13 Electric Circuits

Chapter Outline

1 Electric Circuits and Electric Current

2 Ohm’s Law and Resistance

3 Series and Parallel Circuits

4 Electric Energy and Power

5 Alternating currents and Household Current

Everyday Phenomenon:

The Hidden Switch in Your Toaster

How to Get the Bulb to Light?

How to Get the Bulb to Light?

Electric Current

The electric current 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.

.t

qI

Direction of Current Flow

Electric current is a flow of electrons. In a circuit, electrons 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.

Direction of Current Flow

What Limits the Flow of Current?

What Limits the Flow of Current?A: Resistance

Electric Current Is Analogous to Water Flow

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:

V = I R

What Is the Current?

Electromotive Force (emf)The energy needed to run electrical devices 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.

Circuits

Series Circuit

Parallel Circuit

Try This Box 13.2

Use of Voltmeter

Voltmeter is connected across the resistance.

Use of Ammeter

Ammeter is inserted into the circuit to measure current.

Electrical Energy

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

Exercises1. State Ohm’s law in an equation form in terms of voltage and current.

2. Define power in an equation form in terms of voltage and current.

3. When an appliance is plugged in a 120-volt outlet, it draws a current of 8 amperes. Calculate the power of the appliance.

4. If the above appliance is used 10 hours a day for 28 days per month, and if the cost of electricity is 12 cents per kilowatt‑hour, how much does it cost to operate the appliance for a year?

Electrical Power Transmission

AC adapter

                      

INPUT: AC 120 V, 60 Hz, 15 W

OUTPUT: DC 9V, 1A

Alternating Current

Alternating Voltage

Effective voltage = 115 V

Household Circuits

Power and Current Ratings of some common Appliances

Appliance Power (W) Current (A)

Stove 6000 (220V) 27

Clothes dryer 5400 (220V) 25

Water heater 4500 (220V) 20

Clothes washer 1200 10

Dishwasher 1200 10

Iron 1100 9

Coffeemaker 1000 8

TV 100 0.8

Everyday Phenomenon

Bimetallic Thermostat

CP5