Electricity

Electricity

The Shocking History of Electricity

• Around 600 BC Greeks found that by rubbing a hard fossilized resin (Amber) against a fur cloth, it would attract particles of straw. This strange effect remained a mystery for over 2000 years.

• The word Electricity comes from the Greek name for Amber or “elecktron”

What is Electricity?• Electricity is the movement of electrons

associated with electrical charge. • Electricity is a secondary energy source which

means that we get it from the conversion of other sources of energy, like coal, natural gas, oil, nuclear power and other natural sources, which are called primary sources.

• The energy sources we use to make electricity can be renewable or non-renewable, but electricity itself is neither renewable or non-renewable.

Electric Charge

• Electrons can be stripped off of atoms or added to atoms resulting in a charge

• An atom can have three types of electric charge

– Negative: extra electrons (anion)

– Neutral: just the right amount of electrons

– Positive: not enough electrons (cation)

(A) A neutral atom has no net charge because the numbers of electrons and protons are balanced.

(B) Removing an electron produces a net positive charge; the charged atom is called a cation.

(C) The addition of an electron produces a net negative charge an anion.

Electrical Conductors and Insulators

• Electrical conductors are materials that can move electrons easily. – Good conductors include metals.

• Insulators (nonconductors) are materials that do not move electrons easily. – Plastics are an example if an insulator.

• Semiconductors are materials that vary in their conduction, sometimes conducting sometimes not conducting. – This is usually determined by the temperature of the

material.

Electrostatic Charge• The charge on an ion is called an

electrostatic charge.

• An object becomes electrostatically charged by

1. Friction ,which transfers electrons between two objects in contact

2. Induction which produces a charge redistribution of electrons in a material

Charging by Friction

• Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing.

• Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair

• Hair “stands up” because the positive charges of each hair strand repel each other

Charging by Induction

• The comb has become charged by friction, acquiring an excess of electrons.

• The paper (A) normally has a random distribution of (+) and (-) charges.

• When the charged comb is held close to the paper (B), there is a reorientation of charges because of the repulsion of the charges.

• This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb

Beware of Door Knobs That Bite

More apt to happen in dry weather…why?

In dry weather, air acts and an insulator and an electric charge can build up your body. When your hand gets close enough to the door, the charge is conducted (discharged) from your hand to the knob. Humid air does not allow a large charge to build. The electricity is discharged into the air because water (humidity) is a good conductor.

Electric Force Fields

• A field is a force that is applied over a distance. – Recall the earth gravitational field that extends

form the surface of the earth.

• The space surrounding an electric charge has a property called an electric field. This electric field exerts a force (push or pull) on other electrically charged objects. – like charges experience repulsive force– opposite charges attracted to each other

Electrical Circuits• An electrical circuit contains some device that

acts as a source of energy as it gives charges a higher potential against an electrical field.

• The following three components of a simple electric circuit are illustrated on the next slide. 1) voltage source (such as a generator or battery)

that maintains the electrical potential,

2) some device that offers resistance (such as a lamp or motor ) where work is done by the potential (voltage drop), and

3) A continuous pathways (a conducting material in the wires) for the current to follow.

Electrical Circuits

Note: It is the electron field, and not the electrons, which does the work. The field that accelerates the electrons that are

already in the conducting material.

The Water Pressure Model

• A helpful way of thinking about current, voltage and resistance is to compare electricity to the flow of water– voltage (i.e. potential difference) is

analogous to the water pressure

– current is analogous to the flow of water

– resistance is analogous to the diameter of the pipe (e.g. a small pipe allows much less water to flow than a large pipe)

Electric Current

• The flow of electrons is called electric current

• The unit of electric current is the ampere or amp for short. – The symbol for amp is A

• An amp is the measure of how much charge flows by per second (C/s)

• Coulomb (C) is the unit of charge. – 1 C = 6.25 X 1018 electrons

The Water Pressure Model - Current

• Water Current: How much water flows per second in each pipe.

• Electrical Current: How many electrons flow per second in a wire.

Electric Current

• We measure the flow of water in gallons/minute

• We measure the flow of charges in coulombs/sec

• 1 coulomb/sec = 1 ampere– Remember this is 6.25 x

1018 electrons moving past a point in a wire per second

Electric Current• In the 1700’s people figured out that charges

could move• They had two choices

– Positive charge moves or Negative charge moves

• They guessed WRONG!!!!!– They thought that current was a positive charge flow– This is called “Conventional Current”

• Just the opposite – Hook up a battery and electrons flow from negative

terminal to positive terminal (“Electron Current”)

• A conventional current describes positive charges moving from the positive terminal (+) to the negative terminal (-).

• An electron current describes negative charges (-) moving from the negative terminal (-) to the positive terminal (+)

Potential Difference (Voltage)

• In order to have an electric current there must be a separation of the charge maintaining the electrical field (a potential difference).– This field or potential difference can push a

charge through a conductor.

