ch. 19 electric charges, forces, and fields. the atom the atom has positive charge in the nucleus,...

Ch. 19 Electric Charges, Forces,

and Fields

The atom

The atom has positive charge in the nucleus, located in the protons. The positive charge cannot move from the atom unless there is a nuclear reaction.

The atom has negative charge in the electron cloud on the outside of the atom. Electrons can move from atom to atom without all that much difficulty.

Question

• You charge the balloon by rubbing it on hair or on a sweater, and the balloon becomes negative. How can it pick up a neutral tissue?

This is an electroscope

ChargeCharge comes in two forms, which Ben Franklin designated

as positive (+) and negative(–).Charge is quantized.

– The smallest possible stable charge, which we designate as e, is the magnitude of the charge on 1 electron or 1 proton.

– We say a proton has charge of e, and an electron has a charge of –e.

– e is referred to as the “elementary” charge.– e = 1.602 × 10-19 Coulombs.– The coulomb is the SI unit of charge.

Particle Charge Mass

Proton 1.6x10-19 C 1.67 x10-27 kg

Electron -1.6x10-19 C 9.11 x10-31 kg

Neutron 0 1.67 x10-27 kg

Sample Problem: A certain static discharge delivers -0.5 Coulombs of electrical charge. How many electrons are in this discharge?

Sample Problem: How much positive charge resides in two moles of hydrogen gas (H2)?

How much negative charge? How much net charge?

Sample Problem: The total charge of a system composed of 1800 particles, all of which are protons or electrons, is 31x10-18 C. How many protons are in the system?

How many electrons are in the system?

Electric Force

• Charges exert forces on each other.• Like charges (two positives, or two negatives)

repel each other, resulting in a repulsive force.• Opposite charges (a positive and a negative)

attract each other, resulting in an attractive force.

Coulomb’s Law – form 1

• Coulomb’s law tells us how the magnitude of the force between two particles varies with their charge and with the distance between them.

• Coulomb’s law applies directly only to spherically symmetric charges.

• k = 8.99 × 109 N m2 / C2

• q1, q2 are charges (C)• r is distance between the charges (m)• F is force (N)

Electric ForceThe electric force between 2 objects is symbolic of the

gravitational force between 2 objects. RECALL:

MmFg 2

1

rFg

LawsCoulombr

qqkF

C

Nmx.k

kr

qqF

rFqqF

E

EEE

'

10998 constant Coulomb

alityproportion ofconstant

1

221

2

29

221

221

Coulomb’s Law – form 2• Sometimes you see Coulomb’s Law written in a

slightly different form

• eo = 8.85 × 10-12 C2/ N m2

• q1, q2 are charges (C)• r is distance between the charges (m)• F is force (N)• This version is theoretically derived and less• practical that form 1

Sample Problem: A point charge of positive 12.0 μC experiences an attractive force of 51 mN when it is placed 15 cm from another point charge. What is the other charge?

Sample Problem: Calculate the mass of ball B, which is suspended in midair.

A

B

R = 1.3 m

qA = 1.50 nC

qB = -0.50 nC

Superposition• Electrical force, like all forces, is a vector

quantity.• If a charge is subjected to forces from more

than one other charge, vector addition must be performed.

• Vector addition to find the resultant vector is sometimes called superposition.

Sample Problem

What is the force on the 4 C charge?

y (m)

1.0

2.0

2 C-3 C 4 C

x (m)2.01.0

Sample Problem

What is the force on the 4 C charge?

y (m)

1.0

2.0

2 C

-3 C

4 C

x (m)2.01.0

The Electric Field• The presence of + or – charge modifies empty space.• This enables the electrical force to act on charged particles

without actually touching them.• We say that an “electric field” is created in the space around a

charged particle or a configuration of charges.• If a charged particle is placed in an electric field created by

other charges, it will experience a force as a result of the field.• Sometimes we know about the electric field without knowing

much about the charge configuration that created it.• We can easily calculate the electric force from the electric

field.

Why use fields?• Forces exist only when two or

more particles are present.• Fields exist even if no force is

present.• The field of one particle only can

be calculated.

Field between charged plates

++++++++++++++++++++++++++++

----------------------------------------------

Field Vectors from Field Lines

• The electric field at a given point is not the field line itself, but can be determined from the field line.

• The electric field vectors is always tangent to the line of force at that point.

• Vectors of any kind are never curvy!

Field Vectors from Field Lines

+ -

Force from Electric Field• The force on a charged particle placed in an

electric field is easily calculated.• F = E q• F: Force (N)• E: Electric Field (N/C)• q: Charge (C)

Sample Problem: The electric field in a given region is 4000 N/C pointed toward the north. What is the force exerted on a 400 μg Styrofoam bead bearing 600 excess electrons when placed in the field?

Sample Problem: A proton traveling at 440 m/s in the +x direction enters an electric field of magnitude 5400 N/C directed in the +y direction. Find the acceleration.

For Spherical Electric Fields

• The Electric Field surrounding a point charge or a spherical charge can be calculated by:

• E = k q / r2

• E: Electric Field (N/C)• k: 8.99 x 109 N m2/C2

• q: Charge (C)• r: distance from center of charge q (m)• Remember that k = 1/4peo

Sample Problem: A particle bearing -5.0 μC is placed at -2.0 cm, and a particle bearing 5.0 μC is placed at 2.0 cm. What is the field at the origin?

Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?

Sample Problem

What is the charge on the bead? It’s mass is 32 mg.

E = 5000 N/C

40o

Excess Charges on Conductors• Where does the excess charge reside on a charged

conductor? (Van de Graf Generator)

Field within a Conductor

• When the electric charges are at rest, the electric field within the conductor is zero.

Electric Fields at Conductor Surfaces

Electric field lines contact conductor surfaces a right angles.

ch. 19 electric charges, forces, and fields. the atom the atom has positive charge in the nucleus,...

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