Electricity
Static Electricity
Review of the Atomic Structure of Matter:
(1) protons = + charge, nucleus
(2) neutrons = no charge, nucleus
(3) electrons = - charge, probability cloud
Electrons are the only particles that are free to move from atom to atom. This movement of electrons gives rise to electricity.
Any object that has an unequal number of protons and electrons is said to have a static charge.
Examples of static electricity:
Van de Graaff Generators
If you walk across a nylon carpet with a rubber-soled shoe you can generate a static charge when you touch a doorknob (the electrons are transferred from the tips of your fingers to the doorknob).
Other examples of static electricity include:
(1) the static cling of laundry
(2) the static cling of plastic wraps
(3) lightning – a sudden static discharge produced when the charge on an object is too great and electrons rapidly move from the object to the air molecules that touch the object
Cloud to ground lightning occurs when a negatively charged cloud induces a positive charge at the earth’s surface.
When the cloud becomes sufficiently charged, electrons are explosively transferred to the earth’s surface.
Thunder is produced when the gases in the atmosphere are heated by the lightning and expand rapidly.
Laws of Static Electricity
(1) Opposite static charges attract. Like static charges repel.
(2) Charged objects can induce a charge in uncharged objects by attracting or repelling electrons.
(3) Electric charges concentrate and leak from points.
(4) Positive charges that result from a loss of electrons are only temporary. Gradually, electrons flow from other sources to balance the excess positive charges.
Electroscopes are used to detect static charges.
The pictures above are of a gold-leaf electroscopes.
The amount of charge that an object has is measured in coulombs, c. One coulomb is defined as an electric charged of 6.25 x 1018 electrons or protons.
If an object gains 6.25 x 1018 electrons or loses 6.25 x 1018 electrons (and has an excess of 6.25 x 1018 protons) it has a charge of one coulomb.
Current Electricity
Most nonmetals are insulators (poor conductors). They can hold an electric charge for some time but the charge does not flow away because the electrons in most nonmetals do not readily move from atom to atom.
Metals are good conductors because they have one, two, or three electrons in their outermost energy levels. These electrons are not as tightly held and are free to move from one atom to another.
Metals such as Au, Pt, and Cu are some of the best conductors of electricity.
Materials like plastic, rubber, glass, and porcelain are some of the best insulators or nonconductors.
Semiconductors are materials that have a conductivity between conductors and insulators.
Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide.
In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material.
Although many electronic devices could be made using vacuum tube technology, the developments in semiconductor technology during the past 50 years have made electronic devices smaller, faster, and more reliable.
Semiconductors are used in:
microwave oven / electronic balances / video gamesradio / television / VCR
watch / CD player / stereo
computer / lights / air conditioner
calculator / telephone / musical greeting cards
diagnostic equipment / clock / refrigerator
car / security devices / stove
The flow of electrons through a conductor is referred to as current electricity or electric current.
The path along which the movement takes place is called an electric circuit.
Open vs. Closed Circuits
A circuit is said to be open when a break exists in a complete conducting pathway.
Example: turn a light switch off
A closed circuit (light switch on) allows for the free flow of electron along a conduction pathway.
Iboth switches a
The flow of electrons can be compared to the flow of water.
Electric current is a measure of the number of electric charges, or coulombs (6.25 x 1018 e-), that pass a given point in a wire per second.
One coulomb of electrons flowing through a wire per second is an ampere, A, of current (amp).
The ampere is measured with an instrument called an ammeter.
Current is usually represented by the letter I in equations and graphs.
What keeps electrons moving in a circuit?
In an electric circuit, a battery or generator acts as a “pump”, taking electrons from atoms.
The electrons create a difference in the level of charge between the two binding posts of a cell or battery creating a potential difference between the two posts.
Electrons flow through a wire because the excess electrons at the negative terminal have a higher potential energy than electrons at the positive terminal.
The difference in potential energy between an electron at the negative terminal and one at the positive terminal is called potential difference.
Potential difference between terminals is measured in volts, V.
Potential difference is often referred to as voltage.
Examples:
flashlight battery = 1.5 volts
car battery = 12 volts
electrical outlets = 110 volts
A voltmeter measures potential difference.
Modern batteries use a variety of chemicals to power their reactions. Typical battery chemistries include:
· Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte.
· Alkaline battery - Used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte.
· Lithium photo battery - Lithium, lithium-iodide and lead-iodide are used in cameras because of their ability to supply power surges.
· Lead-acid battery - Used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable).
· Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable).
· Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable).
· Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable).
· Zinc-air battery - This battery is lightweight and rechargeable.
· Zinc-mercury oxide battery - This is often used in hearing-aids.
· Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio is good.
· Metal-chloride battery - This is used in electric vehicles.
When electrons have a steady flow in one direction, the flow is called direct current (dc).