• An electrical current is maintained by pumping charges to a higher electrical potential and the then do work as they flow back to a lower potential

Potential Difference (Voltage)

• The unit of potential difference is a volt (V)– The potential difference (PD) that is created

by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt

• Thus, the voltage of an electrical charge is the energy transfer per coulomb.

• The energy transfer can be measured by the work that is done to move the charge or by the work that the charge can do because of the position of the field

Example of Potential Difference

• Excess electrons will be attracted to the positive plate because of the potential difference, or

• The electric field generated by the charge difference will “push” the electrons from where there is too many (- charge) to where there is too few (+ charge).

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - -

e-

PUSH

The Water Pressure Model

- Voltage

• Water: The higher the tanks the more “push” or stronger force behind water current.

• Electricity: The higher the voltage, the more “push” behind the electron current.

Outlet

• In a household electric outlet, the potential difference (voltage) between the two slots is 120 volts

• Because of a power plant many miles away, electrons are under “pressure” or PUSHED to move from one slot to the next

Batteries

• Create a Potential Difference that causes a flow of electrons (electric current)

• A Battery “pumps” charge from negative end to positive end– Chemical potential

energy is consumed– Electrostatic potential

energy is produced

The Battery

• A simple battery or cell consists of two electrodes of different metals immersed in a weak acid

• Both electrodes slowly dissolve in the acid• At the positive electrode (cathode), electrons are

used in chemical reactions as the metal dissolves• At the negative electrode (anode), electrons are

absorbed into the electrode as the metal dissolves• The net result is a buildup of electrons at the

anode• The flow of electrons is from anode to cathode

outside of the cell or device

Primary vs. Secondary Cells

• In a primary cell or disposable battery, the chemical reactions that power a battery cannot be reversed– Example: alkaline dry cells

• In a secondary cell or rechargeable battery, the flow of electrons can be reversed when it is plugged into an electrical outlet. Electrons are “sent back” to the anode and a new potential is formed. The cathode gains electrons, while the anode gives up electrons thus increasing the potential. – Examples: lead acid, nickel cadmium, lithium, etc.

Dry Cell Battery (Primary Cell)• A typical battery

contains a carbon (graphite) electrode (cathode) and a zinc electrode (anode)

• The electrolyte is a moist paste of ammonium chloride

• As the electrolyte dries out, the voltage decreases.

The Lead Acid Battery (Secondary Cell)

• Two electrodes, one of lead, the other of lead dioxide (PbO2) immersed in sulfuric acid

• Lead ions (Pb++) dissolve, leaving two electrons behind

• Two electrons flow through the circuit and are used to help lead dioxide dissolve

• Automotive batteries are Lead Acid

Hydrogen Fuel Cells• In a fuel cell hydrogen is “burned” by mixing with

oxygen in such a way that it creates a voltage across two electrodes (cathode and anode)

• Hydrogen fuel is channel to the anode on one side of the cell, while oxygen is channel to the cathode on the other side of the cell.

• Hydrogen can be fed in directly or as part of molecules such as water (H2O) or methane (CH4).– Note: It takes energy (thermal or electrical) to

separate the H from these molecules!!!

Hydrogen Fuel Cells

Hydrogen Fuel Cells

• Hydrogen molecules give up their electrons to the platinum anode (H2 2H+ + 2e-)

• A membrane in the center of the cell allows only the 2H+ to pass through. The electrons are forced through the circuit to the second electrode (cathode)

• At the cathode, the electrons are returned to the molecules when hydrogen and oxygen combine to make water

(½O2 + 2H+ + 2e- H2O)

Electrical Resistance

• Electrical resistance is the measure of the resistance of part of a circuit to the flow of electrons (current)

• Materials having the property of reducing a current and that is electrical resistance.

• When resistance occurs work is done: Electrical energy is converted to thermal, radiant sound, etc….

• Electrical resistance is measured in ohms ()

Electrical Resistance

• When the current flows through the light bulb (a resistor), there is a voltage drop as electrical energy is converted to thermal and radiant energy.

• The voltage drop is proportional to the light bulbs resistance.

The Water Pressure Model -

Resistance

Work being done

Electrical Resistance of a Wire

• Resistance factors– Type of material– Length– Cross sectional area – Temperature

• The higher the temperature, the more the atoms vibrate and the more electrons make collisions with the atoms

• 10% of the electrical energy in the United states is lost as heat due to resistance in transmission lines.

The Water Pressure Model - Resistance

More Resistance

More Resistance

Superconductors

• Superconductivity refers to the loss of electrical resistance by certain materials at very low temperatures

• This properties reduces the amount of energy lost as heat.

Ohm’s Law

• Voltage, current and resistance in an electric circuit can be related using Ohm’s Law

• Ohm’s law states that

V = I x Rwhere

– V is the voltage (in volts)– I is the current (in amps)

– R is the resistance (in ohms)