Presentation on theme: "When I was a primary student,my teacher told me to rub my plastic ruler by using a piece of cloth. As a result, the ruler attracted pieces of small."— Presentation transcript:
When I was a primary student,my teacher told me to rub my plastic ruler by using a piece of cloth. As a result, the ruler attracted pieces of small paper automatically.
Matter is made up of atoms. An atom contains positive charges and negative charges. The negative charges are due to the electrons. The atom as a whole is neutral; the quantity of positive charge and negative charge are equal. There is empty space. The electrons are easily to lose away.
Electrons can be transferred from one atom to another. When an electron is removed from a molecule, the molecule contains more positive charges than negative charges. In other words, it contains net positive charges. It becomes a positive ion ( 離子 ). Neutral Atom + Positive ion electron
When an electron is added to an atom, the atom contains more negative charges than positive charges. It has a net negative charge. We say the atom is negatively charged; it becomes a negative ion. + Neutral Atom Electron Negative ion
The quantity of electric charge is expressed in the unit coulomb (C). An electron carries a charge of e = -1.6 x C The negative sign means the charge is negative. -19
Charging by friction Rub two acetate strips with a dry woolen cloth and hold them side by side. The strips repel each other. Repeat with two rubbed polythene strips. The strips also repel each other.(Fig 1.4) Repeat with an acetate strip and polythene strip. The strips attract each other.
Before an acetate strips is rubbed, it contains the same quantity of positive and negative charges. The strip is neutral because there is no net charge. The same is true for the woolen cloth. When the acetate strip is rubbed, some electrons in the strip are transferred to the cloth. As a result, the strip contains more positive charges than negative charges. It has a net positive charge. On the other hand, the cloth is negatively charged because it contains more negative charges than positive charges. When a polythene strip is rubbed, it is negatively charged while the cloth is positively charged. Some electrons in the cloth have been transferred to the strip.
Same material strips repel each other. Different material strips attract each other. *Fig 1.5 There are forces called electric forces ( 電力 ), which cause the strips repel or attract each other. Like charges repel; unlike charges attract.
In all cases, no new charges are produced in the rubbing process. The charges are merely separated. They are transferred from one object to the other through direct contact of the two objects. The quantity of charge obtained by one object is equal to the quantity of opposite charge obtained by the other. The net charge of the two objects taken together remains zero. Charge cannot be created or destroyed.
Connect one strip to the positive terminal and the other to the negative terminal (Fig 1.7). The strips attract each other. Connect two aluminium strips to the positive terminal of the extra high tension (EHT) power supply as shown below. The strips repel each other. *Fig 1.6 *Fig 1.7 Charging by pushing
When both strips are connected to the positive terminal, the EHT power supply draws electrons from the strips, which then become positively charged. For simplicity, we may say that the EHT power supply pushes positive charges onto the strips. When the strips are connected to different terminals, the power supply pushes positive charges onto one strip through the positive terminal and negative charges onto the other strip through the negative terminal. The charge are transferred by conduction.
Charging by sharing *Fig 1.7
The Van de Graff generator generates charges. The Hairs share the same type of charge with the generator by conduction.
ground *Fig 1.8 Fig 1.8 shows a charged metal ball electrically connected to the ground. This can be done by connecting the metal ball and the ground with a wire, or simply by touching the ball with the finger. Since the earth is a huge object, it practically obtains all the charges from the ball, which then becomes neutral. In fact, any charged metal will be completely discharged as result of earthing or grounding. *Fig 1.9 Symbol for earth
*Fig 1.10 An uncharged sphere becomes positive after touching positively charged sphere *Fig 1.11 An uncharged sphere becomes negative after touching negatively charged sphere
Air contains positive ions and free electrons. When a positively charged rods is left in the air, the rod gradually attracts free electrons into it. The positive charge is then neutralized by the electrons. When a negatively charged rod is left in the air, the excess electrons are gradually lost into the air and neutralize the positive ions. In both cases, the rod is said to discharge through air Positive rod Negative rod Positive ions and free electrons in air *Fig 1.12
A charged object discharges easily when the air contains more ions. This can be done by heating the air with a flame(Fig l.12) or hot body, or irradiating it with a radioactive source. In these processes, many ions and free electrons are produced. *Fig 1.12 A charged object also discharges more easily when the air contains a lot of moisture. It discharges less easily when the air is dry.
Objects are classified as electrical conductors ( 導體 ) or insulators ( 絕緣體 ). In a conductor, the charges can move easily from one place to another On the hand, in an insulator, the charges cannot move easily. *Fig 2.1 Electrons are free to move in conductor.
Conductors and insulators are not two distinct class of material. Good insulators are bad conductors and vice versa. Even the best insulator can allow a very small flow of charge. Similarly, charges are not completely free in a conductor. Metals are the most common conductors while non-metals are usually insulators. Some common conductors and insulators are listed below. Conductors Insulator Silver copper aluminium graphite water Vacuum plastics wax dry air wood Earth, human body In decending order of goodness
*Fig 2.2 It is the same of positive ions For a conducting sphere, the excess charges settle evenly on the surface (Fig 2.2). For a conductor of irregular shape, the charge density is higher at sharp points.
The distribution of charges on a conductor is also affected by charges outside. Fig 2.3 shows a charged rod (which may be a conductor or an insulator) brought near a neutral conductor. The positive charges in the rod attract unlike charges in the conductor to the side closer to the rod and push the like charges to the far side. The conductor is charged locally. But on the whole it still remains neutral. + + rod conductor *Fig 2.3 This phenomenon is called electric induction ( 電感應 ). The charges separated in the conductor are called induced charges. The more charges the cod contains and closer is the rod to the conductor, the more charges are induced
Charging by induction: Alternatively, *Fig 2.4 *Fig 2.5
Negative charges are induced on the side closer to the rod and positive charges on the far side. The attractive force between the positively charged rod and the negative charges on the aluminium piece is larger than the repulsive force between the rod and the positive charges. As a result, there is a net attractive force between the rod and the aluminium piece. *Fig 2.6 The pieces are attracted towards the rod
For the same reason, tiny paper strips are attracted towards a charged object. I understand now. Thank you! *Fig 2.7
I know the same phenomenon can be observed when a charged rod is brought near a stream of running water.
Electrostatic precipitation *Fig 3.1
Photocopying *Fig 3.2
Static charges accumulate easily on insulators due to friction. This causes a nuisance in many respects. ~ When you comb your hair on a dry day. You can feel the pull on you hair by the static charges produced. No need to mention this point!!
#Flying aeroplanes and moving vehicles accumulate charges too. Aeroplanes are fitted with conductive tyres so that the charges are carried to the earth upon landing. If not, a spark may occur during refuel and may cause fires and even explosions. Oil trucks prevent such accidents by carrying a conductive belt or metal chain touching the ground.(Fig 3.4)
#Charges accumulated in clouds are carried to the earth through lightning during a thunderstorm. To prevent the lightning from hitting tall buildings, lightning conductors ( 避雷針 ) are installed on the rooftop. Help!