Download The Electric Field

Bell Ringer Proton and Electron
A proton and an electron are held
1) proton
apart a distance of 1 m and then
2) electron
released. Which particle has the
3) both the same
larger acceleration at any one
moment?
p
e
Bell Ringer Proton and Electron
A proton and an electron are held
1) proton
apart a distance of 1 m and then
2) electron
released. Which particle has the
3) both the same
larger acceleration at any one
moment?
p
The two particles feel the same force.
Since F = ma, the particle with the smaller
mass will have the larger acceleration.
This would be the electron.
e
Q1Q 2
Fk
r2
Electric Charge and Electric Field
© 2014 Pearson Education, Inc.
The Electric Field
• According to Michael Faraday, an
electric field extends outward from
every charge and permeates all of
space.
• If a second charge (Q2) is placed
near the first charge, it feels a
force exerted by the electric field
that is there.
• The electric field at point P is
considered to interact directly with
charge Q2 to produce the force on
Q2.
© 2014 Pearson Education, Inc.
The Electric Field
• The electric field surrounding a charge can be determined by
measuring the force on a small positive test charge which is at
rest.
• Test charge: A so small charge that the force it exerts does not
significantly affect the charges that create the field.
The electric field is the force on a
small charge, divided by the
charge:
© 2014 Pearson Education, Inc.
The Electric Field
• The electric field at any point in space is a
vector whose direction is the direction of
the force on a tiny positive test charge at
that point.
• The magnitude is the force per unit charge.
• SI units N/C
• If q is positive, 𝑭 and 𝑬 point in the same
direction. If q is negative, 𝑭 and 𝑬 point in
opposite directions.
© 2014 Pearson Education, Inc.
The Electric Field
For a point charge:
The Electric Field
Problem solving in electrostatics: electric forces and
electric fields
1. Draw a diagram; show all charges, with signs, and
electric fields and forces with directions
2. Calculate forces using Coulomb’s law
3. Add forces vectorially to get result
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Electric Field Lines
• Since the electric field is a vector, it is sometimes
referred to as a vector field
• The electric field can be represented by field lines.
These lines start on a positive charge and end on a
negative charge.
© 2014 Pearson Education, Inc.
Electric Field Lines
• The number of field lines starting (ending) on a
positive (negative) charge is proportional to the
magnitude of the charge.
• The electric field is stronger where the field lines are
closer together.
• The direction of the electric field at any point is
tangent to the field line at that point.
Electric dipole: two equal
charges, opposite in sign:
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Electric Field Lines
Electric field lines for two
equal positive charges
• Unequal charges –Q, and +2Q.
• Note that twice as many lines
leave +2Q as enter –Q (number
of lines is proportional to
magnitude of Q).
© 2014 Pearson Education, Inc.
Electric Field Lines
The electric field between two
closely spaced, oppositely charged
parallel plates is constant.
© 2014 Pearson Education, Inc.
Electric Field Lines
Summary of field lines:
1. Field lines indicate the direction of the field;
the field is tangent to the line.
2. The magnitude of the field is proportional to
the density of the lines.
3. Field lines start on positive charges and end
on negative charges; the number is
proportional to the magnitude of the charge.
© 2014 Pearson Education, Inc.