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Chapter 7 Sound Recording and
Reproduction1 Electric Circuits 2 AC and DC3 Alternating current and voltage4 Electric Current5 Resistance6 Voltage7 Ohm’s Law8 Series and Parallel Circuits9 Electric Energy and Power
Electric Circuits
Demonstration: AC versus DC with a Capacitor.
AC adapter
INPUT: AC 120 V, 60 Hz, 15 W
OUTPUT: DC 9V, 1A
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).
Alternating Current
Alternating Voltage
Effective voltage = 115 V.
Demo: Measure the outlet voltage.
How to Get the Bulb to Light?
How to Get the Bulb to Light?
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.
.arg
Time
eChI
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.
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.
Electric Current Is Analogous to Water Flow
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.
Ohm’s LawGeorg 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
UnitsQuantity Symbol Unit
NameUnit Abbreviation
Current I ampere A
Voltage V volt V
Resistance R ohm Ω
Power P watt W
Energy E joule J
What Is the Current?
Circuits
Series Circuit
Parallel Circuit
Demonstration: Series and Parallel circuits
Household Circuits
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,
.time
EnergyP
Electric Power:.
time
EnergyP
Since the electrical energy is charge times voltage (QV), the above equation becomes,
.t
QVP
Since the current is charge flow per unit time (Q/t), the above equation becomes,
.VIVt
Q
t
QVP
Since V = IR, the above equation can also be written as,
.2
2
R
VRIIVP
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.
.sec s
J
ond
jouleWwatt