Electric Currents and Charges · Force of repulsion pushes objects apart. For comparison, this...

Electric Currents and Charges

Chapter 6

Electricity

• The conventional definition of electricity as a practical use is…

the phenomenon of energy transformation associated with electrons that have moved from one place to another.

Electricity:

However, the science of electricity must be more

precisely defined by the following terms:

Electric field – an influence produced by an electric charge.

Electric charge - the property of subatomic particles that give rise to

the electromagnetic force.

Electromagnetism – the interaction between the magnetic field and

the motion of an electric charge.

Electric current – the flow of electrically charged particles (most

often electrons).

Electric potential – the capacity of an electric field to do work.

Early History of Electricity:• Egyptians, ca. 2750 BC, referred to certain

electric fish as “Thunderer of the Nile”, indicating an awareness of electric shock.

• Thales of Miletus, ca. 600 BC, made the first observations that amber (electricus is Latin for “amber-like”) rubbed on cat’s fur gave rise to magnetic properties.

• The Parthians, ca. 600 – 330 BC, may have had knowledge of electroplating based on a 1936 discovery of a device in Babylonia.

Review of Subatomic Particles:

1. Protons have a positive

electrical charge. (p+)

2. Electrons have a negative

electrical charge. (e-)

3. Neutrons have a neutral

electrical charge. (n0)

Review of Electromagnetic

Forces:

1. Force of attraction pulls objects

together.

2. Force of repulsion pushes

objects apart. For comparison,

this force of repulsion is 1042

times stronger than the pull of

gravity.

--Like charges repel, unlike charges

attract.

• An atom that has a balanced sum of protons

and electrons is electrically neutral and is

not attracted to objects.

In some materials, electrons are weakly

held and can be removed.

Only electrons can easily move from atoms.

• An object gaining electrons will have a net

negative electrical charge.

• An object losing electrons will have a net

positive electrical charge.

– Any atom with an unbalanced number of

electrons will possess an electric field or region

surrounding a charged particle in which an

electric force affects other charged particles.

– The strength of an electric field depends on the

distance from the charged particle.

– I.e. Like gravity (although much stronger), as

the distance from a charged particle increases,

the strength of its electric field decreases.

– This is why/how refrigerator magnets work!

Static Electricity:

• Static electricity is the buildup of electric charges

on an object.

– I.e. Electricity at rest.

Methods of Static Electric Charging:

1. Friction removes electrons from an object onto another.

e.g. Walking on wool carpet will remove electrons onto you.

2. Conduction is electric charging by direct contact with a charged object.

e.g. Touch someone after walking on wool carpet and static electricity is conducted to that person.

-In heat conduction, metals are good conductors and wood, plastic, rubber and glass are poor conductions (insulators)

-Static free rooms are coated with vinyl or glass.

https://phet.colorado.edu/sims/html/john-travoltage/latest/john-travoltage_en.html

Methods of Static Electric Charging, cont.:

3. Induction is the rearrangement of electric charges.

-Electrons in a neutral object are moved by the influence of a nearby charged object - direct contact is not necessary.https://phet.colorado.edu/sims/html/balloons-and-static-electricity/latest/balloons-and-static-electricity_en.html

-E.g. Lightning rods attract lightning because the metal rod becomes positive when the electric field of a nearby storm cloud forces the electrons to rearrange.

Lightning:

• Lightning is a powerful static electric

discharge.

Lightning:

• Steps to produce lightning:

1. From 15,000 to 25,000 ft above sea level, electric charges form from turbulent winds in a matrix of water droplets and ice crystals.

2. The electric charge becomes concentrated in a region of a cloud.

3. The charge moves downward from the cloud in a series of 50-m stepladders.

4. Eventually it encounters a conductor on the ground and creates a large spark between the charge and the ground!

Facts about Lightning:

• The fear of lightning is called astraphobia.

• Lightning tends to be attracted to:

-Metals: induction occurs easily, creating a positively charged ground.

• Benjamin Franklin, ca. 1752, invented the lightning rod based on this principle.

-Tall objects: electrons take the shortest path from one object to another object.

