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Introduction to static electricity
An object is positively charged if there are more positive charges than negative charges on the object.
An object is negatively charged if there are more negative charges than positive charges on the object.
An object is electrically neutral if there are equal amount of positive and negative charges on the object.
Like charges repel but unlike charges attract.
Electrical conductors such as metals have free moving electrons that can help to conduct electricity.
Electrical insulators such as rubber and wood do not have free electrons so are not able to conduct electricity.
An object is negatively charged if there are more negative charges than positive charges on the object.
An object is electrically neutral if there are equal amount of positive and negative charges on the object.
Like charges repel but unlike charges attract.
Electrical conductors such as metals have free moving electrons that can help to conduct electricity.
Electrical insulators such as rubber and wood do not have free electrons so are not able to conduct electricity.
Charging by friction (rubbing)
In this video, it demonstrates charging by friction. By using a plastic comb to comb the hair, the hair loses electrons to the comb. The comb becomes negatively charged since it has more electrons (negative charges) than protons (positive charges). This method of charging the comb is by friction. The negatively charged comb can then be used to attract a flowing stream of water causing it to bend.
For O level questions, a common example include rubbing polythene rod with cloth causing the rod to be negatively charged while the cloth becomes positively charged. This is because the cloth has lost some electrons to the polythene rod. This is charging by friction.
*Note that ONLY electrons can be transferred between objects! Protons CANNOT BE TRANSFERRED because they are inside the nucleus of atoms and cannot come out of the nucleus so easily! Some students are confused over this important concept! An object becomes positively charged because it loses electrons, NOT BECAUSE it gains protons (positive charges). Remember that protons CANNOT move out of the nucleus of atoms so CANNOT BE TRANSFERRED between objects!
For O level questions, a common example include rubbing polythene rod with cloth causing the rod to be negatively charged while the cloth becomes positively charged. This is because the cloth has lost some electrons to the polythene rod. This is charging by friction.
*Note that ONLY electrons can be transferred between objects! Protons CANNOT BE TRANSFERRED because they are inside the nucleus of atoms and cannot come out of the nucleus so easily! Some students are confused over this important concept! An object becomes positively charged because it loses electrons, NOT BECAUSE it gains protons (positive charges). Remember that protons CANNOT move out of the nucleus of atoms so CANNOT BE TRANSFERRED between objects!
Charging by induction
Take note that there is an error in one of the diagram in this video at time 6:42. The rod should be drawn as positively charged.
The first part in this video shows how to make a metal sphere become negatively charged by induction.
Step 1: Bring a positively charged rod near to left side of a metal sphere. <Electrons are attracted to the left side of sphere facing the positively charged rod. This leaves the right side of the sphere to become positively charged>.
Step 2: Earth the right side of metal sphere by touching with finger or connecting a wire to the ground. <Electrons flow from earth to the right side of metal sphere making the right side neutral>.
Step 3: Remove the finger or earth wire.
Step 4: Remove the charged rod. <Once the charged rod is removed, electrons on the left side redistribute themselves over the metal sphere. Since there is now more electrons than protons in the metal sphere, the metal sphere becomes negatively charged (net negative charge)>.
The second part in this video shows how to make a metal sphere become positively charged by induction.
Step 1: Bring a negatively charged rod near to left side of a metal sphere. <Electrons are repelled away from left to go to right side of metal sphere. This leaves the left side of sphere to become positively charged while the right side becomes negatively charged since there are more electrons now on right side than left side>.
Step 2: Earth the right side of metal sphere by touching with finger or connecting a wire to the ground. <Electrons flow from right side of sphere to earth making the right side of sphere to become neutral>.
Step 3: Remove finger or earth wire.
Step 4: Remove the charged rod. <Once the charged rod is removed, some electrons from right side of sphere move over to left side of sphere. The effect of this causes the whole sphere to have equal distribution of electrons on both right and left side. But, remember now that the sphere has lesser electrons than before as some electrons had already flowed to the earth (see step 2). Thus, the whole sphere is now positively charged. It now has more protons than electrons.>
Important note: Some students are confused over the effect of earthing the metal sphere. The earth is actually a very enormous pool of many positive and negative charges. When earthing the right side of the metal sphere in the above examples, it depends on what charge the right side of sphere has. If it is positively charged, then electrons (negative charges) from earth will be flowing upwards to the right side of sphere since unlike charges attract. However, if it is negatively charged on right side of sphere, earthing it will cause electrons from the right side to flow downwards to the earth since electrons are attracted to the earth which contains many positive charges.
