Electric fields

• The electric field is the force per unit charge that would be experienced by an infinitesimal positive test charge placed at any point.
• Field strength (E) = Force/Charge
• Electric field strength decreases with increasing distance from the source charges.
• The greater the charge magnitude, the stronger the electric field intensity.
• Electric field strength depends on the magnitude of the electric charge producing the field.
• E = V/R Energy = voltage/resistance)
• two negative/positive charges make a positive electric force meaning repulsion
• One positive and one negative charge means a negative force meaning attraction
• Er 


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• Electric field strength is known as the potential gradient
• Electric field strength = potential/distance
• Electric field strength, E = KQ/r^2 where k = 1 /4(pi)(permittivity of free space)
• Coulomb’s law? (force exerted on charge due to field)

  F = k Qq/r^2 where k = 1 /4(pi)(permittivity of free space)
• F = E (energy) x Q
• Electric potential energy?

  E = kQq / r
• Potential = k Q/r (JC-1)
• Distance of closest approach is a way of discovering radius size (not very accurately) and is due to like charges repelling.
• At distance of closest approach, the speed of particle is 0. so Ek= Ep (KQq/r)
• Parallel plates create a uniform electric field. field Strength constant: force is constant; acceleration of particles is constant.
• Equipotentials are equally spaced between parallel plates.
• For parallel plates, electric field strength = V/d
• If a charged particle enters parallel plates at right angle it’s trajectory will be parabolic so can use suvat as if it were a projectile motion question. Horizontal velocity is constant. vertical acceleration = F/m = EQ/m = VQ/ md (E=V/d) ( F=ma) (F=EQ)
• If an object is levitating between parallel plates, the force due to gravity and force due to electric field are equal and opposite. EQ=mg VQ/d = mg Q=mgd/V