Objectives -- state that there are positive and negative charges and that charge is measured in coulombs -- state that unlike charges attract and that like charges repel --describe an electric field as a region in which an electric charge experiences a force --draw the field of an isolated point charge and show understanding that the direction of the field lines gives the direction of the force acting on a positive test charge -- draw the electric field pattern between 2 isolated point charges -- show understanding that electrostatic charging by rubbing involves a transfer of electrons --describe experiments to show electrostatic charging by induction -- describe examples where electrostatic charging may be a potential hazard -- describe an example of the use electrostatic charging e.g. photocopier, spraying of paint, electrostatic precipitator and laser printer
Rub a plastic ruler on your sleeve/hair and then hold it near some tiny pieces of paper/tissue. What happens ? Static Electricity
Nucleus: contains positively-charged protons and non-charged neutrons Surrounding: negatively-charged electrons In an uncharged atom, the number of protons = the number of electrons
Charging is the process of adding or removing electrons into or from a conductor (or insulator) which results in an imbalance of number of electrons in the charged conductor (or insulator).
Positively-charged object (excess positive charges): More protons than electrons Negatively-charged object (excess negative charges): More electrons than protons
Unit: Coulomb Electric charge is quantized as a multiple of the electron ( x C) or proton ( x C) charge. Q) How many electrons are there in 1 coulomb of charge? x 10 18
The figs. below show the law of force for charges: (a) Opposite charges attract (b) Like charges repel
Unlike charges attract Like charges repel
Coulomb’s law F=(k q a q b )/ r 2 ; –where k= 8.99 x 10 9 (Nm 2 /C 2 ) (i.e. q n =q h =1C & r=1m; F = [8.99 x 10 9 x 1 x 1]/ (1) 2 = 8.99 x 10 9 N)
Materials that do not allow electrons to move freely inside them are called electrical insulators. electrons are all in fixed positions The addition or removal of electrons at any one part of the insulator does not result in the electrons in other parts of the same insulator to move. charge is localised (or confined) to the region.
Examples of insulators are wood, plastics, ebonite, glass, fur, silk. The method of charging by friction will only work when two insulators are rubbed against each other.charging by friction When an insulator is charged by the friction method the charge remains on the surface of the material. This is because the charge cannot move through the insulator.
Some materials allow electrons to move about easily inside them. These are called electrical conductors. outer electrons (valence electrons) are loosely bound, relatively free from individual atoms We say that these electrons are delocalised.
When electrons are gained/lost by the conductors, the other electrons will flow automatically so that electron re-distribution in the conductors occur. Examples are all metals like copper, iron, steel Charged by induction
Electric Field Lines Electric field lines are patterns of lines that point in the direction that a positive test charge would accelerate if placed upon the line. As such, the lines are directed away from positively charged source charges and toward negatively charged source charges. To communicate information about the direction of the field, each line must include an arrowhead that points in the appropriate direction. An electric field line pattern could include an infinite number of lines. Because drawing such large quantities of lines tends to decrease the readability of the patterns, the number of lines is usually limited. The presence of a few lines around a charge is typically sufficient to convey the nature of the electric field in the space surrounding the lines.
Rules for drawing electric field lines The rules for drawing electric field lines for any static configuration of charges are: 1)The lines begin on positive charges and terminate on negative charges. 2) The number of lines drawn emerging from or terminating on a charge is proportional to the magnitude of the charge. 3) No two field-lines ever cross in a charge-free region. (Because the tangent to the field line represents the direction of the resultant force, only one line can be at every point.) 4) The line approaches the conducting surface perpendicularly.
Electric Field of a single positive and negative charge + -
Electric Field between 2 unlike point charges + - A B C Comment on the distances between the electric field lines A, B and C
Electric Field between 2 like charges + + X The point X in the diagram is a null point. There is no electric field at point X.
Electric Field between 2 like charges X - - The point X in the diagram is a null point. There is no electric field at point X.
