Introduction to Static Electricity

In Static Electricity course, you will expect to handle lines and points as well as the related calculations. It is important for you to keep your accurate geometrical instruments close. Specific outcomes

• Demonstrate the existence of static charges
• Explain how to detect electric charges.
• Describe the properties and uses of static charges
• Describe the electric charging and discharging of objects.
• Explain the relationship between current and static electricity.
• Investigate effects of static charges on the environment.

CONTENT

Static electricity is stationary (i.e. not moving) and in all the examples above, it is produced by friction. It is usually produced when two surfaces are in close contact rub against each other. Electrostatics is the study of static electricity or electric charge which is static (not moving). Remind learners that all objects surrounding us (including people!) contain large amounts of electric charge. There are two types of electric charge: positive charge and negative charge. If the same amounts of negative and positive charge are brought together, they neutralise each other and there is no net charge; the object is neutral. However, if there is a little bit more of one type of charge than the other on the object, then the object is electrically charged. The concepts: positively charged (an electron deficient) and negatively charged (an excess of electrons). The unit in which charge is measured is coulomb (C). A coulomb is a very large charge. In electrostatics we often work with charge in microcoulombs electric charging). and nanocoulombs Objects may become charged by contact or when rubbed by other objects. Charge, like energy, cannot be created or destroyed – charge is conserved. When a ruler is rubbed with a cotton cloth, negative charge is transferred from the cloth to the ruler. The ruler is now negatively charged and the cloth is positively charged. If you count up all the positive and negative charges at the beginning and the end, there is still the same amount, i.e. total charge has been conserved! An electrostatic force is exerted by charges on each other. The electrostatic force between:

• like charges are repulsive
• opposite (unlike) charges are attractive.

The closer together the charges are, the stronger the electrostatic force between them. Perform the suggested experiment to test that like charges repel and unlike charges attract each other. The electrostatic force also determines the arrangement of charge on the surface of conductors, because charges can move inside a conductive material. On a spherical conductor the repulsive forces between the individual like charges cause them to spread uniformly over the surface of the sphere, however , for conductors with non-regular shapes, there is a concentration of charge near the point or points of the object. Conductors and insulators: All the matter and materials on earth are made up of atoms. All atoms are electrically neutral i.e. they have the same amounts of negative and positive charge inside them. Some materials allow electrons to move relatively freely through them (e.g. most metals, the human body). These materials are called conductors. Other materials do not allow the charge carriers, the electrons, to move through them (e.g. plastic, glass). The electrons are bound to the atoms in the material. These materials are called non-conductors or insulators. There can be a force of attraction between a charged and an uncharged neutral insulator due to a phenomenon called polarisation. The latter is explained in terms of the movement of polarised molecules in insulators. Learners are also introduced to the electroscope, a very sensitive instrument which can be used to detect electric charge. The principles of electrostatics are made use of in various ways. Voltage differences exist between the pins of special electronic microchip devices and as such get charged. These pins, getting charged by induction destroy the inner circuits of the chips Hence at places where these microchips are being produced and sold, the floor dresses and handling equipment are all made of antistatic materials. A lightning conductor is made by assembling a number of sharply pointed copper spikes. It is then fixed at the highest position of a building and a copper conductor connected to it comes down the building to be connected to the Earth.

Electrostatic static precipitators Particle impurities such as dust enters the atmosphere from coal power plants, iron and cement industries and other factories emitting black smoke. This is a hazardous situation, but these particles can be removed by electrostatic precipitators.

Figure1.1: The structure of Electrostatic dust precipitator

Air containing dust particles flow between the wires of about 50,000V voltage. These particles then stick to the iron strips and 20 -50 tons of dust can be collected by hammering the strips for about one hour.

Photocopy machine A powerful beam of light is directed to the letters and diagrams on the paper inserted into the photocopy machine. The reflected light forms its image in the drum of the machine. The black portions retain the charges while the white portions loose the charges by neutralisation. The black carbon powder (toner) spreads across the drum and sticks to the charged places and when the photo copy paper passes through the drum the charged particles stick to it. Finally, when the paper passes through a heater the powder fuses to print a permanent figure on the paper.

Electrostatic paint sprayer Paint drops are sprayed as fine particles and are charged negatively. The body of the motor car or any other object to be painted is charged positively. Then the negative particles spread as a thin layer and sticks on to the metal surface. Paint drops do not spread over to the atmosphere and hence environment pollution is prevented.

Fuel filling Filling of fuel into air planes, ships etc. is done at high speeds. The vapour formed during the filling can get charged and charged fuel particles are extremely harmful as they can form sparks and catch fire. The formation of electric charges takes place due to rubbing during the filling and emptying of fuel. Metal strips are used to earth the pipes and neutralise the charges thereby preventing any explosions.

Expected Knowledge about Static Electricity

• Existence of static charge: Positive and negative charges (Law of electrostatics)
• Detection of charge: charging by contact, testing the sign of charge using gold- leaf electroscope etc.
• Properties and uses of static charges: -Properties; like charges repel, unlike charges attract
• -Uses: dust precipitators, ink jet printers, photocopiers.
• Electric charging and discharging of objects.
• Relationship between current and static electricity in terms of effects as static electricity producers same effect as current electricity.
• Effects of static charges on an environment: e.g. lightning etc.

SKILLS

• Experimenting the existence of charges by rubbing some materials
• Detecting charge using an electroscope
• Communicating properties and uses of static charge
• Experimenting charging and discharging of objects
• Communicating knowledge on the relationship between current and static electricity
• Investigating the effects of static charges on the environment e.g. lightning

Remember, mathematics is a practical subject, follow the instructor and tutor’s explanations and try to implement on your own, before you try to tackle the practical exercises.

Also, don’t forget to explore the Syllabus and  Pastpaper page for more exercise and revisions

All the best!