BMAT Physics - P1 Electricity Practice Questions and Answers

Know and understand that insulators can be charged by friction When two insulators move relative to each other, friction between the two can result in both objects becoming electrically charged. One of the objects becomes positively charged, and the other becomes negatively charged. Charging by Induction When a neutral object is places near to a charged object, the object can become charged. If one end of an object charged by induction is momentarily earthed, the charge leaks away, and the object becomes permanently charged. Know and understand that charging is caused by gain or loss of electrons When two objects are rubbed together, friction can result in electrons being transferred from one to the other. This results in the two objects becoming electrostatically charged. One object (the one that loses electrons) becomes charged positive, and the other (the one that gains electrons) becomes charged negative. It is always electrons that are responsible for electrostatic charge Know and understand that like charges repel and unlike charges attract. Two charged objects placed near each other exert an electrostatic force on each other. The size of this force depends on the size of the two charges and the distance between them. The larger the charges, the larger the force, and the larger the distance of separation the smaller the force. Like charges repel. Unlike charges attract. Understand applications and hazards associated with electrostatics, including the role of earthing Photocopying, laser printing and electrostatic air cleaning. Sparking - why it occurs, and the eliminating the risk. Sparking occurs when the air between two objects becomes ionised by a large voltage and starts conducting. Two charged objects, that have air between them, can discharge by a spark between them. This will happen either when the charge becomes large enough or when the distance between the objects becomes small enough. - Risk of fires being caused by sparking. An example of this is the risk of explosion when aircraft are refuelled. The risk of sparking can be eliminated by earthing. If two objects, that would otherwise cause each other to become charged by friction, are connected together by a wire (or if one of them is connected to Earth by a wire) then electrostatic charging cannot take place and the risk is averted. Know the difference between AC and DC Direct current (dc) is current that is always in the same direction. Examples: cells or batteries. Alternating current (ac) is current that repeatedly changes direction, usually very rapidly. Examples: generators in power stations. AC graph is y = sin x