Electricity

Understanding electricity requires a basic knowledge of physics. Everything we see, touch, feel or smell is composed of matter. All matter is made of atoms.

The three principle parts of an atom are the electron, neutron and the proton.

The proton has a positive charge. The electron has a negative charge. The neutron has no charge.

Electrons orbit the nucleus of the atom in orbits known as shells. The outer shell of an atom is known as the valence shell. The electrons in the valence shell are the important ones in the study of electricity. The valence shell of atom can contain up to eight electrons.

The conductivity of the atom depends on the number of electrons that are in the valence shell. When an atom has only one electron in valence shell, it is almost a perfect conductor. When an atom has eight valence electrons the valence shell is said to be complete and the atom is an insulator.

Electron Flow Electrical current is the flow of electrons. If an external electrical force, or electrical charge, is applied to the atom, the delicate balance holding the valence electrons in orbit is upset and the valence electrons are freed from orbit. They move in response to the external electrical force, moving away from the negative polarity towards the positive polarity of the external force.

Conductors: When an atom contains only one valence electron, that electron is easily given up. Atoms that easily give up and take on electrons are known as conductors.

Insulators: Materials containing 7 or 8 valence electron are known as insulators. Insulators are materials that resist the flow of electricity.

When the valence shell of an atom is full, the electrons are held tightly and are not given up easily.

Conventional Theory Vs. Electron Theory There are two theories of how current flows through a direct current circuit. Conventional Theory of how current flows says that electrons flow from the positive terminal of the battery to the negative terminal. This theory, though incorrect, was accepted for many years, and is still used in some service literature.

Electron Theory is the currently accepted theory regarding electricity. This theory states that the flow of electrons is caused by an excess number in one area and a shortage in the other. The area with the surplus electrons is considered negative, and the area with too few electrons is considered positive.

In a vehicle this means that electrons flow from the battery’s negative terminal to it’s positive terminal.

Voltage: Current flow is produced by having a difference in electrical potential, or pressure, at each end of a conductor (one with a surplus of electrons and the other lacking electrons) This pressure differential is what causes the current to flow. To keep current flowing, it is necessary to maintain an electrical pressure or, as it is commonly called, VOLTAGE

Voltage refers to the electromotive force (EMF) that moves electrons. Electrical pressure is measured in volts and uses the symbol E.

Current: Current flow is created when voltage moves through a conductor. This flow is measures in Amperes (symbol A) There must be a complete circuit before current can flow. Even though voltage may be present, current cannot flow unless there is a return path to the current source.

Resistance: Resistance is the opposition to current flow. The amount of resistance is measured in units called ohms. The symbol R or the Greek symbol Ω are used to represent resistance.

OHM’S Law: All electrical theory is based on Ohm’s law. The three electrical values – voltage, current and resistance – are mathematically related. This relationship is defined as Ohm’s law. Ohm’s law can be used to find an one unknown factor (voltage, current or resistance) if the others are known.

In Ohm’s law E = voltage I = current R = resistance Voltage is equal to the current times the resistance. Current is equal to the voltage divided by the resistance. Resistance is equal to the voltage divided by the current.

Electric Circuits: An electric circuit consists of a power source, a load (device to be operated) and the necessary conductors to provide a path for the electricity to flow.

The circuit must be complete (connected to make a complete path for the current) in order to function. In a complete circuit all of the components and wiring are connected electrically.

A basic circuit consists of a power source (battery), a device to be operated (ie. Light bulb), and connecting wires. The current flows from the negative terminal of the battery through the conductor, through the device, and then through the second conductor and back to the positive terminal of the battery. It then passes from the positive terminal to the negative terminal through the inside of the battery.

Circuit = a closed path that electric current follows This flow of electrons can power things like light bulbs

Series Circuit: A series circuit consists of one or more electrical devices (resistances) that are wired so that the current has only one path to follow. That means that it must flow through every resistance to get back to the power source and complete the circuit.

Because each electrical device impedes the rate of current flow, the total resistance offered by all of the resistances determines the amperage flow. In a series circuit the Total Resistance is equal to the sum of all resistances in the circuit. RT= r1=r2=r3=r4…………

RT= R1+R2+R3 RT=10Ω+20Ω+30Ω RT=60Ω

RT= 3kΩ+10kΩ+5kΩ RT=18kΩ or 18000Ω

Electric Current (I) “Current” in a river is the flow of water past a point. Unit = #gallons/sec? Electric Current (I) is the flow of electric charge past a point. Unit = #Coulombs/second This unit is called an “Ampere” or “Amp” (A) DEF: 1 Ampere = 1 Coulomb/second “I” is used as the symbol for electric current. EX: I = 4.5A means that 4.5 Coulombs of charge are passing by every second André Marie Ampére (1775-1836)

Voltage (V) (aka “Potential”) Units are Volts, V: Voltage is the electrical “pressure” that produces current. 1 volt = 1 Joule(EPE) per Coulomb EX: V = 12V Read as: “Potential equals 12 Volts” or “The voltage is 12 Volts”

What Is a Voltage Difference? - the force that causes electrons to flow electric charge flows from a higher voltage to a lower voltage (This is the same way water works when it comes out of the hose. When you turn the faucet, the water naturally flows from high water pressure to a low pressure).

In English, resistance = tendency for material to slow down the flow of electrons  -- electric current loses energy as it flows -- a light bulb acts as a resistor because t is causing the current to do work   * Copper has a low resistance, hence it is used for household wiring and other electronics- it is a conductor.

