PHY 110, Introduction to Physics

Dr. Henry SC 453, 572-6164 (alt 572-5309)

E-Mail: henryh1@nku.edu

Web Site: www.nku.edu/~henryh1/

© Hugh Henry, 2008

If you are Tardy

Initial beside Your Name on the Sheet on the Front Table

Chapter 7

Electricity

© Hugh Henry, 2008 Electricity

4

Electric charge

Electric charge is an inherent physical property of certain subatomic particles that is responsible for electrical and magnetic phenomena. Charge is represented by the symbol q. The SI units of charge is the coulomb (C).

Like mass, electric charge is a fundamental property of matter. Unlike mass, there are two types of charge:

Positive and Negative.

5

Electric charge, cont’d

Recall that every atom is composed of electrons surrounding a nucleus. The nucleus contains:

protons — positive charge. neutrons — no charge.

Electrons — negative charge. An atom’s atomic

number is the number of protons in the nucleus.

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Electric charge, cont’d

Consider how remarkable it is that the electron charge and proton charge are virtually identical

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Electric charge, cont’d

An atom’s charge is neutral if it has the same number of protons and electrons.

An atom is said to be ionized if the number of electrons and protons is different.

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Electric charge, cont’d

If the protons outnumber the electrons, there is more positive charge. We call such an atom a positive ion.

If the electrons outnumber the protons, there is more negative charge. We call such an atom a negative ion.

Negative Ion Positive Ion

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Electric charge, cont’d

Compounds dissolved in liquid break into oppositely charged ions

Table Salt dissolved in water

10

Electric force and Coulomb’s lawLike charges repel, unlike charges attract. When we bring electric charges together, they

exert a force on each other. Positive charges attract negative charges. Positive charges repel positive charges. Negative charges attract positive charges. Negative charges repel negative charges.

11

Electric force and Coulomb’s law, cont’d

Coulomb’s law describes the forces between electric charges.

Coulomb’s Law states that the force acting on each of two charged objects is directly proportional to the net charges on the object and inversely proportional to the square of the distance between them:

12

Electric force and Coulomb’s law, cont’d

The force on q1 is equal and opposite to the force on q2. If the charges increase, the force increases. If the distance increases, the force decreases.

13

Electric force and Coulomb’s law, cont’d

The proportionality constant has a value of 9×109 N-m2/C2.

Coulomb’s law is then:

F is in newtons, q1 & q2 are in coulombs, and d is in meters.

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Electric force and Coulomb’s law, cont’d

The electrostatic force is similar to gravity.

However, the electric force constant is 100 trillion trillion times larger than gravity:Gravitational Force constant = 7 * 10-11

Electrical Force constant = 9 * 109

15

Electric field

Just as we talk about the gravitational field, we can also define an electric field.

The electric field lines indicate the direction of the force on a positive charge.

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Electric field, cont’d

If the field points to the right: The force on a positive charge is to the right. The force on a negative charge is to the left. The charges act as if there is a positive

charge at the start

of the line.

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Electric field, cont’d

An electrostatic precipitator (or your furnace electrostatic air cleaner) uses electrostatics. Negatively charged wire charge the dirt

passing between the plates.

The now negative dirt particles are attracted to the positively charged plates.

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Electric field, cont’d

Lightning is caused by charge buildups in a thundercloud When the charge concentrations get large

enough, electricity can break across

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Electric currents

An electric current is a flow of charged particles. It is the rate of flow of electric charge. The amount of charge that flows by per

second:

The SI unit is the ampere (A or amp): 1 amp is 1 coulomb per second.

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Electric currents, cont’d

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Electric currents, cont’d

Here are some examples of electric current. Positive current is in the direction of positive

charge flow – which can be confusing, because most current flow is negative electrons

a) Electrons flow through metal wire

b) Electrons flowthrough vacuum

c) Ions flow through a liquid

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Resistance

Resistance is a measure of the opposition to current flow. Resistance is represented by R. The SI unit of resistance is the ohm (). A conductor is any substance that readily

allows charge to flow through it. An insulator is any substance through which

charge does not readily flow.