Facts about Lightning:• An average negative lightning strike can transfer

6.0x1018 electrons.

• The current flow is usually between 10,000 and 200,000 amperes and transfers on average 5 coulombs (500MJ) of energy.

• An average positive lightning carries 10 times more energy than an average negative strike.

• The entire flow of the charge (current) lasts less than ½ second.

• Lightning heats the surrounding air to about 20,000ºC (36,000ºF), over 3 times hotter than the Sun’s surface.

https://youtu.be/uAOSu9LFb3g

Facts about Lightning:• A supersonic shock wave that decays to an acoustic wave

(thunder) forms from the rapid expansion of air from heat

produced by lightning.

• Florida holds the U.S. record for the most cloud to ground

lightning strikes per year.

Electric Potential, Current and Resistance:

• Voltage () is the potential difference between the charges.

-Voltage is a measure of energy available to move electric charges.

-i.e. Voltage is the amount of work that charged particles can do, or the amount of electromotive force (emf or ).

Voc =

Where:

Voc =Voltage on an open circuit

= Electromotive force

Volt (V) is the unit for voltage.

*Electrons cannot move without energy input.

• Current (I) is the flow of charged particles (most often electrons).

-Electrons in motion will remain in motion if they have a path and source.

-A path is any conducting material and a source is any device that transmits electrons.

-Electric current is measured in amperes (A) or the amount of electrons that pass a given point per second.

• Resistance (R): is the opposition to the flow of

charged particles.

-Resistance is measured in ohms (Ω).

-Resistance produces heat, light, or

mechanical energy.

Ohm’s Law

Factors that Affect Resistance:

1. Temperature:

-Temperature has a minor effect on resistance.

-Resistance increases with a rise in

temperature in pure metals and metal alloys.

-Resistance decreases with a rise in

temperature in carbon, semiconductors and

electrolytic solutions (acids or alkalis)

Factors that Affect Resistance:

2. Length:-Resistance of a uniform conductor is directly

proportional to the length of the conductor.

i.e. The longer the conducting wires, the more resistance.

3. Cross-Sectional Area-The resistance of a uniform conductor is inversely

proportional to its cross-sectional area.

i.e. A thick conducting wire has a low resistance;

a thin conducting wire has a high resistance.

Factors that Affect Resistance:

4. Nature of the Materials:

-The resistance of a given conductor depends on

the material of which it was made.

-Insulators have very high resistance.

-Some metals have very low resistance.

e.g. Silver, Copper, Mercury, Aluminum

-Some metals have high resistance.

e.g. Iron, Calcium, Magensium and Tungsten

Producing an Electric Current Using

Electrochemical Cells:

• Count Alessandro

Giuseppe Antonio

Anastasio Volta,

Italian Physicist, 1791,

invented the first

electrochemical cell

using zinc, silver and

sulfuric acid.

Producing an Electric Current Using

Electrochemical Cells:

• All electrochemical cells produce an electric

current by using chemical potential energy.

-A chemical reaction converts chemical energy

to electric energy.

Dry Cell:

• Dry cells contain an acid or alkaline paste

surrounding a carbon core encased in a

metal, usually zinc can.

• Electrons are released in a reaction with the

zinc and paste.

• Acid or alkaline cells are most common in

1.5V potential.

Dry Cell:• A series of cells connected is a battery.

– E.g. Flashlight batteries

Dry Cell:

• Nickel-cadmium rechargeable batteries use

nickel oxide hydroxide paste and cadmium

electrodes, but only carry a potential of

1.2V.

• Lithium-ion rechargeable batteries carry a

potential of 3.6V, but are more volatile,

especially if undercharged.

Wet Cell or Voltaic Cell:

Voltaic Cell Contains:

-Electrodes – usually copper

and zinc or lead plates or

rods.

-Electrolyte – Any ionizing

solution (usually sulfuric

acid, H2SO4).

Wet Cell or Voltaic Cell:

• Zinc reacts with H2SO4

and releases 2 electrons.

• When conductors are

connected to the

electrodes, the excess

electrons from the zinc

(negative pole) are

pushed to the copper

(positive pole).

Thermocouple:

• Thermocouples convert heat energy into electric energy.

