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Chapter 17 . Electric Forces & Fields. Terms to Know. Electrostatics: Study of electric charge Charged object: has unequal #s of protons and electrons positively charged object negatively charged object Opposite charges: attract + and – Like charges: repel

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Chapter 17

Chapter 17

Electric Forces & Fields

Terms to know
Terms to Know

  • Electrostatics: Study of electric charge

  • Charged object: has unequal #s of protons and electrons

    • positively charged object

    • negatively charged object

  • Opposite charges: attract

    • + and –

  • Like charges: repel

    • the same charge: + and +, or – and –

Induction is in
Induction is in…

(1) Electroscope

(2) Grounding

Coulomb charges

  • Unit of charge: C = coulomb

    • 1 proton = 1.602×10-19 C; 1 electron = ‒1.602×10-19 C

  • 1 C = charge of 6.24×1018 electrons or protons

Coulomb s law
Coulomb’s Law charges

  • Coulomb's Law (4 min)

  • the force, F, between two charged particles, qAand qB over a distance r

    • Called electrostatic force

    • Mutual attraction/repulsion

      • Notations: FAB= force of A exerting on B

        FBA = force of B exerting on A

      • |FAB | = | FBA |

    • A vector: has a magnitude and a direction

Electrostatic force coulomb s law
Electrostatic Force = Coulomb’s Law charges

  • Magnitude:

    F = electrostatic force

    kc = Coulomb constant = 8.99×109 N· m2/C2

    qA = the charge of particle A

    qB = the charge of particle B

    r = the distance between A and B

  • Analogous to Universal Law of Gravitation:

    G = 6.67×10‒11 N·m2/kg2

Know that
Know that.... charges

  • Coulomb’s law allows us to

    • determine the magnitude of the force between two charged particles

    • determine if the two charges are attracting (positive F value) or repelling (negative F value)

  • Coulomb’s law doesn’t tell to which direction a particle is moving

    • Must be done “manually”

Example charges

  • qA = +3 C located at +1 m from the origin and qB = -2 C located at -2m.

    • The negative F = Particles A and B are attracting

    • The particle A moves to left; B moves to right

      • How do we express the directions?

        • Don’t use a sign for attraction or repulsion

        • Use the signs to indicate the directions of force (N=+; S= ‒, etc)

Problem solving strategy
Problem-Solving Strategy charges

  • Drawing always helps

  • Indicate the direction of force

    • Use the signs for the directions of F, not for the charges of particles

    • Vectors are algebraically additive if they lie in the same dimension (Principal of superposition)

      • Use Trig functions to break down a vector to x- and y- component

  • Use the Coulomb’s law to get the magnitude of the force

  • Consider if the answer is reasonable

Sample problem pg 17a pg 635
Sample Problem, Pg 17A, Pg 635 charges

The electron and proton of a hydrogen atom are separated, on average, by a distance of 5.3×10−11 m. Find the magnitude of the electric force and the gravitational force that each particle exerts on the other.

To get the resultant force on a particle
To get the resultant force on a particle charges

  • Identify all forces acting on the particle

    • Don’t include the forces the particle exerting on other particles


  • Resolve each force into x- and y-component

    • Get the sum of each component

    • Using the Pythagorean theorem, get the hypotenuse, which is the resultant force

Example 17b pg 638
Example 17B, Pg 638 charges

Consider three point charges at the corners of a triangle, as shown right, where q1=6.00×10-9 C, q2=-2.00×10-9 C,

and q3=5.00×10-9 C. Find the magnitude and

direction of the resultant force on q3.

Example problem 2
Example Problem 2 charges

  • A charged particle, A (+6.0 μC) is located near another charged particle B (-3.0 μC) and is located 4.0 cm from the right of A.

  • What is the force of B on A?

(b) A third particle (+1.5 chargesμC) is added to the configuration. If it is located 3.0 cm directly beneath A, what is the new net force on A?

Equilibrium charges

  • the state in which the net force = 0

    • Must have at least three charged particles

    • Must have two opposite forces with the same magnitude for the center particle

      (Ex) (1) + - + (2) - + -

      (3) - - - (4) + + +

  • is seen as objects at rest or at constant velocity

Sample problem 17c pg 640
Sample Problem charges17C, Pg 640

Three charges lie along the x-axis. One positive charge, q1 = 15 µC, is at x = 2.0 m, and another positive charge, q2=6.0 µC, is at the origin. At what point on the x-axis must a negative charge, q3, be placed so that the resultant force on it (q3) is zero.