Electric Field between 2 charged parallel plates
The Triboelectric Series lists materials according to how likely they are to let go of electrons or to take on electrons from other materials. It is somewhat parallel to Chemistry's Periodic Table. Charging by Friction
Give up negative charges The following materials will give up electrons when brought in contact with materials, especially those that attract electrons. They are in the order of most apt to give electrons to those that barely give up electrons. Dry human skin ( ) Leather Rabbit fur Glass Human hair Nylon Wool Lead Silk Aluminum Paper Cotton ( + )
Collects negative charges The following list of materials will attract electrons when brought in contact with other materials, especially those that give up electrons. They are in the order of least apt to attract electrons to those that readily attract electrons. Wood ( - ) Amber Hard rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (like Scotch Tape) Polypropylene Vinyl (PVC) Silicon Teflon ( )
The polythene is negatively charged when it rubs with dry cloth, because some of the outer electrons are transferred from the wool surface onto the polythene. Note: Only electrons can move -- the protons remain fixed.
The perspex rod is positively charged when it rubs with dry cloth, because some of the outer electrons are scraped off from the rod and move on to the cloth. Note: Only electrons can move -- the protons remain fixed.
Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object.
Charging by Induction Induction charging is a method used to charge an object without actually touching the object to any other charged object. The overall charge on the system of two objects is the same after the charging process as it was before the charging process. Charge is neither created nor destroyed during this charging process; it is simply transferred from one object to the other object in the form of electrons.
Step 1 Place 2 conductors (e.g. metallic spheres) on insulated stands. Make sure that the 2 conductors are initially uncharged. A B insulating stand Charging 2 spheres with equal but opposite charges A B insulating stand Step 2 Next, put the 2 conductors in contact.
Step 3 (part i) Bring a negatively-charged rod near the 2 conductors. Take care that the negatively-charged rod does not touch any of the 2 conductors. insulating stand A B A B Same number of positive charges & negative charges are formed. Step 3 (part ii) Notice that the negative charges in the rod repel the electrons in A to move further away. Since A touches B, some electrons in A will be repelled to the further end of B. As A lost some electrons, A becomes positively-charged. As B gained some electrons, B becomes negatively-charged.
Step 3 (part iii) Take special note of the charge distribution in both the conductors A & B. insulating stand A B A B Step 4 With the negatively-charged rod still in place, separate the 2 conductors apart. Note how the charge distribution in both the conductors remains unaffected due to the presence of the charged rod.
Step 5 (part i) With the 2 charged conductors still separated, remove the negatively-charged rod. insulating stand A B A B Step 5 (part ii) With the 2 charged conductors still separated, remove the negatively-charged rod away. Now that the charged rod is removed, the charges in A & B re-distribute themselves immediately.
Step 5 (part iii) We started with 2 conductors which are neutral. Now, we have 2 conductors that are oppositely-charged. Also, the number of positive charges in one conductor is the equal to the number of negative charges in the other conductor. insulating stand A B The same experiment can also be repeated with a positively-charged rod. A B insulating stand
Charging a single sphere using earthing method insulating stand Step 1 Place a negatively-charged rod near the single conductor (e.g. metallic sphere) that is sitting on an insulated stand. Make sure that the single conductor is initially uncharged insulating stand Step 2 (part i) Note that the moment the negatively-charged rod is placed near the conductor, the charge distribution in the conductor is disturbed immediately.
Step 2 (part ii) This happens because the negative charges repel the conductor’s electrons to its further end insulating stand Since the left-end of the conductor has “lost” some electrons to the right-end, some positive charges are noted to appear on the left-end too insulating stand Step 3 (part i) Next, touch the conductor for a short while. As the human body is regarded as a good conductor of electricity, the excess charges in the conductor (electrons in this case) will flow through the body down to earth. Touching the charged conductor in this case is called earthing
Step 3 (part ii) Finally, all excess charges are discharged insulating stand Step 4 Once the excess electrons are discharged, remove the finger from the conductor while keeping the negatively-charged rod stationary.
