ELECTROTATIC Learner ote: In Grade 10 learners are introduced to electric charges. There are two types of electric charge, positive and negative. Equal numbers of positive and negative charges result in an object that is neutral. Relate electric fields to magnetic fields. Emphasise that electric and magnetic forces may attract or repel. Qualitative work is done in grade 10 which forms the base for grade 11 for calculating the magnitude of the force between charges when they study Coulomb s Law. Learner ote: imple experiments can be performed to show that like charges repel and opposite charges attract. Rub a ruler on a jersey; pick up a small piece of paper. Rub over an inflated balloon; it will stick to a smooth wall. A thin stream of water of water can be deflected by a charged rod or balloon. ECTIO A: TYPICAL EXAM QUETIO Question 1: 10 minutes A glass rod is charged negatively by rubbing it with a cloth. The rod is held near a neutral graphite coated, insulated sphere. 1.1 Explain how the glass obtained a positive charge (2) 1.2 What is the charge on the cloth? (1) 1.3 The sphere is neutral. What does it mean? (1) 1.4 Draw a diagram showing the neutral sphere before the rod was near it. (1) 1.5 Draw a diagram and explain the effect of bringing the positive glass rod close to the sphere. (4) 1.6 What is this phenomena called? (1) 1.7 Comment on the number of charges on the sphere when it is polarised. (2) 1.8 tate two ways that can be used to increase the electrostatic force between the glass rod and the sphere (2) Learner note: Be sure that you know that all objects contain both positive and negative charges, but we only say an object is charged when it has extra positive or negative charges. (14) Page 1 of 7
Question 2 10 minutes (When the liquid is sprayed it moves through the nozzle this causes friction. Objects are charges by means of friction) 2.1 When a sprinkler system is used to spray plants, the droplets will be positively charged due to friction. Did the droplets gain protons or loose electrons? (1) 2.2 Explain how the drops became charged (4) 2.3 What is the advantage of spraying charged drops (3) 2.4 Use this example to illustrate the concept of conservation of charge (5) (13) Question 3 (Taken from GDE June 2009) 10 minutes A lightning conductor made of copper is usually attached to a tall building. This reduces the risk of damage by a lightning strike. 3.1 Explain why the lightning conductor is made of copper (2) 3.2 Would it be advisable to be in a car when lightning strikes? Explain (4) 3.3 Define the Law of conservation of charge (2) 3.4 A plastic ruler is charged positively. By referring to atomic structure, explain how the ruler becomes charged (3) (11) ECTIO B: OLUTIO AD HIT Question 1: 1.1 The glass rod was neutral, when rubbed (friction) electrons are transferred from the glass rod to the cloth. Learner note: It is important to emphasize that the glass still contains electrons, all were not transferred. There are more positive charges than negative charges in the positively charged object. 1.2 egative 1.3 It has an equal number of negative charges (electrons) as positive charges (protons) 1.4 + - + - + - + - + - + - + - + - + - Page 2 of 7
1.5 + + + - - - + + - + - - + + - + - - + + + + + + Positive rod attracts negative charges towards it Uneven distribution of charges in sphere, it is now polarised although it is still neutral, no electrons were transferred. 1.6 Electric polarisation 1.7 The number of positive and negative charges is equal, no transfer occurred 1.8 Rub rod more, a greater charge increases the force, bring the rod closer to the sphere inverse proportion between charge and distance of rod Question 2 2.1 loose 2.2 As drops move through the nozzle when sprayed, the electrons are transferred from the drops to the nozzle due to friction. The drop becomes positively charged as it has fewer electrons than protons, the nozzle becomes negatively charged 2.3 The drops form a fine mist as each drop repels the other because their charges are similar, the plants and drops are attracted the plants are not charged. Water clings to the plant. 2.4 Conservation of charge charges cannot be made or destroyed, they are only transferred from one object to another. egative charges rubbed off the drops are taken up by the sprayer Question 3 3.1 Copper is a good conductor of electricity and will allow the charges to be conducted easily 3.2 Yes, the tyres are made of rubber an insulator, it will not conduct the electricity to the ground 3.3 Charges can not be made or destroyed, they are only transferred from one object to another 3.4 Electrons are rubbed off the ruler by means of friction between the ruler and the cloth. The plastic ruler is an insulator as it has few free electrons to carry the charges, it therefore becomes positively charged Page 3 of 7