Resistance (R) When an electric current is made to do work, resistance (R) results. (Energy  heat, light, motion, etc) The units: Ohms (W) EX: R = 58 kW Read as “Resistance equals 58 kilo-Ohms or 58,000 Ohms”” Georg Ohm (1789-1854) Symbols for resistors

A battery is a voltage source The battery acts as a “pump” to give electrons higher potential energy. Current will only flow if there is a Potential difference, or voltage, between 2 points Voltage is the electrical potential energy per coulomb of charge Joules Volts = --------------- Coulomb Alessandro Volta (1745-1827)

A Simple Circuit Ohm’s Law!!! V = 15 Volts R = 3 Ohms The current is determined by the Voltage and the Resistance (1) Bigger voltage means bigger current: The current is directly proportional to the Voltage (2) Bigger Resistance means smaller current: The current is inversely proportional to the Resistance Together, these 2 give us……. Ohm’s Law!!!

Ohm's Law --  the current (I) in a circuit equals the voltage difference  (V) divided by resistance (R)

V I = ---- = R 15V = 5 A 3Ω For this circuit, Ohm’s Law tells us that: Volts V = 15 Volts R = 3 Ohms For this circuit, Ohm’s Law tells us that: V I = ---- = R Amperes Ohms 15V = 5 A 3Ω So the current is 5 Amps (or 5 Coulombs per second)

Symbols used when drawing circuits (You will draw these during our lab Symbols used when drawing circuits (You will draw these during our lab... Beware!)

Puttin’ it together! Electric current is directly proportional to voltage Electric current is reduced by “resistance” in a circuit. Any time the circuit performs work this means that there is resistance to the flow of current. The resistance could be from: An electric motor (Energy  motion) A light bulb (Energy  light) A resistor (Energy  heat) Georg Ohm (1789-1854)

In a Series Circuit: Current has only one path to follow. There is a voltage drop across each device (bulb, motor, resistor, etc): VR1 , VR2 , VR3 , ……… These voltage drops add up to the voltage rise in the battery: VBatt = VR1 + VR2 + VR3 + …..

In a Series Circuit: VBatt I = Rtotal Adding more devices increases the resistance of the circuit: RTotal = R1 + R2 + R3 + …. The Current (I) is the same in all devices. This Current is: VBatt I = Rtotal

Current (I) is the same for all!!! Series Circuit: The voltage may be different for each device. (recall from Ohm’s Law: V = IR) VR1 = I x R1 VR2 = I x R2 VR3 = I x R3 Current (I) is the same for all!!! R1 = 2Ω R2 = 9Ω R3 = 7Ω Question: Which bulb has the greatest voltage drop?

In a Series Circuit: The Power consumed by each device may also be different: (P = IV) PR1 = I x VR1 PR2 = I x VR2 PR3 = I x VR3 Q: Which bulb has the greatest power consumption? R1 = 2Ω R2 = 9Ω R3 = 7Ω

Example of a Series Circuit: The total resistance: Rtotal The current through all devices: I = 3) The Voltage drop in each device is: VR1 = I x R1 VR2 = I x R2 VR3 = I x R3 = 2Ω + 9Ω + 7Ω = 18Ω VBatt / Rtotal = 9V/18Ω = 0.5Amps Note: these add up to exactly 9 volts!! = .5A x 2Ω = 1 V = .5A x 9Ω = 4.5 V = .5A x 7Ω = 3.5 V R1 = 2Ω R2 = 9Ω R3 = 7Ω VBatt = 9V

Example of a Series Circuit: The Power consumed by each device is: PR1 = I x V1 PR2 = I x V2 PR3 = I x V3 Note: these add up to 4.5W = 9V x 0.5A the power of the battery = .5A x 1V = .5 Watts = .5A x 4.5V = 2.25 Watts = .5A x 3.5V = 1.75 Watts R1 = 2Ω R2 = 9Ω R3 = 7Ω VBatt = 9V

Ohm’s Law in a Parallel Circuit: The current can follow more than one path to return to the battery. The total current is just the sum of the branches: Itotal = I1 + I2 + I3 + … Each branch may have a different current, and can be controlled by its own switch:

In a Parallel Circuit: All devices have the same voltage drop, which is always equal to the battery’s voltage: Vbattery = V1 = V2 = V3 = … All three bulbs receive the same voltage: The current in each branch depends on the resistance: I1 = VBatt /R1 I2 = VBatt /R2 I3 = VBatt /R3 Ohm’s Law applied to each device! The less resistance a branch has, the more current it will carry.

In a Parallel Circuit: Ohm’s Law applied to whole circuit: The more devices you add to a parallel circuit, the greater the total current (in and out of the battery), and the less the total resistance (opposite of series). Ohm’s Law applied to whole circuit: Rtotal = VBatt /Itotal Question: As I total gets bigger, what happens to Rtotal?

Example of a Parallel Circuit: The voltage across each bulb is the same: 6V = Vbattery The currents in each branch are: I1 = Vbatt /R1 = 6V ÷ 2Ω = 3 Amps I2 = Vbatt/ R2 = 6V ÷ 9Ω = .67 Amps I3 = Vbatt / R3 = 6V ÷ 7Ω = .86 Amps The total current from the battery Is…. Itotal = I1 + I2 + I3 = 4.53 A Note: these add up to 4.53 A R1 = 2Ω R2 = 9Ω R3 = 7Ω VBatt = 9V Vbatt = 6V

Example of a Parallel Circuit: Itotal = I1 + I2 + I3 = 4.53 A Ohm’s Law gives us the total resistance of the circuit: Rtotal = Vbatt/Itotal = 6V/4.53A Rtotal = 1.32 Ω Note: if this were a series circuit, the resistance would add to 18Ω! --much larger! Also, adding more bulbs Would add more branches, decreasing resistance (unlike for series circuits!) R1 = 2Ω R2 = 9Ω R3 = 7Ω VBatt = 9V Vbatt = 6V