The important concept again is that ONLY electrons can move between places. Either electrons move from right side of sphere to earth (if right side of sphere had more electrons: negatively charged) or earth to right side of sphere (if right side of sphere had lesser electrons: positively charged).
The first part in this video shows how to make a metal sphere become negatively charged by induction.
Step 1: Bring a positively charged rod near to left side of a metal sphere. <Electrons are attracted to the left side of sphere facing the positively charged rod. This leaves the right side of the sphere to become positively charged>.
Step 2: Earth the right side of metal sphere by touching with finger or connecting a wire to the ground. <Electrons flow from earth to the right side of metal sphere making the right side neutral>.
Step 3: Remove the finger or earth wire.
Step 4: Remove the charged rod. <Once the charged rod is removed, electrons on the left side redistribute themselves over the metal sphere. Since there is now more electrons than protons in the metal sphere, the metal sphere becomes negatively charged (net negative charge)>.
The second part in this video shows how to make a metal sphere become positively charged by induction.
Step 1: Bring a negatively charged rod near to left side of a metal sphere. <Electrons are repelled away from left to go to right side of metal sphere. This leaves the left side of sphere to become positively charged while the right side becomes negatively charged since there are more electrons now on right side than left side>.
Step 2: Earth the right side of metal sphere by touching with finger or connecting a wire to the ground. <Electrons flow from right side of sphere to earth making the right side of sphere to become neutral>.
Step 3: Remove finger or earth wire.
Step 4: Remove the charged rod. <Once the charged rod is removed, some electrons from right side of sphere move over to left side of sphere. The effect of this causes the whole sphere to have equal distribution of electrons on both right and left side. But, remember now that the sphere has lesser electrons than before as some electrons had already flowed to the earth (see step 2). Thus, the whole sphere is now positively charged. It now has more protons than electrons.>
Important note: Some students are confused over the effect of earthing the metal sphere. The earth is actually a very enormous pool of many positive and negative charges. When earthing the right side of the metal sphere in the above examples, it depends on what charge the right side of sphere has. If it is positively charged, then electrons (negative charges) from earth will be flowing upwards to the right side of sphere since unlike charges attract. However, if it is negatively charged on right side of sphere, earthing it will cause electrons from the right side to flow downwards to the earth since electrons are attracted to the earth which contains many positive charges.
The important concept again is that ONLY electrons can move between places. Either electrons move from right side of sphere to earth (if right side of sphere had more electrons: negatively charged) or earth to right side of sphere (if right side of sphere had lesser electrons: positively charged).
Electric field
An electric field is the region in which a positive charge experiences a force.
The direction of electric field points away from a positive charge but it points towards a negative charge.
The direction of electric field points away from a positive charge but it points towards a negative charge.
Electric field lines between like and unlike charges, and two conducting plates
Experiment to show how a light conducting sphere moves between two oppositely charged metal plates
In this expriment, the many styrofoam balls wrapped in aluminium foil act as light conducting spheres. The balls will accelerate upwards and downwards continuously as long as the parallel metal plates remain oppositely charged.
Explanation: At first, each of the ball was neutral in charge. Assume that the top metal plate is positively charged while bottom metal plate is negatively charged.
As soon as both metal plates become oppositely charged, the upper side of each ball facing the top plate will be induced to become negatively charged while underside of the ball will be induced to become positively charged <electrons within each ball are repelled to the upper side of ball making the upper side to be negatively charged while underside is positively charged>.
Since the balls were resting at the bottom plate in the beginning and the underside of each ball was induced to be positively charged, the bottom side immediately gain electrons (unlike charges attract) from the negatively charged bottom plate.
As they continue to gain electrons, the originally positively charged underside of each ball will be neutralised and then the ball eventually becomes negatively charged as it continues to gain electrons. Now, the whole ball is negatively charged and repelled away from the bottom plate since ball and bottom plate are now negatively charged (like charges repel).
