Physics

• Electric charge


  Intrinsic property of the particles that make up matter
• Electric charge


  • Can be positive or negative
  • Atoms are composed of negatively-charged electrons and positively-charged protons
• Coulomb


  Unit of electric charge (C)
• Proton and electron have equal and opposite elementary charge = 1.6 x 10^-19 C
• Charge on proton = +1.6 x 10^-19 C
• Charge on electron = -1.6 x 10^-19 C
• Protons and neutrons are made up of quarks with 2/3 and -1/3 charges (electrons are still fundamental)
• Charge cannot be created or destroyed (it is conserved) but it can be moved around
• Charges feel electrostatic forces
• Balloon rubbed against nylon jumper

  • Causes a force of attraction to human hair
• Positive charge on balloon


  Friction moves electrons from hair to balloon, making balloon negatively charged and hair positively charged
• Rubbing balloon on hair


  1. Balloon becomes negatively charged

  2. Hair becomes positively charged

  3. Hair stands on end (like charges repel)

  4. Balloon sticks to hair (opposite charges attract)

  5. Balloon near wall, wall's electrons repelled making it positively charged, balloon sticks to wall (opposite charges attract)
• Coulomb's law


  Describes the strength and direction of the electrostatic force between two charges
• Electrostatic force


  • Is a vector, with magnitude and direction
  • Direction can be indicated by components or a unit vector
• Two 0.5 kg spheres with 100 μC charges, 25 cm apart


  • Calculate electrostatic force and compare to weight
• Where multiple charges are present, the forces sum as vectors (principle of superposition)
• The combined force on a charge from multiple other charges is the vector sum of the individual forces
• Two protons 3.6 nm apart, electron 1.2 nm from one proton


  • Calculate total force on electron
• Electric field


  The force a unit positive charge would experience if placed at a point
• Electric field


  • Is a vector, with direction represented by field lines
  • Field lines start on positive charges and end on negative charges
  • Field lines are closer together where the field is stronger
• Electric field around a positive charge


  • Field lines radially outward
• Electric field around a negative charge


  • Field lines radially inward
• Electric field between two charges

  • Unlike charges: field lines between
  • Like charges: field lines repel
• Electric field between charged plates

  • Constant electric field
• Positive 5 μC charge, negative 2 μC charge 0.74 m apart on x-axis, point P 0.6 m above positive charge


  • Calculate electric field and force on a +1.5 μC charge at P
• Electric dipole


  A pair of positive and negative charges
• An electric dipole in an electric field will feel a torque but no net force
• Electrostatic analyzer


  Selects velocities of charged particles based on the force they experience in an electric field
• Conductors


  • Metals where some electrons are weakly held and can move freely, creating electric current
• Insulators


  • Non-metals where electrons are strongly held and cannot move freely
• Semiconductors


  • Half-way between conductors and insulators
• Matter is made up of positive and negative charges
• Electrons/protons carry the elementary charge 1.6 x 10^-19 C
• Forces between charges are described by Coulomb's Law
• Forces from multiple charges sum as vectors
• Electric field describes the force-field around charges
• Two 0.5 kg spheres with 60 μC charges, 1400 N force

  • Calculate distance between spheres
• Electric field of 1.25 x 10^6 N/C at 0.15 m from point charge

  • (a) Calculate electric flux through sphere of 0.15 m radius

  (b) Calculate magnitude of point charge
• Parallel-plate air capacitor


  • (a) Calculate plate separation to store 240 pC at 42 V

  (b) Calculate potential difference to store 240 pC if plate separation is doubled
• Carbon rod resistance at 25.8°C


  • Calculate resistance given resistance at 0°C and temperature coefficient