Step 5 (part i) Finally, the negatively-charged rod is removed. insulating stand Step 5 (part ii) The positive charges remaining in the single conductor will re-distribute themselves uniformly. insulating stand
Discharging Discharging is the process in which a charged body is removed of excess charges from it. When a charged body is discharged, it is said to be neutralised. insulating stand Discharging a Negatively-Charged Conductor (by earthing) To discharge a negatively-charged conductor, you only need to touch it. Touching the conductor to discharge it is known as Earthing. When the conductor is earthed, the free electrons will flow out of the conductor, through the body, to the earth.
Discharging a Positively-Charged Conductor (by earthing) To discharge a positively-charged conductor, you only need to touch it too. Touching the conductor to discharge it is also known as Earthing. When the conductor is earthed, the positive charges in the conductor attract electrons from the earth to flow towards them. The electrons will neutralise the positive charges in the conductor. insulating stand
A small conducting sphere Y which is positively charged is suspended in air with an insulating thread. Y is held stationary by a force F. It is brought near to a large conducting sphere X which is negatively charged. X is resting on an insulating stand. Describe & explain what will happen when force F is removed X Y Question
The Gold Leaf Electroscope The gold leaf electroscope can be used to find out if an object is charged and what type of charge it carries. When the electroscope is charged at A, B becomes charged the same as the metal plate C. Like charges repel, so the leaf rises at B.
Lightning Conductors Large amounts of electric charge are built up in clouds because of friction from winds. When the charge on a cloud is sufficiently large, the air ionizes and becomes conducting. The charge flows throughout the air to earth as lightning and strikes the nearest or sharpest object. (continue on next slide) Dangers of Static Electricity
Fires and Explosions Charge can build up on many objects such as planes and petrol tankers. If not discharged carefully, a spark (similar to that produced by lightning) can start a fire or cause an explosion. A “anti static strip" will periodically touches the road discharging any built up static electricity.
The Van de Graff generator is a machine for charging things up. When it is switched on, charge builds up on its dome. Charge is deposited on the bottom of the belt and is carried up to the dome by the belt. If too much charge builds up on the dome, the dome discharges itself by letting sparks fly to any nearby object. belt To power + supply - insulator Hollow metal sphere The Van de Graff Generator Uses of Static Electricity
Photocopier Step 1: Inside the photocopier, a light-sensitive plate ( or drum) is given a negative charge. Step 2: An image from the original document is projected onto the plate. The bright areas lose their charge but the dark areas keep it.
Step 4: A blank sheet of paper is pressed against the plate and picks up toner. B Step 5: The paper is heated so that the toner melts and sticks to it. The result is a copy of the original document. B B Step 3: Toner is attracted to the charged areas ( the dark areas). B
Spray Painting Many mass produced objects such as cars are spray painted. To increase efficiency and reduce paint usage the paint particles and the car body are given opposite charges. Thus, the paint will be attracted to parts of the car not yet covered by paint.
Electrostatic Precipitator Flue ash is a mixture of dust and smoke produced by many factories and power stations. Charged metal plates in the chimney attract these particles and remove them from the exhaust gases.
Laser Printer Inside a laser printer there is a drum which holds an electric charge. Next to the drum is a transfer corona roller, which can negatively or positively charge the drum as needed, as well as a toner unit. In most laser printers, the drum starts out positively charged, although this process can also work in reverse. The controller manipulates a small laser to “write” on the drum with a negative charge, creating an electrostatic image.
Then, the drum is rolled through the toner, which is positively charged so that it will cling to the areas of negative charge on the printer drum. The printer feeds a piece of paper, which is given an even stronger negative charge by the transfer corona wire before being rolled past the drum. The electrostatic image on the drum will transfer to the paper, which is then discharged to prevent it from clinging to the drum. Then it is fed through a fuser which heats the toner and causes it to bind with the fibers in the paper. Meanwhile, the drum passes a discharge lamp, which will expose the entire surface of the drum and erase the electrostatic image. The transfer corona wire applies another positive charge, and the printer is ready for the next page or job.