The negatively charged balls move upwards away from bottom plate towards the positively charged top plate as unlike charges attract.
Once the balls touch the top metal plate, electrons are lost to the positively charged top metal plate. As electrons continue to be lost, the ball first becomes neutral and then becomes positively charged as more electrons continue to be lost to the top metal plate.
Now that the ball has becomed positively charged, it will be repelled away from positively charged top metal plate (like charges repel) towards bottom metal plate.
The whole process repeat again as the ball touch the negatively charged bottom metal plate (see earlier description when ball was first resting on bottom metal plate). The whole effect of this is that the balls will keep bouncing between the two metal plates continuously as long as the metal plates remain oppositely charged.
Explanation: At first, each of the ball was neutral in charge. Assume that the top metal plate is positively charged while bottom metal plate is negatively charged.
As soon as both metal plates become oppositely charged, the upper side of each ball facing the top plate will be induced to become negatively charged while underside of the ball will be induced to become positively charged <electrons within each ball are repelled to the upper side of ball making the upper side to be negatively charged while underside is positively charged>.
Since the balls were resting at the bottom plate in the beginning and the underside of each ball was induced to be positively charged, the bottom side immediately gain electrons (unlike charges attract) from the negatively charged bottom plate.
As they continue to gain electrons, the originally positively charged underside of each ball will be neutralised and then the ball eventually becomes negatively charged as it continues to gain electrons. Now, the whole ball is negatively charged and repelled away from the bottom plate since ball and bottom plate are now negatively charged (like charges repel).
The negatively charged balls move upwards away from bottom plate towards the positively charged top plate as unlike charges attract.
Once the balls touch the top metal plate, electrons are lost to the positively charged top metal plate. As electrons continue to be lost, the ball first becomes neutral and then becomes positively charged as more electrons continue to be lost to the top metal plate.
Now that the ball has becomed positively charged, it will be repelled away from positively charged top metal plate (like charges repel) towards bottom metal plate.
The whole process repeat again as the ball touch the negatively charged bottom metal plate (see earlier description when ball was first resting on bottom metal plate). The whole effect of this is that the balls will keep bouncing between the two metal plates continuously as long as the metal plates remain oppositely charged.
Hazard due to electrostatic charging
This video shows the example of lightning as a hazard due to electrostatic charging.
Clouds contain ice particles as temperature at high altitudes is cold. When cloud drift through air, ice particles collide against one another as well as with air and there is transfer of electrons between the ice particles and also air causing ice particles and air to become ionised.
The negatively charged ice particles and air will release a large quantity of electrons (negative charges) attracted to and flowing to the ground (which is a large pool of positive and negative charges). This movement of large quantity of negative charges to the ground is seen as lightning.
Clouds contain ice particles as temperature at high altitudes is cold. When cloud drift through air, ice particles collide against one another as well as with air and there is transfer of electrons between the ice particles and also air causing ice particles and air to become ionised.
The negatively charged ice particles and air will release a large quantity of electrons (negative charges) attracted to and flowing to the ground (which is a large pool of positive and negative charges). This movement of large quantity of negative charges to the ground is seen as lightning.
Application of electrostatic charging
This video shows the benefits of using electrostatic spraying.
The object to be sprayed on is earthed. A spray gun is used to spray droplets. As spray droplets emerge from the spray gun, the droplets are charged with same charges. The charged spray droplets having same charges repel one another and at the same time are attracted to the object to be sprayed on. The effect of this is that the spray droplets which repel one another will land evenly on the object and give a uniform coating on the object. This also reduces wastage of the spray.
This way of using electrostatic charging is found in electrostatic spray painting of cars and electrostatic spraying of insecticides on crop plants.
The object to be sprayed on is earthed. A spray gun is used to spray droplets. As spray droplets emerge from the spray gun, the droplets are charged with same charges. The charged spray droplets having same charges repel one another and at the same time are attracted to the object to be sprayed on. The effect of this is that the spray droplets which repel one another will land evenly on the object and give a uniform coating on the object. This also reduces wastage of the spray.
This way of using electrostatic charging is found in electrostatic spray painting of cars and electrostatic spraying of insecticides on crop plants.