• Thermocouples are used as an electrical source for temperature gages and valve switches.

• The differences in specific heat of copper/iron wire creates a low voltage electric current.

Current Direction:

• Direct Current (DC) – Electrons flow in the

same continuous direction.

– E.g. Cell phones, flashlights. From batteries = DC

• Alternating Current (AC) – electrons flow

back and forth through the conductor.

– Your Refrigerator, washer and dryer, oven, lights,

use AC. From the wall = AC

Current Direction:

• Nikola Tesla, 1882, Croatian

born American, is responsible

for the invention of the AC

indcution motor; George

Westinghouse bought Tesla’s

patent and promoted AC use in

the United States.

• Standard AC frequency in the

US is 60Hz (cycles per second).

Electric Circuits:

• Electrons cannot flow unless they have a

closed path.

– i.e. Any break in the current creating an open

path does not allow electrons to flow.

Parts of a Circuit:

1. A source is any generator of electromotive force.

e.g. Battery, Inductive Generator, etc

2. A conducting path is usually a metallic wire.

3. The resistance (load) is any appliance

e.g. Lamp, Machine, or Motor

• In the absence of a load, a short circuit results and often results in a malfunction of the source or path.

4. A switch is necessary to open or close a circuit.

Series Circuit: all devices are connected one after the other. There is only one path.

Advantages:

• Several devices may be switched on simultaneously.

• Less wiring is required.

• Voltage may be divided without the use of a transformer, therefore less energy loss by unwanted resistance.

Disadvantages:

• If a break occurs at any point in the

circuit, the entire circuit is open.

e.g. If one light on a Christmas tree light

string goes out, they all go out!

Parallel Circuit: all devices are on different branches of the path. There is more than one path.

Advantages:

• Any device can be operated independently of other devices.

Disadvantages:

• The circuit requires more conducting paths (more wires involved).

Types of Circuits:

Series and Parallel:

Electric Power:

• Electric power is a measure of the rate of

work (energy) provided by electricity.

Electric Power:• All electric appliances convert electric energy into

light energy, heat energy, and/or mechanical energy.

• Electrical power is rate of this conversion.

• The amount of electricity an appliance uses depends on its power, which is determined by voltage, current flow, and time of usage.

i.e. P = VI

Where: P = Power (W)

V = Voltage (V)

I = Current (A)

Electric Power:• The amount of energy used is calculated by:

e = Pt

since P = e/t

or work/time

• Large quantities of electric power sold by electric power companies are measured in kilowatt-hours (kWhr).

• Current cost is about 10-12 cents/kWhr depending on where you live.

Electrical Safety:

Electrical Safety:

• Fuses and circuit breakers are devices used

to protect a circuit from overload.

– They switch off the power automatically if

amperage (causing excessive resistance in the

path) exceeds the allowed rate.

• A fuse is a thin strip of wire that melts

(opening the circuit) if too much current is

drawn through the wire.

Electrical Safety:

• A circuit breaker or

reset switch is a device

in the form of a relay

that can open or close

an electric circuit.

Electrical Safety:• A ground fault circuit

interrupter (GFCI) is a

specialized circuit breaker

designed to open the

circuit if current leakage

exceeds .006A (6MA)

– GFCIs should always be

used where the potential

for wet conditions are

present.

e.g. outside, kitchen and

bathroom receptacles

Electrical Safety:

• Electricity can be dangerous and should be

respected!

Rules of Electrical Safety:

1. Always use electrical devices in dry conditions.

2. Stay away from power service lines, especially during wet weather.

3. Use a fiberglass or wooden ladder when working near electric lines.

Rules of Electrical Safety:

4. Never run wires under rugs or carpet.

5. Never modify a receptacle for multiple

appliances.

6. Correctly repair frayed electrical wires or

throw away the device.

7. Never stick objects in receptacles or

appliances.

Rules of Electrical Safety:

8. Avoid using extension cords inside a home.

9. Call a knowledgeable electrician if you are unsure if a condition is safe.

-Don’t do it yourself if you are unsure of what you are doing!

10. Never by-pass fuses or circuit breakers.