Physics121

• Electrostatic force


  Attractive and repulsive force between particles caused by their electric charges

  
• Coulomb's force


  Another term for the electrostatic force
• Electrostatic force


  • When iron is placed on silk or cotton cloth, they cling to the body
  • Lightning
• ANY charged object, whether positively charged or negatively charged, will have an ATTRACTIVE interaction with a neutral object
• How a balloon becomes charged

  1. Balloon is rubbed on hair, picking up extra electrons

  2. Balloon is now charged and attracts neutral objects like paper

  
• When a balloon is placed against a wall

  The excess electrons in the balloon repel the electrons in the wall and are attracted to the positive charges

  
• When a balloon is rubbed on hair

  The balloon removes some of the electrons in the hair, giving each strand a positive charge, causing them to repel each other and "stick up"
• As you walk across a carpet

  Electrons are transferred from the rug to you, giving you extra electrons
• Capacitors


  Devices that store electrical charges
• When you touch a door knob (conductor)


  The extra electrons move from you to the knob, causing a shock
• The basic schematic form of a capacitor is below

  
• Positive charges tend to go up, negative charges tend to go down in lightning
• Capacitance


  The ability of an isolated conductor to store the charge given to it
• When the attraction reaches a critical level, you get a lightning bolt
• Capacitor


  • A device which stores electric charge
  • Consists of two conductors carrying equal but opposite charges and separated by an insulator
  • Has many important applications in electronics

  
• Applications of capacitors


  • Storing electric potential energy
  • Delaying voltage changes when coupled with resistors
  • Filtering out unwanted frequency signals
  • Forming resonant circuits
  • Making frequency-dependent and independent voltage dividers when combined with resistors
• Once a capacitor is charged, it has the same voltage as the battery
• The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge itself
• Charging a capacitor


  1. Current flows through the capacitor and each side becomes charged with equal and opposite charges

  2. When the capacitor is fully charged, the current stops flowing
• Parallel plate capacitor


  Two conducting plates, each having a surface area A, separated by distance d, and a voltage V is applied

  
• The capacitance of a capacitor can be changed by physically changing the geometry of the capacitor itself
• This is not a practical capacitor to build
• Dielectric


  An insulating material placed between the conductors to help store the charge
• Capacitors in series


  • Charge each other by induction, so they have the same charge
  • The electric potential is divided up amongst them

  
• Capacitors in parallel


  • Have the same voltage because they touch both ends of the battery
  • Split up the charge amongst them

  
• The slope of the V vs Q graph is the inverse of the capacitance
• The forms of the symbol of the capacitor can be shown in different ways

  
• The energy density of a capacitor is related to the capacitance and voltage
• All electric circuits have one thing in common—they depend on the flow of electric charge
• Electric current


  When electric charge flows from one place to another

  
• The more charge that flows, and the faster it flows, the greater the electric current
• Electric charge is carried through a circuit by electrons

  
• Electric current (I)


  Amount of charge (ΔQ) divided by the amount of time (Δt)

  
• Ampere (A)


  Unit of current, defined as the flow of 1 coulomb of charge in 1 second

  
• A 1-amp current is fairly strong. Many electronic devices operate on currents that are a fraction of an amp
• When charge flows through a closed path and returns to its starting point, we say that the closed path is an electric circuit

  
• Direct-current (DC) circuit


  Current always flows in the same direction
• Alternating-current (AC) circuit


  Currents that periodically reverse their direction
• Electrons move fairly freely in metal wires, but something has to push on them to get them going and keep them going
• No water flow if both ends of the garden hose are held at the same level


  Water flows from the end where the gravitational potential energy is high to the end where it is low