applied physics chap 61 electricity and magnetism

Applied Physics Chap 6 1

Electricity and Magnetism


Video: Electricity


Static Electricity: Electrical charges that collect on the surface of some object.

Static electricity is most noticeable when:

Air is dry

Friction is involved.


Discovery of electricity

The Greek work for Amber is “Electron” hence our word “electricity”

Early experimenters knew that a rubber rod rubbed with fur could accumulate static electricity

Similarly, a glass rod rubbed with silk would accumulate static electricity

Amber: a clear, resinous, yellowish rock, known since ancient times, made of petrified tree sap would build up static electricity when rubbed on fur.

Amber


Benjamin Franklin

Believed that all things held electricity.

Friction rubbed electricity off of the rubber rod and onto the fur. Thus the rod had less or “negative” electricity and the fur had more or “positive” electricity.

Franklin determined that charges moved from positive, having more electricity to negative, having less.

American scientific experimenter.


Charge: the fundamental electrical property to which the mutual attractions or repulsions between electrons or protons is attributed.

Electrical Charge comes in two forms:

Positive: carried by particles called Protons that are usually found in the nucleus of an atom.

Negative: carried by particles called Electrons that are usually found in a cloud surrounding an atomic nucleus.

Electrical Charge


Electrically charged particles exert forces on other electrically charged particles.

Like charges repel

Unlike Charges attract

– +

Unlike

+ +

Like


Charge: a fundamental electrical property which causes like charges to repel and unlike charges to attract.

Atoms have a nucleus containing positively charged protons.

The nucleus is surrounded by a cloud of negatively charged electrons.

The protons mass is 2000 times more massive than the electron’.

The protons positive electrical charge is of equal magnitude but opposite sign from the electrons electrical charge.

Conservation of Electrical Charge: Charge can be transferred from one object to another but the total number of charges always remains unchanged.

Charge can neither be created nor destroyed.


Electrical Force: a force that one charge exerts on another. when the charges are the same sign, they repel; when the charges are opposite, they attract.

+ -

+ +

Like Charges Repel

-+

Unlike charges Attract


The strength of the electrical force

The strength of the electrical force decreases as the inverse of the square of the distance between the two charges

- -+ +Stronger Weaker

-+++

- -- -+ -+ +

and gets stronger as the amount of charge increases.


Electrical Charge Induction

Induction: the charging of an object without direct contact.

Like charges repel and unlike charges attract even when objects are not touching.


Van de Graff Generator: a device that uses friction to build up a charge on surface of the hallow metal sphere.

1.) Hallow Metal sphere

2.) Copper brush

4/5.) Rubber belt

7.) drive motor

8.) Wand

9.) Spark.


Electric Field: a “force field” that fills the space around every electric charge or group of charges.

Electrical fields surround all charged particles and interact with the electrical fields of other charged particles causing push (repulsion) or pull(attraction) forces to occur.

Michael Faraday: 1791 –1867 an English experimenter who first proposed the existence of electrical fields and discovered that electrical and magnetic force fields were related.


Electroscope: A device that uses electrical fields and induction to detect nearby electrical charges.

Negative charges on the rubber rod push away negative charges on the electroscope causing the gold leaves to repel

When charged the two foil leaves are pushed far apart since both leaves carry the same charge.


Winds carry charged particles of ice upward causing the upper cloud to become positively charged and the lower cloud to be negatively charged

The cloud bottom induces a positive charge on the ground. When the charges grow large enough we get lightning.

Induction in Thunderstorms


Conductors and Insulators:

In metals, free electrons can move freely through the metal without being attracted by other atoms.

Insulator: a material, usually a non-metal, like rubber, glass or plastic that is a poor conductor of electricity.

In insulators, free electrons are strongly attracted by adjacent atoms and cannot move freely.

Conductor: a material, usually a metal, through which electrons can move without hindrance.


Electrical Potential Energy: energy is stored as a result of the distance between two or more electrical charges and their strengths.

Electric Potential: the potential energy available per unit of charge, measured in “volts”

Coulomb

Joules

eargch

energypotentialelectricPEe

Is equal to an electrical potential of 1 Volt

Voltage: the difference in electric potential between any two points in an electric field.

Voltage = Vneg –Vpos


Voltmeter: An instrument used to measure the difference in electric potential between two points

Voltmeters are connected in parallel with voltage sources.


Selector switch

Voltage Scale


Andre Ampere (1775 – 1836)

was a French physicist who is generally credited as one of the main discoverers of electromagnetism. The SI measurement unit for electrical current, the ampere, is named after him.

Electric current: is a measurement of how many electrical charges are moving through a conductor each second in units called Ampere’s,

Ampere1C second1

Coulomb1

time

chargeelectricurrent


Voltage sources and Batteries:

Storage batteries accumulate charge from a chemical reaction. An electrolyte, usually an acid, reacts with the different metal electrodes located at the two ends of the battery.

Negative electrons accumulate at the Cathode (the negative electrode) and positively charged atoms accumulate at the anode (the positive electrode).

This potential difference between the positive and negative charges at the two electrodes creates the voltage.

The positive anode is said to be at 0 volts.


Wet Cell Battery

Automobile batteries have a liquid electrolyte solution composed of water and Sulfuric Acid, with lead and zinc electrodes.


Dry Cell battery

DRY CELL” batteries or Flashlight type batteries are sealed at the factory and have a thick paste that serves as the electrolyte.

