Thursday, Jan. 20, 2011 1 PHYS 1444-02 Dr. Andrew Brandt PHYS 1444 – Section 02 Lecture #2 Chapter...

Thursday, Jan. 20, 2011 1PHYS 1444-02 Dr. Andrew Brandt

PHYS 1444 – Section 02Lecture #2

• Chapter 21– Coulomb’s Law– The Electric Field & Field Lines– Electric Fields and Conductors– Motion of a Charged Particle in an Electric Field

•Welcome HW is due Friday at 9pm• Ch. 21Homework is due 9 pm, Thursday, Jan. 27.It’s long so start right away!• Labs start week of 31st

Thursday Jan. 20, 2011Dr. Andrew Brandt

Mastering Physics Grades• For grading purposes, some numeric answers to questions need to be exact. For example, the answer to the question "How many days are in a week?" must be 7.•The typical grading tolerance for most numeric answers in Mastering assignment questions is between 2%-3%. For example, if the grading tolerance is 2% and the correct answer is 1043, both 1042 or 1045 are also graded as correct.•When an answer is within tolerance, but doesn't match the correct answer: The officially correct answer displays in a purple box (provided that Show Whether Answer is Correct is set to Always). Students should use this answer if subsequent parts of an assignment item require calculations based on this answer.•Students should use at least three digits or significant figures in answers, unless otherwise specified or unless the exact answer can be expressed using fewer than three significant figures. If higher precision is required, or lower precision is allowed, this is specified in the question or its instructions. When students must do multiple calculations to get an answer they should use more significant figures than required during each calculation and round off at the end•You are allowed 4 attempts at a question (with each attempt you lose some points). Reread problem, could you have made a sign error or a unit error or a round-off error?



• The value of the proportionality constant, k, in SI units is

• Thus, if two 1C charges were placed 1m apart the force would be 9x109N.

1 2Q QCoulomb’s Law – The Formula

• Is Coulomb force a scalar quantity or a vector quantity? Unit?– A vector quantity. Newtons

• Direction of electric (Coulomb) force is always along the line joining the two objects.– If two charges have the same sign: forces are directed away from each other.– If two charges are of opposite sign: forces are directed toward each other.

• Coulomb’s Law is accurate to 1 part in 1016.• Unit of charge is called Coulomb, C, in SI.

F 1Q 1 22

QQF k

r

2r 2Q Formula

9 2 28.988 10 N mk C


Electric Force

• Does the electric force look similar to another force? What is it?– Gravitational Force

• What are the sources of the forces?– Electric Force: Charge, fundamental property of matter– Gravitational Force: Mass, fundamental property of matter

• What else is similar?– Inversely proportional to the square of the distance between the sources of the force

What is this kind law called?• Inverse Square Law

• What is different?– Gravitational force is always attractive.– Electric force depends on the sign of the two charges.– Magnitude

1 22

QQF k

r

and Gravitational Force1 2

2

M MF G

rExtremely

Similar


The Elementary Charge and Permittivity• Elementary charge, the smallest charge, is that of an

electron: – Since electron is a negatively charged particle, its charge is –e.

• Object cannot gain or lose fraction of an electron.– Electric charge is quantized.

• It always occurs in integer multiples of e.

• The proportionality constant k is often written in terms of another constant, 0, the permittivity of free space. They are related and .

• Thus the electric force can be written:• Note that this force is for “point” charges at rest.

191.602 10e C

01 4k 12 2 20 1 4 8.85 10k C N m

1 22

0

1

4

QQF

r


Example 21 – 1 • Electric force on electron due to proton. Determine the

magnitude of the electric force on the electron in a hydrogen atom exerted by the single proton (Q2=+e) that is its nucleus. Assume the electron “orbits” the proton at its average distance of r = 0.53 x10-10 m. (0.5 Angstrom)

191 1.602 10Q e C

1 2 1 22 2

0

1

4

QQ QQF k

r r Using Coulomb’s law

Each charge is 192 1.602 10Q e C and

So the magnitude of the force is

1 22

QQF k

r

19 19

9 2 2210

1.6 10 1.6 109.0 10

0.53 10

C CN m C

m

88.2 10 N

Which direction? Towards each other… Example on board


Example 21 – 2 • Which charge exerts greater force? Two positive

point charges, Q1=50C and Q2=1C, are separated by a distance L. Which is larger in magnitude, the force that Q1 exerts on Q2 or the force that Q2 exerts on Q1?

1 212 2

QQF k

LWhat is the force that Q1 exerts on Q2?

Therefore the magnitudes of the two forces are identical!

Is there any difference? The direction.

What is the force that Q2 exerts on Q1? 2 121 2

Q QF k

L

What is this law?

What is the direction? Opposite to each other!

Newton’s third law, the law of action and reaction


Solving Problems • Read and re-read problems carefully• Draw a diagram using arrows to represent vectors• Choose a convenient coordinate system• Note the known and unknown quantities• Write down the relevant relationships• Do an approximate calculation• Solve, substituting numbers only at the end• Keep track of units• Consider if answer is reasonable


Vector Problems • Calculate magnitude of vectors (Ex.

force using Coulomb’s Law)• Split vectors into x and y components

and add these separately, using diagram to help determine sign

• Calculate magnitude of resultant |F|=(Fx

2+Fy2)

• Use = tan-1(Fy/Fx) to get angle

Example on board

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