow

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Electromotive Force (emf) or Voltage The energy needed to operate an electrical device comes from batteries. Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to another terminal (leaving it negatively charged). Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. The maximum potential difference is called the electromotive force* (emf) of the battery. The electric potential difference is also known as the voltage, V. The SI unit for voltage is the volt, after Alessandro Volta (1745- 1827) who invented the electric battery. 1 volt = 1 J/C.

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Ohm’s Law Georg Simon Ohm (1787-1854), a German physicist, discovered Ohm’s law in 1826. This is an experimental law, valid for both alternating current (ac) and direct current (dc) circuits. When you pass an electric current (I) through a resistance (R) there will be a potential difference or voltage (V) created across the resistance. Ohm’s law gives a relationship between the voltage (V), current (I), and resistance (R) as follows: Voltage = Current X Resistance V = I R

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Units QuantitySymbolUnit Name Unit Abbreviation CurrentIampereA VoltageVvoltV ResistanceRohmΩ PowerPwattW EnergyEjouleJ

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What Is the Current?

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Circuits

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Series Circuit

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Parallel Circuit Demonstration: Series and Parallel circuits

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Household Circuits

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Electrical Energy and Power Our daily life depends on electrical energy. We use many electrical devices that transform electrical energy into other forms of energy. For example, a light bulb transforms electrical energy into light and heat. Electrical devices have various power requirements. Electrical power, P is defined as the electrical energy transfer per unit time,

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Electric Power: Since the electrical energy is charge times voltage (QV), the above equation becomes, Since the current is charge flow per unit time (Q/t), the above equation becomes, Since V = IR, the above equation can also be written as,

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Killowatt-hour (kWh) The SI unit of power is watt, after James Watt (1736- 1819), who developed steam engines. Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatt- hour, kWh is the energy consumed for one hour at a power rate of 1 kW.

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