chapter 15

• electric charge


  si unit of electric charge is the coulomb

  each electron and proton has the same amount of charge of 1.6x10^-19

  like charges repel each other while unlike charges attract each other
• electric field

  an electric charge can exert a force on another nearby charge, this is known as electric force

  the electric force can be attractive or repulsive, it gets weaker as the nearby charge moves further away

  electric field is a region in which an electric charge experiences an electric force
• electric field diagram


  arrows always points towards a negative charge and away from a positive charge

  lines are closer together when electric field or electric force is stronger

  field lines never cross one another
• different attraction of electrons by different atoms

  when two objects come into contact, the less tightly held electrons from object A are attracted and transferred to object B. the positively charged object A and the negatively charged object B will attract each other and thus they stick together
• charging one conductor


  placed a negatively charged rod close to the conducting sphere. the unlike charges will accumulate on the side nearer to the charged rod while the like charges accumulate on the other side. touching the sphere on the right allows electrons to be repelled further away into the person's body. remove the hand and the charged rod you will now get a positively charged conductor
• charging two conductors


  place a charged rod close to one of the conducting spheres that are in contact with one another. positive charges in the rod attracts free electrons to the left, leaving the right side positively charged. keeping the rode in position, separate the two spheres to isolate the two groups of apposite charges. remove the charged rod, both spheres now have equal but opposite charges
• the charged rod induces both negative and positive charges in the object. however, the negative charges are nearer to the rod compared to the positive charges. hence, attraction is stronger than repulsion and the paper bits are attracted to the rod
• when both conducting spheres are in contact, nothing happens as the charged particles will not flow out of the sphere

  when in contact, redistribution will occur
• dry bulk flows through a hose or pipe

  friction against the inner walls can lead to electrostatic charging of the hose and the substance. at the outlet, the opposite charges between the hose and the substance might cause sparking. if substance is combustible, the spark can cause a fire or even an explosion
• earthing/grounding

  due to the danger of electrostatic charging and discharge in presence of combustible substances, preventive measures are made

  a metal chain or strip is dangled to the ground at the rear end to conduct electrons to the ground. with grounding, the electrostatically generated charges are prevented from building up by being diverted to the ground
• spray painting

  the paint is charged positively, the pain droplets will repel one another, enabling a more uniform mist. usually, a car part being painted is metallic. when the positive droplets gets attracted to the free electrons on the surface of the car, paint droplets will also be able to land on targeted areas.

  compared to uncharged paint, there will be less wasted paint and resulting coat will be more uniform

  however, charged paint might cause sparking, turning mist into a fire spray
• electrostatic precipitator


  incoming gas with unwanted particles enters the electrostatic precipitator

  the unwanted particles are charged by free electrons and negative ions when they pass through the plate

  the negatively charged particles will be attracted to the positive plates

  the clean gas, where the unwanted particles are removed, exits the electrostatic precipitator

  a hammer regularly knocks the plate so that the accumulated particles on the plates drop into a bin below the plates