Section 4 Electricity and Magnetism Lecture

Section 4 Electricity and Magnetism 4.2 Electrical Quantities Positive and Negative Charges 4.2.1 Electric Charge An atom as being made up of a small central nucleus containing positively charged particles called protons, surrounded by an equal number of negatively charged electrons. Every nucleus except hydrogen also contains uncharged particles called neutrons. The charges on a proton and an electron are equal and opposite so an atom as a whole is normally electrically neutral, i.e. has no net • When a strip of polythene is rubbed with a cloth it becomes charged. If it is hung up and another rubbed polythene strip is brought near, repulsion occurs. • Attraction occurs when a rubbed strip of cellulose acetate is brought near. • This shows there are two kinds of electric charge, positive (+) and negative (-). • Like charges (+ and +, or – and –) repel, while unlike charges (+ and –) attract. • The production of charges by rubbing can be explained by supposing that friction causes IGCSE Question: 4.2 Electrical Quantities Positive and Negative Charges Markscheme: 1. rods repel OR move apart / away (1) like / same (type of) charges on both rods (1) 4.2 Electrical Quantities Positive and Negative Charges IGCSE Question: Markscheme: B Explanation: Static electricity is the build-up of electric charge on the surface of objects. This occurs when certain materials are rubbed against each other, causing electrons to be transferred from one material to the other. Concept of Charge Transfer: • When two different materials come into contact and are then separated, electrons may transfer from one material to the other. • The material that loses electrons becomes positively charged. • The material that gains electrons becomes Gold-Leaf Electroscope The device is used for detecting electric charge and can also identify its polarity An electroscope can be used for (read Coursebook pg 186): • Detecting a charge • Charging by contact • Determine conductors and insulators Electrons, Insulators and Conductors Insulator • • In an insulator all electrons are bound firmly to their atoms; (Good insulators include plastics such as polythene, cellulose acetate, Perspex and nylon) An insulator can be charged by rubbing because the charge produced cannot move from where the rubbing occurs, i.e. the electric charge is static. Conductor • In a conductor some electrons can move freely from atom to atom. (All metals and carbon are good conductors) • A conductor will become charged only if it is held with an insulating handle; otherwise electrons are transferred between the conductor and the ground via the person’s body. Electric Fields • • • Electric field is the region of space where an electric charge experiences a force due to other charges. If the electric force felt by a charge is the same everywhere in a region, the field is uniform; a uniform electric field is produced between two oppositely charged parallel metal plates (see Fig 4.2.6). An electric field is a vector quantity as it has both magnitude (strength) and direction. The electric field lines radiating from an isolated positively charged conducting sphere and a point charge are shown in Figures 4.2.7a and b: the field lines again emerge at right angles to the conducting surface. The direction of an electric field at a point, denoted by arrows, is the direction of the force on a small positive charge placed in the field Dangers of Static Electricity See Youtube Video: The Dangers of Static Electricity https://youtu.be/SAhrPa0XB88 The Dangers Of Electrostatic Electricity https://youtu.be/XKAhx4NdJTs Sparks occur between electrostatic charges when the electric field is strong enough. Solution: Providing an easy path for electrons to flow safely to and from the Earth. Mindmap 1. Ground is positively charged relative to the clouds. Because of this there is now a huge potential 2. Lightning happens when charged particles travel from the clouds to earth. Lightning has potential to damage. 3. To prevent this damage, so tall buildings has lightning conductors (or thick metal rods) at the top of the building. This rods allow the lightning to discharged safely through them and flow through a metal rod straight through the building and into the ground.