The field model
In the force model, a particle A exerts a force directly on particle B.
In the field model, A alters the space around it.
Particle B then responds to the altered space. The altered space is the agent that exerts
the force on B.
Magnitude of electric field = q
Field Field strength (N/C)
Inside a current-carrying wire 10 -2
Near the earth’s surface 10 (typical field: 100 N/C, directed down)
Near objects charged by rubbing 10 to 10 6
Needed to cause a spark in the air 10 6
Inside a cell membrane 10 (enough to cause spark in air!)
Inside an atom 10 11
The field of a single charge
The field vectors point away from a positive charge
Electric field of charge q at a distance r
, [away from q if q > 0, toward q if q < 0])
The field lines are closest together near the charge, where the field strength is greatest.
Start on +
End on – Tangent to field
Close = strong, far = weak
Magnetic field is uniform in a parallel plate capacitor
The electric field of the heart is a dipole
Two parallel plates have charges of equal magnitude but opposite sign. Which of the following
changes would not increase the field strength between the plates?
Increasing the magnitude of the charge on both plates
Decreasing the distance between the plates
Decreasing the area of the plates (while keeping the magnitude of the charges the
Field strength depends on density of charge.
A small sphere is suspended from a string in a uniform electric field. Several different cases of
sphere mass and sphere charge are presented in the following table. In which case is the angle
at which th