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Resistance, cont’d

Resistance of a wire depends on: Composition. The particular substance from

which the object is made. Length. The longer the wire, the higher the

resistance. Diameter. The thinner the wire, the higher the

resistance. Temperature. The higher the temperature, the

higher the resistance.

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Resistance, cont’d

Resistance is similar to friction. Resistance inhibits the flow of electric charge. Electrons typically produce the current in

metals. The electrons collide with the atoms of the

metal, which slows them down. They also lose some energy to the atoms.

The metal gets hotter.

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Resistance, cont’d

Superconductivity is a phenomenon which occurs at temperatures near 0oK Certain substances provides zero resistance

to the flow of electric charge. The temperature at which a

substance superconducts is its transition temperature.

This graph shows the superconducting transitionfor Hg, ~4.2K (-269ºC).

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Electric current and Ohm’s law

An electric current will flow through a wire only if an electric field is present to exert a force on the charges. A device providing the electric field – and

therefore the force – is called a power supply. The power supply forces

charges out one terminal, through the wire, and back into the second terminal.

Car battery is shown here as a power supply

27

Electric current and Ohm’s law, cont’d

Voltage is the work that a charged particle can do divided by the size of the charge. It is the energy per unit charge given to

charged particles by a power supply.

The SI unit is the volt (V). One volt equals one joule per coulomb.

28

Electric current and Ohm’s law, cont’d

The flow of charge in an electric circuit is similar to the flow of water through a closed path. The power supply acts like the

water pump. It adds energy to make the

current flow. The resistance corresponds to

the narrow section of pipe. The current is like the flow of

water.

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Electric current and Ohm’s law, cont’d

Ohm’s law specifies that the current in a conductor is equal to the voltage applied to it divided by the resistance:

V is the voltage through the conductor, I is the current passing through the conductor,

and R is the conductor’s resistance.

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Electric current and Ohm’s law, cont’d

According to Ohm’s law, V = IR

Larger Voltage is produced by Same Resistance

with Larger Current Larger Resistance

with Same Current

31

Electric current and Ohm’s law, cont’d

According to Ohm’s law,

Larger Current is produced by Same Resistance with Larger Voltage

Smaller Resistance with Same Voltage

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Electric current and Ohm’s law, cont’d

Electric Circuits are represented schematically by boxes (left) or symbols (right)

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ExampleExample 7.1

A light bulb in a 3-volt flashlight has a resistance of 6 ohms. What is the current in the bulb when it is switched on?

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ANSWER:

The problem gives us:

The current through the bulb is

ExampleExample 7.1

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ExampleExample 7.2

A small electric heater has a resistance of 15 ohms. What voltage is required to produce a current of 2 amperes?

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ANSWER:

The problem gives us:

The necessary voltage is

ExampleExample 7.2

37

Electric current and Ohm’s law, cont’d

A series circuit has only one path for the current to flow. The voltage across the first bulb, plus the

voltage across the second, etc., equals the voltage of the battery.

The current through each bulb is the same as the current passing through the battery.

38

Electric current and Ohm’s law, cont’d

Every electric circuits must be a closed loop If the series circuit is interrupted, then current

no longer flows through the circuit. If one bulb goes bad, then all the bulbs go out.

39

Electric current and Ohm’s law, cont’d

A parallel circuit has multiple paths for the current to flow. The current through the first bulb, plus the

current through the second, etc., equals the current through the battery.

The voltage on each bulb is the battery voltage.

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Electric current and Ohm’s law, cont’d

If one bulb goes out, the other bulbs remain lit. There is still a closed path for the electricity to

flow through the circuit.

