- Slides: 15
Static Electricity and Electric Fields
Static Electricity Review • Static electrical charge is created when insulators ( or conductors insulated from Earth) are subjected to friction. • There are two types of static electrical charge positive charge and negative charge.
Static electricity A polythene rod rubbed on a duster acquires a negative charge while an acetate rod rubbed on a duster gets a positive charge A negative charge is produced when excess electrons are present. A positive charge is produced when there are too few electrons leaving an excess of positive ions.
----- +++++++ What is happening is that some negatively charged electrons from the surfaces of atoms are being redistributed from the rod to the cloth. CHARGE IS CONSERVED When a plastic rod are rubbed with a dry cloth. It acquires an electrostatic charge. In the case above the rod has acquired a positive charge. The cloth has acquired a corresponding negative charge The sort of charge acquired depends on the material of the rod and the cloth. E. g. a polythene rod rubbed with a cloth acquires a negative charge ++ ++ ++ -- Like charges attract and unlike charges repel
Test this Unlike charged materials attract each other and like charged materials repel each other.
Conductors and non conductors With a poor conductor, the charge acquired is localized ---- + + + With a metal conductor the charge can be distributed. This is becausemetals have electrons which can move freely about the structure Insulating plastic stand
Conductors and non conductors Bringing charged rods next to a metal surface results in the movement of charge on the conducting surface. This is known as INDUCTION. - -- -- - ++++ + + ++ + +
Conductors and non conductors Inducing charge on a conductor works even if there is no initial overall charge on it. - -- -- ++++ + -- --
Induced charge and picking up paper If a positively charged rod is brought near to a piece of paper, the paper sticks to the rod. +-+-++-+-+- As the positively charged rod approaches the paper, the electrons in the paper are attracted towards the rod. This makes one side of the paper negative and one side positive. A charge has been induced on the paper and the negative side of the paper is attracted to the positive rod.
Electric Fields Just as bar magnets have an associated magnetic field objects with an electrical charge have an associated “electric field”. 0 -5000 V EHT
Observing Electric Fields Grains of semolina in castor oil. - + The grains of semolina behave like electric compass needles (electric dipoles), and line up to show the direction of the electric field.
Observing Electric Fields - +
Convention: The field lines are given a direction away from a positive charge and towards a negative charge. Field lines never cross each other.