Electric Fields and Electric Potential

Electric Fields and Electric Potential

1. Coulombs law can be used to find the force between two point charges

   • If the charges are opposite, the force is attractive and F will be nagative
   • The force on Q is always equal and opposite to the force on q
   • Further apart charges, weaker force
   • The force also depends on the permittivity of the material the charges are in

2. There is an electric field around a charged object

   • Can be attractive or repulsive, but are still radial
     • Charged spheres can be treated as point charges
     • Can be represented by field lines
   • Charge is measured in coulombs
     • It can be positive or nagative
   • Opposite charges attract, like charges repel
     • If a charged object is placed in an electric field it will feel a force

3. Electric field strength is force per unit charge

   • For this equation, you find the force acting on a charge Q which is in an electric field. Q does not cause the electric field.
   • E is a vector pointing in the direction that a positive charge would move in
     • Newtons per Coulomb
     • Depends on where you are in the field
     • A point charge has a radial field
   • In a radial field, E is inversely proportional to r squared
     • When the electric field is generated by a point charge, we redefine Q as the charge causing the field, and q as the charge experiencing the force.
       • In a radial field, E depends on the distance R from point charge Q
         • Area under a graph of E against r would give the electric potential

4. Field strength id the same everywhere in a uniform field

   • A uniform field can be produced by connecting two parallel plates to opposite poles of a battery
     • Field strength is the same between any two points and is the voltages per unit distance
       • Volts per metre
     • Charge particles will move through a uniform electric field like projectiles
       • A particle of charge Q will experience a constant force given by E=F/Q
         • The work done on a particle by this force (W=Fd) increases its kinetic energy and causes it to accelerate at a constant rate in the direction of the force
           • If the particle has a velocity perpendicular to the field lines, the horizontal velocity will remain constant
           • Curved path
   • Parallel plate capacitors generate a uniform electric field
     • The capacitance of a capacitor depends on how easy it is to generate an electric field between the plates and on the dimensions of the capacitor

5. Electric potential is potential energy per unit charge

   • All points in an electric field have a potential, V
     • This is the work done to bring a unit positive charge from infinity to that point in the field. (At infinity, electric potential would be zero)
       • The sign of V depends on the charge of Q
         • The absolute magnitude of V is is greatest on the surface of the charge
   • Electric potential energy is just the electric potential that a unit positive charge would have at a certain point. Therefore you can use electric potential to find electric potential energy
   • Electric potential can be used to figure out capacitance, I can't find a picture of the equation but it's capacitance is equal to 4(pi)(Epsilon)r

6. Comparisons between electric and gravitational fields:

   • gravitational field strength, g, is force per unit mass and electric field strength, E, is force per unit positive charge
     • The size of an electric force depends on the medium between the charges
   • Newtons law of gravitation for the force between two point masses and coulombs law for force between two point charges are inverse square laws
   • They have radial fields
   • Gravitational potential is potential energy per unit mass, and electric potential is potential energy per unit positive charge. Both are zero at infinity.
   • Gravitational forces are always attractive but electric forces can be attractive or repulsive
   • Objects can be shielded from electric fields but not gravitational fields

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