Electrostatics

• Charge

  One of the basic properties of the elementary particles of matter giving rise to all electric and magnetic forces and interactions
• Electrostatic charge

  Electric charge at rest on the surface of an insulated body
• Electric Force Field

  The region around a charged particle or object in which it exerts a force on other charged particles or objects
• Electrostatic Force

  The force exerted by non-moving (static) charges on each other
• Basic Law of Electrical Charges

  Like charges repel each other while unlike charges attract each other
• Law of Conservation of Charge

  The net charge of an isolated system remains constant
• Atoms are made of charged particles: Electrons have a negative charge, Protons have a positive charge, The magnitude of these charges is the same, and until now we have expressed these values in atomic units of – 1 and + 1
• Coulomb (C)

  The unit of Charge (Q)
• Charge of 1 electron (e-) = -1, 6 x 10-19 C, Charge of 1 proton (p+) = +1, 6 x 10-19 C
• To calculate the number of electrons or protons that are present in a given amount of charge

  No. particles = total charge / charge on one particle
• Charging large objects by friction

  1. Two objects made of different materials can be electrically charged by rubbing them against each other

  2. During this process, the atoms from the different substances interact. The substance with atoms with a greater electron affinity attracts some loosely bound electrons from the surface of the other substance to itself. On separation, the objects will have opposite electrostatic charges – charge separation has occurred

  
• Triboelectric series

  Compares the abilities of substances to lose or gain electrons by friction

  
• Discharge

  When oppositely charged objects come into contact, the charge imbalance minimizes by electrons flowing from negative onto positive objects – sometimes this can happen very quickly and even through the air!

  
• Polarisation

  If a positively charged rod is brought close to a non-conductor such as paper, it can cause the electrons on the surface of the paper to move round to the side of the paper which is closest to the rod i.e. the paper is now polarised. The positive charges on the paper do not move.

  
• Charging large objects by contact

  1. When a conductor becomes charged through contact, the electrons spread across the conductor such that the electrons are spaced as far away from each other as possible to minimize the repulsive forces between them

  2. The two objects will have the same charge when separated – see the Law of Conservation of Charge

  3. For insulators, the charge does not get distributed but remains localised where the contact or friction occurred

  
• It is sometimes useful to charge a non-conductor (like balloons) BECAUSE the charge will not distribute and so a greater effect can be seen
• Electrostatic forces

  • They are non-contact (field) forces
  • They get weaker the further apart the charges are
  • They depend on the strength of the charges
  • They only occur when there is an interaction between two or more objects

  
• Electroscope

  An instrument which can indicate the presence of charge. Usually an electroscope is composed of a metal disc or sphere at the top of the wire or rod and thin foil leaves at the bottom which is placed in a transparent container.

  
• Polarising an electroscope

  As soon as a charged object is held near to the electroscope, the leaves will move apart as the charge on the disc causes the leaves to become polarised

  
• Electrostatic force

  • Non-contact (field) force
  • Gets weaker the further apart the charges are
  • Depends on the strength of the charges
  • Only occurs when there is an interaction between two or more objects

  
• Charge

  Two types: positive and negative
• Like charges

  Repel
• Unlike charges

  Attract
• Electroscope

  • Instrument which can indicate the presence of charge
  • Usually composed of a metal disc or sphere at the top of the wire or rod and thin foil leaves at the bottom which is placed in a transparent container
• Polarising an electroscope

  1. Bringing a charged object near the disc

  2. Leaves repel each other

  3. Greater the charge, stronger the repulsive force, further the leaves move

  
• Charging an electroscope through contact

  1. Bringing a charged rod near a charged electroscope

  2. Can cause the leaves to collapse or repel further
• It is only the electrons that move
• Two charged hollow metal spheres

  • If the left sphere swings towards the right sphere, the left sphere is oppositely charged
  • If the left sphere swings away from the right sphere, the left sphere is similarly charged
• Two positively charged metal balls

  • Electrostatic force between them is repulsive
  • As the distance between them decreases, the electrostatic force increases

  
• Two charged spheres hanging from strings

  They will swing away from each other due to the repulsive electrostatic force between them
• Law of Conservation of Charge

  • The net charge of an isolated system remains constant
  • If isolated, charged objects come into contact, the charge will redistribute but the total charge will remain the same

  
• Neutral pith ball brought near positively charged rod

  • Electrostatic process is polarisation
  • Attractive force will occur between the ball and the rod
  • When they touch and then suddenly repel, it is because the electrons have redistributed
• +6 mC pith ball and -10 mC pith ball

  • They will attract each other
  • Combined charge will be -4 mC
  • The -10 mC ball will gain electrons
  • After separation, the +6 mC ball will have 4 mC and the -10 mC ball will have -4 mC
  • 4 mC of charge will have moved
  • 4 x 10^6 electrons will have moved
  • They will now repel each other
• When identical, spherical, insulated charged conductors come into contact, electrons flow to redistribute evenly over both objects
• Sphere A had +2.4 nC, Sphere B was uncharged

  • After contact, each sphere will have +1.2 nC
  • Sphere A will have lost electrons
• Pith ball X (+3.5 μC), Y (+2 μC), Z (-6 μC)

  • X will lose electrons when it contacts Y
  • After separation, X will have +2.75 μC
  • 0.75 μC of charge moved when X contacted Y
  • When X contacts Z, Z will end up with -6.75 μC
• Electric field

  The region around a charged object in which a force would be exerted on another charged object
• Pith ball A (+3 nC), Pith ball B (-7 nC)

  • Ball B will gain electrons when they touch
  • After separation, Ball A will have +1 nC, Ball B will have -5 nC
  • 4 nC of charge has moved
  • 2.5 x 10^7 electrons have transferred
  • The balls will now attract each other
• Ball A (+10 nC), Ball B (+5 nC)

  • They will repel each other
  • After contact, the combined charge will be +7.5 nC
  • Ball B will gain electrons
  • After separation, Ball A will have +7.5 nC, Ball B will have +7.5 nC
  • 2.5 nC of charge will have transferred
  • 1.56 x 10^7 electrons will have moved
  • The balls will now repel each other
• Ball A (-1 pC), Ball B (-5 pC)

  • They will attract each other
  • After contact, the combined charge will be -6 pC
  • Ball A will lose electrons
  • Ball A will gain protons
  • After separation, Ball A will have -3 pC, Ball B will have -3 pC
  • 2 pC of charge will have transferred
  • 1.25 x 10^6 electrons will have moved
  • The balls will now attract each other