Fields

• Field


  The indivisible area of influence around an object. Causes to experience a force
• Source


  The object that produces the field. Anything with mass
• Test object


  Affected by the source
• Fields have a source, direction and strength

  • theres 3 types of fields

  1. Gravitational fields
  2. electrical fields
  3. magnetic fields
• Electrical fields. Test object - any charged object

  1. static electricity
  2. lightning
• Gravitational fields. test object - anything with mass

  1. things falling towards earth
  2. the earth orbiting the sun
• Magnetic fields. Test Object - magnet

  1. magnets
  2. using a compass
• Gravitational fields

  1. Exist around all objects with mass
  2. direction of force is always towards the center of the object creating the field
• Newtons 2nd Law: F = ma


  When an object is in a gravitational field, it will experience an acceleration due to gravity. Therefore we can rewrite Newtons 2nd Law to calculate the force of gravity

  Fg = mg
• Acceleration due to gravity is known as gravitational field strength

  GFS- a measure of how strong a gravitational field at a given point in space

  • formula g= Gm/r^2
• What happens to the gravitational field strength (g) if mass increases?


  It has greater gravitational field strength
• what happens to the gravitational field strength if the radius increases?


  The gravitational field strength decreases
• If the distance from the source doubles what happens to the grav field strength 


  The strength decreases

  • calculation - g= 1/2^2 =1/4
  • changes by a factor of 4
• If the distance triples, what happens to the grav field strength?


  It decreases

  • calculation g = 1/3^2 = 1/9
  • changes by a factor of 9
• If the distance from the source quadruples, what happens to the grav field strength?


  It decreases

  • calculation g = 1/4^2 = 1/16
  • changes by a factor of 16
• If the distance from the source is cut in half what happens to the grav. field strength?


  It increases

  • calculation g= 1/0.5^2 = 1/0.25
  • chabges by a factor of 0.25
• every charged object (positive or negative) creates an electrical field of force in space around it
• How do objects become charged

  • recall atoms made a positive nucleus and negative electrons orbiting the nucleus
  • charges are neither created nor destroyed, just transferred
  • when one object rubs against another, electrons are transferred from one material to another
• Opposite charges attract, like charges repel
• The more charged an object is, the greater the force of repulsion or attraction
• A positively charged object creates an electric field with vectors pointing away from the object
• A negatively charged object creates an electric field with vectors pointing towards the object
• Remember away from the positive, towards the negative
• The direction in which the positive test body will move is the direction of the electric fiel. will move towards the negative object because opposite charges attract
• Electrical field strength equation 


  |E| = kg/r^2

  • |E| - magnitude of electrical field strength
  • kg - electrostatic charge of source
  • r - radius or center to center distance (m)
• “+” C means an object lost electrons

  ”-“ C means an object gained electrons
• Magnetic fields are caused by moving electrons
• Electricity and magnetic fields are intertwined - one creates the other
• All magnets have 2 poles : North and South. this makes magnets dipolar
• Attraction and repulsion occurs as with electric fields
• Some elements have stranger magnetic properties than others - iron, cobalt, nickel
• To determine the direction of magnetic field lines we would use a “north” test body to see which direction it will move
• Magnetic field lines always move out of the north and into the south pole
• North on a compass points to south on a magnet
• The geographical North Pole is actually the magnetic south pole
• Conductor substances that allow electrons to move freely - iron, cobalt, nickel, copper
• Insulators substances that inhibit the flow of electrons - wood, plastic rubber

  • movement of a magnet through a coiled conductor creates electricity
• electromagnet - a conductor that becomes magnetized when electric current runs