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.

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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,

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Q

t

QVP

Since V = IR, the above equation can also be written as,

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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

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jouleWwatt