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Electric current and Ohm’s law, cont’d

A 3-way light bulb is a combination of switch-selected series and parallel circuits

50 W: A closed, B open

100 W: A open, B closed 150 W: A closed, B closed

(Parallel Circuit)

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ExampleExample 7.3

Three light bulbs are connected in a parallel circuit with a 12-volt battery. The resistance of each bulb is 24 ohms. What is the current produce by the battery?

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ANSWER:

The problem gives us:

Since the bulbs are connected in parallel, they each have the same voltage.

So the current through each bulb is:

ExampleExample 7.3

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ANSWER:

The total current is the sum of the currents through each bulb.

There are three bulbs, so:

ExampleExample 7.3

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ExampleExample 7.3A

Three light bulbs are connected in a series circuit with a 12-volt battery. The resistance of each bulb is 24 ohms. What is the current produce by the battery?

46

ANSWER:

The problem gives us: V = 12VSince the bulbs are connected in series, the

current is the same through all the bulbs.

We add the resistance of each bulb to get the total resistance:

R = 24Ω + 24Ω + 24Ω = 72Ω

ExampleExample 7.3A

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Practical Electric currents and Ohm’s law

Current I is measured with an Ammeter in Series with the Load

Voltage V is measured with a Voltmeter in Parallel with the Load

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Practical Electric currents and Ohm’s law, cont’d

Resistor Color Codes show resistance and tolerance

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Human“Electrical System”

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ECG (Electrocardiogram) OR

EKG (Elektrokardiogram)

Human“Electrical System,” cont’d

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Cardiac Pacemaker

Human“Electrical System,”

cont’d

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Power and energy in electric circuits The power output of a circuit is the rate at

which energy is delivered to the circuit.

Rewrite the equation, multiplying 1 (q/q): This represents the energy given to each

coulomb of charge, multiplied by the number of coulombs that pass per second.

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Power and energy in electric circuits, cont’d

Since: The energy given to each

coulomb of charge is voltage.

The number of coulombs that pass per second is the current.

The power output expression

≡ VI

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Power and energy in electric circuits, cont’d

Hence power through a circuit can be written

And since from Ohm’s Law, V=IR this can also be written as

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Power and energy in electric circuits, cont’d

The electrical resistance of many substances causes them to get hot.

This type of heating is called ohmic heating.

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ExampleExample 7.4

In Example 7.1, we computed the current that flows in a flashlight bulb. What is the power output of the batteries?

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ANSWER:

The problem gives us:

So the power consumed is

ExampleExample 7.4

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DISCUSSION:

This means the batteries supply 1.5 joules of energy each second.

ExampleExample 7.4

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Power and energy in electric circuits, cont’d

The current through the filament causes it to get very hot. The filament is a very

thin wire, which hence has higher resistance.

The thicker supporting wires have lower

resistance and do not

get as hot.

60

ExampleExample 7.5

An electric hair dryer is rated at 1,875 watts when operating on 120 volts. What is the current flowing through it?

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ANSWER:

The problem gives us:

The power is given by

Since we want the current, we divide by V:

ExampleExample 7.5

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DISCUSSION:

The wiring in the house and the hair dryer’s cord must be large enough to handle 15.6 A without overheating.

ExampleExample 7.5

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Power and energy in electric circuits, cont’d

Recall our definition of power: The change in energy per unit time.

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Power and energy in electric circuits, cont’d

So we can write the energy in terms of the power:

The energy change equals the rate at which energy is transferred times how long it is transferred. So if we know the power a circuit uses and

how long the circuit operates, we can determine the energy used by the circuit.

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Power and energy in electric circuits, cont’d

Power companies do not charge for power. They charge for energy. (Duke Power Duke Energy)

A kilowatt-hour is energy.

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ExampleExample 7.6

If the hair dryer discussed in Example 7.5 is used for 3 minutes, how much energy does it use?

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ANSWER:

The problem gives us:

The energy is given in terms of power as:

ExampleExample 7.6

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DISCUSSION:

This amount the energy will also: Accelerate a small car to 60 mph, or Brew 5 cups of coffee.

