Name: ________________ Class: _________________ Index: ________________ STATIC ELECTRICITY

Download Name: ________________ Class: _________________ Index: ________________ STATIC ELECTRICITY

Post on 15-Dec-2015




0 download

Embed Size (px)


<ul><li>Slide 1</li></ul> <p>Name: ________________ Class: _________________ Index: ________________ STATIC ELECTRICITY Slide 2 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 Slide 3 Rub a plastic ruler on your sleeve/hair and then hold it near some tiny pieces of paper/tissue. What happens ? Static Electricity Slide 4 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 Slide 5 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). Slide 6 Positively-charged object (excess positive charges): More protons than electrons Negatively-charged object (excess negative charges): More electrons than protons Slide 7 Unit: Coulomb Electric charge is quantized as a multiple of the electron (-1.602 x 10 -19 C) or proton (+1.602 x 10 -19 C) charge. Q) How many electrons are there in 1 coulomb of charge? 6.242 x 10 18 Slide 8 The figs. below show the law of force for charges: (a) Opposite charges attract (b) Like charges repel Slide 9 Unlike charges attract Like charges repel Slide 10 Coulombs 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 &amp; r=1m; F = [8.99 x 10 9 x 1 x 1]/ (1) 2 = 8.99 x 10 9 N) Slide 11 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. Slide 12 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. Slide 13 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. Slide 14 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 Slide 15 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. Slide 16 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. Slide 17 Electric Field of a single positive and negative charge + - Slide 18 Electric Field between 2 unlike point charges + - A B C Comment on the distances between the electric field lines A, B and C Slide 19 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. Slide 20 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. Slide 21 Electric Field between 2 charged parallel plates ++++ ++++ ---- ---- Slide 22 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 Slide 23 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 ( + ) Slide 24 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 ( - - - - - - ) Slide 25 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. Slide 26 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. Slide 27 Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object. Slide 28 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. Slide 29 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. Slide 30 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 &amp; 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. Slide 31 Step 3 (part iii) Take special note of the charge distribution in both the conductors A &amp; 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. Slide 32 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 &amp; B re-distribute themselves immediately. Slide 33 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 Slide 34 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. Slide 35 Step 2 (part ii) This happens because the negative charges repel the conductors 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 Slide 36 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. Slide 37 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 + + + + + + Slide 38 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. Slide 39 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 + + + + + + + + + + + + Slide 40 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 &amp; explain what will happen when force F is removed. - - - - - - - - - - - - - - - - + + + + X Y Question Slide 41 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. Slide 42 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 Slide 43</p>