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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

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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

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

• Conductor: material in which a charge can move easily

• Insulator: material in which a charge cannot move easily

• Induction: to charge an object without touching it

• Inducing by a Positive Charge

• Induction by a Negative Object

### Induction is in…

(1) Electroscope

(2) Grounding

(3) Polarization = separation of positive and negative charges

### Coulomb

• 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 (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

• 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....

• 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

• 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

• 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

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

• Identify all forces acting on the particle

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

(Ex)

• 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

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

• 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 μC) is added to the configuration. If it is located 3.0 cm directly beneath A, what is the new net force on A?

### Equilibrium

• 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

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.