ExampleExample 7.6

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Power and energy in electric circuits, cont’d

Compact Fluorescent Lights (CFL’s) are being pushed to save energy But these bulbs contain

mercury – a hazardous material – and if they are broken, they almost require a HazMet team to clean up

There is an alternative . . .

http://www.youtube.com/watch?v=o6RJGQvpd60

Made

in C

HINA

70

Power and energy in electric circuits, cont’d

Light Emitting Diodes (LED’s) have been used for low level lights for years.

LED’s with the power required for home lighting are available.

New technology may cut the current cost ($39-49) in half.

http://www.technologyreview.com/computing/35132/?p1=MstRcnt

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AC and DC

There are two types of current flow: direct current (DC) represents current flow

that is always in the same direction. Batteries provide DC.

alternating current (AC) represents current flow that alters direction periodically.

Wall outlets provide AC.

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AC and DC, cont’d

Here is an example of DC provided by a battery to a light bulb. The current always passes from the positive

battery terminal, through the bulb, and then to the negative terminal.

The current is constant in time.

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AC and DC, cont’d

Here is an example of AC provided by a wall outlet to a light bulb. The current always passes from the positive

terminal, through the bulb, and then to the negative terminal.

But the terminals swap position over time.

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AC and DC, cont’d

So the current passing through the bulb changes direction. Our wall outlets operate at 60 Hz. So the current changes direction 120 times

each second.

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Thomas Edison used – and lobbied for acceptance of – Direct Current (DC)

But today we use AC power in our homes – supplied at a standard voltage of ~120 volts, and a frequency of 60 Hz.

Why do we use AC today?

Edison’s Workshop, Greenfield Village

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AC and DC, cont’d

The big advantage of AC over DC is the transformer, a device that can increase or decrease AC voltage. AC can be transformed to high voltage for

power transmission . . . DC cannot

(Voltage transformation discussed, chapter 8)

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AC and DC, cont’d

It is more efficient for the power company to use very high voltage and low current to transmit power.

Cross-country power lines use several hundred-thousand volts, allowing smaller currents to be used.

With smaller current, smaller wires can be used without fear of excessive ohmic heating. This saves money of cable, the supporting structures, etc.

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AC and DC, cont’d

Because of the transformer, AC voltage can be stepped up for efficient transmission – including transmission across power grids – then stepped down for use in our homes. The same amount of power requires 2875 times

more current at 120 VAC than at 345,000 VAC

Electrical Safety

80

We noted earlier that the wiring in the house and cord of an 1875 Watt hair dryer must be large enough to handle 15.6 A without overheating.

With a 20 A circuit, only 4.4 A is left for all other appliances

Electrical Safety, cont’d

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This circuit includes appliances pulling more than 20 A. This can cause the

wires to overheat and cause a fire.

Fuses and circuit breakers are installed to stop current flow if this happens.

Electrical Safety, cont’d

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Electrical Safety, cont’d

This fuse uses ohmic heating to monitor the current through the circuit. If too much current flows, the fuse gets too

hot, and a small wire in the center melts, breaking the circuit.

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Electrical Safety, cont’d

Circuit breakers use bimetallic strips to protect from ohmic overheating If too much current flows, the strip bends,

breaking the circuit and tripping the switch.

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Electrical Safety, cont’d

This 2280W heater requires 19A, but the extension cord is rated for only 10A.

This is a fire hazard!!

2280 watt heater

10 A extensioncord

Cords (and extension cords) must be sized to carry the current without overheating

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Electrical Safety, cont’d

They do not protect against undersized extension cords

2280 watt heater

10 A extensioncord

Fuses and breakers are matched to the current capacity of the internal home wiring

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Electrical Safety, cont’d

3-wire grounded plugs prevent electric shock

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Important Equations, Chapter 7

Understand the relationships reflected in Coulomb’s Law:

END