Dry cells are portable and can be readily carried and changed whenever they discharge.


Electric resistance: the amount of resistance a material offers to the flow of electric charges moving through it. The lower the resistance, the smaller the voltage needed to push electrons to through it.


The resistance, R, of a length of wire is described by the

expression: R = ρL/A where

ρ = resistivity of the material composing the wire,

L = length of the wire in metersA = area of the conducting cross section of

the wire in m2.

Calculating wire resistance


Reading Resistor Codes.

Small resistors have a color code that describes their resistance

First Band10’s place

Second BandOne’s place

Third BandPower of 10

Fourth Bandtolerance

For example: a green first band means “50

A black second band means 0

A red third band means multiply by 102 or 100

Resistance = 50 x 100 or5000


Resistor tolerances:

Since resistors are very small, it is difficult to manufacture an exact resistance amount.

Tolerance gives the highest and lowest resistance that a given resistor may actually have.

Nominal value: the target resistance of a resistor say 5000.

A tolerance of 5% means that an acceptable resistor may actually be 5000 + 5% = 5000 + 250 = 5250 or

5000 – 5% = 5000 – 250 = 4750

Resistors are available with tighter tolerances but they are much more expensive.


Resistance is measured in units called Ohms.

Georg Ohm 1789 - 1854

As a high school teacher, Ohm worked with a battery invented by Alessandro Volta and equipment of his own invention to define the fundamental relationship among voltage, current, and resistance, which we now know as Ohm’s Law.


Ohm’s Law: Current in a simple circuit

R

VI

Ohms

Volts

Amperes

Resistance

VoltageCurrent

I

VR

R

VIIRV


Direct Current: Negative charges that flow continuously from the negative to the positive electrode of a battery.

Even though the electron moves slowly through the wire, the nudging of one electron on another travels through the circuit almost instantaneously.

The electrons in a light bulb instantly feel the full force of the voltage and react to lighting the light.

Negative charge

Positive charge

Push Pull


Conductor

Circuit: a complete continuous path of conductors through which electrical charges can flow

Voltage Source

Resistance or Load element

Control element or switch


Video: Series and Parallel


Voltage source: a battery or other form of power supply that provides a continuous source of electrical potential.

The symbol for a battery is:

Conductor: a metal wire or other device that creates a continuous path for electrons to flow through.

The symbol for a conductor is:


Load, or Resistance: any device that converts electrical energy into heat, light or motion.

Resistor: is a device that converts electrical energy into heat The symbol for a resistor is:

The symbol for a light bulb is:

Control or switch: an element that can interrupt, stop or reduce the flow of current in a circuit. Symbol for a closed switch is:

Symbol for an open switch is:


Series Circuit: A circuit arranged in such a way that there is only one path for current to flow and charges must flow through each element in turn.

Voltages in a Series Circuit Add

1.5 V + 1.5 V + 1.5 V = 4.5 V

If you reverse a battery then:

1.5 V - 1.5 V + 1.5 V = 1.5 V


Resistances in series: ADD UP

5 + 10 + 15 = 30

Resistances don’t care which way you install them.


Calculating Currents in a series Circuit:

Conductor 30 V

5

10

2.) Use Ohm’s Law to calculate current.

.Amperes215

V30

R

VI

1.) Add up any resistances: 5 + 10 = 15


Voltage drop: the voltage used by each load element in a series circuit. The voltage drop is equal to V = IR where I is the series current and R is the resistance of each load element.

Like voltages, voltage drops in a series circuit add up to the total voltage in the battery.

Kirchoff’s law of voltages

The sum of all voltage drops is equal to the voltage of the battery.


Voltage drops: V = IR

For the 10 resistor, V = (2A)(10 ) = 20 V

For the 5 resistor, V = (2A)(5 ) = 10 V

Total voltage = 30 V

Conductor 30 V

5

10

I = 2A


Series circuits:

Have one single path for current to follow

Voltage is used up by each load element in the circuit.

The sum of all voltage drops is equal to the battery voltage.

Current is the same for all load elements and must flow through all load elements in turn.

Resistance adds in a series circuit.


Parallel Circuits: A circuit with 2 or more paths for current to flow.

30 V 5 10

Each load element sees a full 30V of potential

Each load element has a current calculated as:

.Amperes65

V30

R

VI

.Amperes310

V30

R

VI


Voltage sources in parallel do not add.

1.5 V +

1.5 V +

1.5 V +

= 1.5 V

Currents In each of the parallel paths add to equal the total current: IT = I1 + I2

VI1

I2

IT

Kirchoff’s Law of Currents


Parallel Circuits:

Have two or more paths for current to flow.

Each load element sees full system voltage.

Currents divide to each load element in the circuit.

The sum of all individual currents equals the total system current.

Voltage sources in parallel do not add.


Equivalent Resistance: a resistance equal to the effect of placing two or more resistances in parallel.

Equivalent resistance is usually around half of the smaller of the parallel resistances.

Equivalent resistance: of two resistors R1 and R2 is found using the formula:

3333

1

12

1

101

51

111

1

21

....

RR

RT

X15 + 10

X1 =

X1

Calculator:


Calculating Currents in a Parallel Circuit

30V 5 10

Use Ohm’s law to find the current in each parallel path:

For the 2 resistors:

A

IIIcurrentTotal

AV

R

VI

AV

R

VI

T

963

310

30

65

30

21

22

11

applied physics chap 61 electricity and magnetism

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