Intro to Electricity Physics – 6 th Six Weeks

Introduction Atoms contains particles known as protons, neutrons, and electrons. Protons and electrons possess something called “electrical charge”. Charge is the name for the property of attraction and repulsion that occurs between certain subatomic particles. Electric Charge is a physical property. Exactly what causes electrical charge in these particles is not known.

Charge There are 2 kinds of electrical charge: positive and negative. The two types of charge were thus named by Benjamin Franklin in The electrons of all atoms are identical; they have the same mass and charge. The Protons of all atoms are identical to the protons of all other atoms as well. Electrons have a negative electrical charge, while protons have a positive electrical charge. (neutrons do not have any electrical charge) Although protons are nearly 1800 times more massive than electrons, the amount of positive electrical charge of a proton is exactly equal to the amount of negative electrical charge for an electron.

The Flow of Charge…a correction Previously, you heard that Benjamin Franklin named the two types of charge as being “positive” and “negative”. There is a problem…Benjamin Franklin had no knowledge of electrons (which wouldn’t be discovered until more than a century later). Electrons are the basis of electricity. Franklin thought positive charges rubbed off to an object (like when you rub a wax rod with wool) but in fact it was the other way around. By the time anyone discovered the error the terminology was well established. Therefore, even though we know that electrons are the charges that most readily move – they are still called “negative” when they should’ve been called “positive”. Objects he called “negative” – that he believed had a deficit of charge – actually have a surplus of charge (electrons)

The Flow of Charge…a correction Many electrical diagrams and textbooks will show the “conventional flow notation” (that is the technically incorrect Franklin direction) instead of scientifically correct “electron flow notation” that actually matches the true flow of charges. That being said in AC current – flow switches anyway, and in DC – devices and any math using flow – luckily work either way (whew!)

An atom contains equal numbers of protons and electrons so their charges cancel each other out and the atom has no net electrical charge. Objects with no net electrical charge are said to be neutral. Most atoms are neutral because they have the same amount of protons as electrons. If a material gains or loses charged particles (usually it loses or gains electrons) it becomes electrically charged. Gaining electrons makes a substance negatively charged, whereas losing them makes it positively charged. A charged atom is called an ion. Electrically Neutral vs. Electrically Charged

Coulomb The SI unit for charge is the Coulomb, and its symbol is C The charge of 1 Coulomb is the charge of 6.24 billion billion electrons (that is 6.24 x electrons). 1 C of charge is about the same charge that passes through a 100 W light bulb in approximately 1 second.

Coulomb’s Law Discovered by the French physicist Charles Coulomb ( ) in the 1700s. Coulomb’s Law states for charged particles (or objects that are small compared with the distance between them) the force between the charges a) is proportional to the product of charge on both objects, and b) is inversely proportional to the square of the distance between them. Generally speaking: A) B)

Coulomb’s Law In the Coulomb’s Law formula: F means electric force and is in Newtons (N) q means electric charge and is in Coulombs (C) d means distance between the charged objects in meters (m) Coulomb’s Law specifically states these relationships between charged particles: A) B) Force is proportional to the product of the charges Force is inversely proportional to the square of the distance between charges

(note: Coulomb’s Constant is alternately represented by K or k c in formulas and calculations) Coulomb’s Law and the relationship between variables

Coulomb’s Law Coulomb’s Law example situation #1: Force is inversely proportional to the square of the distance between charges 1)If the distance between two charged objects doubles, the force between them will decrease by a factor of four. 2)If the distance between them decreases by a factor of two, then the force between them will increase by a factor of four.

Coulomb’s Law Coulomb’s Law example situation #2: Force is (directly) proportional to the strength of each charge 1)If the charge of two objects is doubled, the force between them will quadruple. 2)If the charge of two of two objects decreases by a factor of two, then the force between them will decrease by a factor of four

Coulomb’s Law calculations – The Formula Note: A negative electric force would mean an attractive force whereas a positive electric force would mean a repulsive force. The “d” in the formula measures from the center of the two objects – not their edges.

Example 1: Calculating the force between two objects Two steel objects contain a net charge of C each. Find the force on the objects if they are 1 m apart. Note: the force is positive, therefore repulsive

Example 2: Calculating the force between two objects One steel object has a net charge of -1 C and another steel object has a net charge of 2 C. Find the force on the objects if they are 2 m apart. Note: the force is negative, therefore attractive

Potential Electron Transfer. Electrons are located on the outside of the atom, and when some surfaces are rubbed by another object they can be transferred. The atoms of different materials hold onto their electrons more tightly than do atoms of other materials.

Potential Electron Transfer. For outer electrons, the attractive force of the nucleus is largely canceled out by the repulsive force of the inner electrons, and thus they are much more likely to be transferred. Since Protons are held deep within the nucleus of the atom, they are almost never transferred. Therefore, electricity can be said to be the transfer or flow of electrons.

Potential Electron Transfer. When an object receives the transferred or excess electrons it becomes negatively charged, while the items that lose electrons now are positively charged. Any atom that does not have a neutral electric charge is an ion Positively charged ions are called cations and negatively charged ions are known as anions

Static Electricity and the Conservation of Charge Static Electricity – the accumulation of excess electric charges on an object. When an object becomes charged, charge is neither created nor destroyed, negatively charged electrons have simply been moved from one object to the other. Law of Conservation of Charge – Charge can be transferred from object to object, but it cannot be created nor destroyed. In all cases, when an object has been electrically charged, electric charges have moved.

Like vs. Opposite Charges There are two rules that electric charges obey: (1) Opposite charges attract and (2) Like charges repel. Those things that gain electrons are negatively charged, those that lose electrons are positively charged. Electric fields surround every electric charge. Any charge that is placed in an electric field will be either pushed or pulled by it. Therefore electrically charged objects do not have to be physically touching for them to interact with each other.

Electric Fields

Conductors Conductors – materials which allow excess electrons to move more easily through them than other materials. One or more of the outer electrons are relatively free to wander. A metallic object has a “sea” of shared electrons that easily move from one atom to the other normally. Therefore metals are the best naturally occurring electrical conductors (and for the same reason, they are also the best naturally occurring conductors of heat) Thus electrical wires are made of copper and not something non-metallic.

Insulators Insulators – any material that does not allow excess electrons to move through it easily. Electrons are tightly bound to their atoms. Good Insulators include plastic, wood, rubber, air, and glass. Since the flowing of excess electric charges in a wire is surrounded by an electric field, to be safe, wires are surrounded by an insulating substance like plastic or rubber.

Transferring Electric Charge Charging by Friction – electrons are transferred when one material rubs against another. Charging by Contact – electrons are transferred from one material to another by simply touching them together (no rubbing is required) Charging by Induction – the process of charging one object without touching it with another object. Recall that electrical forces can act on objects from a distance as well. Static Discharge – a transfer of charge through the air between two objects because of a buildup of static electricity.

A lightning strike is a large example of static discharge between the negatively charged cloud and the comparatively positively charged ground. Excited air particles emit light, you see this light as a spark. The great amount of electrical energy causes air in the bolt’s path to expand rapidly (at temperatures of more than 25,000°C) emitting sound waves heard as thunder.

Charge by Induction