ECTIO C: ADDITIOAL COTET OTE Electrostatics is the investigation of charges that are not moving they are static. ELECTRIC CHARGE We have to look at the composition of an atom first. Inside the nucleus of an atom we find positive protons and neutral neutrons, around the nucleus in the orbitals we find negative electrons. If friction is applied to an object the negative electrons (in the orbitals) can be transferred. That means an object can receive or loose electrons. An object becomes positively charged due to the loss (electrons are transferred) of electrons. An object becomes negatively charged due to the gain of electrons. When substance loose electrons due to friction to become positively charged, the substance it was rubbed with will gain electrons and become negatively charged. When electrons are transferred from one object to another it is referred to as induction. The Law of conservation of charge states that - Charges can not be created or destroyed, but can be transferred from one object to another. FORCE EXERTED BY ELECTROTATICALLY CHARGED OBJECT An Electrostatically charged object has an electric field round it the electric field is an area where another object will experience an electrostatic force it will be attracted or repelled. The force between charges increases when the magnitude (size) of the charge on the object increases. The force between the charged objects increases as the distance between the charged objects decreases. The magnitude of the charges and the electrostatic force is directly proportional. The electrostatic force and the distance between the objects show an inverse proportion Remember that like charges repel and unlike charges attract one another! It is important to understand that a neutral object will be attracted to a positive or a negative substance. If a negative object comes near a neutral object the extra electrons will repel the free electrons on the neutral object and will cause the electrons to move away. This will cause one side to be positive and the positive side will attract the negative object. Page 4 of 7
If a positive object comes near a neutral object, the free electrons in the neutral object will be attracted to the side where the positive object is held. The positive object will be attracted to the negative side of the neutral object. When a neutral object forms a positive and a negative side due to a charged object near it, the phenomena is called electrical polarisation. Materials that consist of polarised molecules can be rotated when brought near to a charged object, so that one side of the object is more positive and the other side more negative. However the object as a whole remains neutral as there was no transfer of electrons. CODUCTOR AD IULATOR An electrostatic force will cause for charges to move. If charges can move through a substance, it is called a conductor. If the charge can not move through a substance it is called an insulator or a non-conductor. Conductors: Metals are good conductors because their electrons are loosely bonded and can easily move. If a solution conducts electricity it is called an electrolyte. Insulators: on-metals are very good insulators, their electrons are very tightly bonded and can not move. ECTIO : D HOMEWORK Question 1 20 minutes 1. Explain why a piece of wood is an insulator. (2) 2. Classify the following substances as insulators and conductors, tabulate your answer Iron, plastic, copper, glass, wood, rubber, ceramic, zinc (8) 3. In the motoring industry the paint drops sprayed onto the car become charged as they leave the spray gun. The car may be oppositely charged during the spray process. Give economical and social advantages of this. (6) 4. Applying spray pesticide to plants uses the same principle as in the previous question. Explain the advantage of electrostatics using this example. (4) (20) Page 5 of 7
ECTIO : E OLUTIO TO EIO 12 HOMEWORK ECTIO A: TYPICAL EXAM QUETIO Question 1 a.) Repel b.) Attract c.) Question 2 Page 6 of 7
Question 3 3.1 oft ironhard - steel 3.2 tronger magnet will have more magnetic field lines than the weak magnet. The field lines will be closer together especially at the end of the magnet 3.3 usceptibility refers to how easily the domains in a ferromagnetic substance are aligned to cause the substance to become magnetised Retentively refers to the ability of the domains in a ferromagnetic substance to remain aligned ie. its ability to become a permanent magnet 3.4 Fridge magnet keeps reminders in a visible spot Fridge magnets are collectables and may be used as decoration Magnetic letters can be used on boards to assist young children with spelling Fridge door magnets help to seal the door and keep the temperature low which saves electricity Compasses can be used in navigation to prevent people from getting lost The IP is supported by Page 7 of 7