An example is a dry cell battery, where current travels in the same direction for the life of the cell.
When the direction of the current keeps jumping back and forth in a circuit, it is called an alternating current (ac).
Electric generators (dynamos) and power plants are sources of alternating current.
Capacitors are metal plates separated by an insulator that store an electrical charge.
When you connect a capacitor to a battery, here’s what happens:
· The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.
· The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.
Resistance is a measure of how much a conductor resists or holds back the flow of electrons.
Current is greater if the resistance of the circuit is reduced.
Good conductors have low resistance, while poor conductors have higher resistance.
The electrical resistance of a conductor depends on:
(1) thickness – Thick wires have less resistance. Resistance varies inversely as the square of its diameter.
(2) length – Longer wires have greater resistance. Resistance varies directly with length.
(3) temperature – Resistance increases as the conductor becomes hotter and decreases as it becomes cooler.
In superconductors molecular motion has been slowed to the point that it does not hinder electron flow.
Resistance is measure in ohms, W.
W = omega
The relationship between electric current, voltage, and resistance is described in Ohm’s Law.
Ohm’s Law states that the amount of electric current flowing in a circuit increases as voltage increases and decreases as resistance decreases.
current = voltage / resistance
I = V / R
Units: I(amps, A), V(volts, V), R(ohms, W)
V = A x R
Examples:
A circuit has a resistance of 100 ohms and a potential difference of 110 volts. What is the resulting current in the circuit in amperes?
(a) How large a current will 120 volts send through a resistance of 20 ohms?
(b) What is the resistance of an electric toaster if 120 volts sends a current of 5 amperes through it?
Electric power is a measure of the rate at which the energy of flowing electrons is used.
A current of one ampere flowing through a circuit with a potential difference of one volt produces one watt of power.
Power = current x voltage
P = IV
Units: P(W), I(A), V(V)
Example:
At 110 volts, 0.25 amperes of current flow through a small black and white TV. How much power is used by the TV?
You pay for the energy you use!
The electric energy you use in your home is measure in units called kilowatt-hours, kWh.
Electric devices are rated in watts according to the amount of energy per second they need to operate.
The charges are based on:
(1) power – how fast electrical devices consumer energy
(2) time – how long the devices are used
energy = power x time
E = Pt
E = VIt
P = VI
Units: E(Wh or kWh), P(W or kW), t(h)
They use a kilowatt-hour meter. The electric bill is calculated on a price per kilowatt-hour plus assorted fees.
Example:
How much energy is used by a 750-watt room air conditioner in 8 hours?
If we used it for 8 hours a day for 15 days, what is the operating cost?
Paths For Electrons
(1) A series circuit is one in which all components are connected in tandem. The current at every point of a series circuit stays the same. In series circuits the current remains the same but the voltage drops may vary. The total resistance is the sum of the parts.
There is only one path for current. If one bulb in a series burned out all of the bulbs would go out.
(2) Parallel circuits are those in which the components are so arranged that the current divides between them. In parallel circuits the voltage remains the same but the current may vary. The circuits in your home are wired in parallel.
Current that passes through one electrical device does not have to pass through another.
Overloaded wiring may overheat and cause a fire.
Short Circuits create a pathway of very low resistance.
In most cases, short circuits are caused by electron flow between two wires that are touching.
Example:
Overloads can occur when the amount of current used by all of the devices exceeds the safe limits of the circuit.
Fuses and circuit breakers offer protection.
A fuse contains a short, Pb-alloy wire that melts if the circuit carries a load that is too large. The circuit breaks and the current stops. They are consumable.
good fuse
blown fuse
A circuit breaker acts like a switch to open a circuit if the current is too great. It can be reset and reused.
Generating Electricity
(1) cells and batteries
(2) Electromagnetic induction – process of producing a current by moving a magnet near a conductor or conductor near a magnet.
A generator produces electricity by turning coils of a wire in a magnetic field.
(1) Steam (burning coal) or falling water
(2) blades of the turbine are turned
(3) coils of wire are turned in a magnetic field
(4) electricity is produced
(5) transmitted at high voltages (step-up transformer) by power lines
(6) voltage decreased (step-down transformer) prior to use
A transformer is a device that changes the voltage of alternating current.
Heat and Light From Electricity
Resistance causes heat. The greater the resistance the greater the heat produced.
Electricity produces light. The following are types of electric light:
(1) electric arc – A small gap is present in a circuit. With sufficient voltage the current jumps the gap and produces a bright light.
(2) incandescent bulb – heat a wire or filament to the point it glows
Example: tungsten filament
(3) gas – At low pressures gases are conductors. A high voltage strips electrons from the gas.
Examples: Ne – red, Na –yellow, Hg – blue/green
(4) fluorescent – bulb is coated with phosphors (powered minerals that convert UV light to visible light – fluorescence)