ch1 Electric Charge

Chapter 1 Electric Charge
1-1 Electromagnetism 1-2 Electric Charge 1-3 Charge Conservation and Quantization
1-4 Coulomb's Law
1-5 The Significance of the Electric Force
Repulsion Attraction both negative or opposite signs both positive k: electrostatic constant
For historical reasons and simplifying many other formulas, k is usually written:
F1 F21 F31 q1
y (m) q3
q2 ˆ ( q3 )r ]. ˆ [( 2 )r21 2 4 0 r21 r31 31
ˆ ˆ i, r j Substituting r21 31
F1 F21
F31
= 45
Faraday discovered electromagnetic induction
In 1865 Maxwell put forward electromagnetic field theory In 1905 Einstein created special relativity
1-2 Electric Charge
★Electric charge is an intrinsic characteristic of fundamental particles making up objects. ★Every object contains equal amounts of positive negative charges. The object is said to be electrically neutral. ★We say that an object is charged to indicate that it has a charge imbalance, or net charge. ★Charged objects interact by exerting forces on one other. ★Charges with the same electrical sign repel each other and charges with opposite electrical sign attract each other.
Example 1-1. The nucleus in a copper atom has a radius of about 41015 m and contains 29 protons.
(a) What is the magnitude of the repulsive electrostatic force between two of these protons that happen to be separated by 41015 m?
Where,
Where,
0
Example 1-2 Consider three point charges q1 = q2 = 2.0 nC and q3 = 3.0 nC placed as in the figure. Find the forces on q1 and q3. Solution: From the principle of superposition of electric forces, the force on q1 is
q2
9 9 F1 9.0 10 i 13.5 10 j ,
O q1
x (m)
1 13.5 2 2 9 123.7 F1 F1 x F1 y 16.2 10 N, 1 tan
9.0
The force on q3 is calculated in much the same way,
k 1 4 0 8.99 109 N m 2 /C2 ,
permitivity constant真空介电常数
Coulomb’s law
Vector form Unit vector
Discussion
Positive, negative charges and the direction of electric force
q3 q1 ˆ ( q2 )r ] ˆ F3 F13 F23 [( 2 )r13 2 4 0 r13 r23 23
ˆ cos i sin j , r , = 45 ˆ r23 j 13
9 9 F3 4.8 10 i 18.3 10 j
Solution: From Coulomb’s law
This is a small force to be acting on a macroscopic object, but an enormous force to be acting on a proton. Such repulsive forces should blow apart the nucleus of any element(but hydrogen).nuclear force to why? There must be some attractive But they don’t, counter this repulsive electrostatic force.
Electromagnetic Fields
• Electrostatic field 静电场 • Magnetostatic field 稳恒磁场 • Electric and magnetic field in dielectric or magnetic medium 介质中的电场和磁场 • Electromagnetic induction 电磁感应 • Electromagnetic field and electromagnetic waves 电磁场和电磁波
In summary, we can say that: Charge is conserved absolutely and free charge is quantized in positive or negative integer multiples of e.
1-4 Coulomb’s Law
Radioactive decay of nuclei 238 U 92 (原子核的放射性衰变)

234 90
Th He
4 2
Annihilation process (湮灭过程) e +e
+
Pair production (电子对产生)
e e


Charge is Quantized Experiment shows that any positive or negative charge q is made up of a certain elementary charge: q = ne, n = 1, 2, 3, ….. The elementary charge : e = 1.60 1019 C When a physical quantity such as charge can only be discrete values rather than any value, we say that the quantity is quantized.
(b) What is the magnitude of the gravitational force between those same two protons ?
Solution: From the law of Gravitation
This result tells us that the (attractive) gravitational force is far too weak to counter the repulsive electrostatic forces(14N) between protons in a nucleus. The protons are bound together by the strong nuclear force. The ratio of the two forces:
??????ymq3rr21310314??21r?i????31r?j????substitutingfij9919010?13510?????????xyfff22911116210n??????11135?tan123790?????xmoq1q2?45?31f?21f?1f?theforceonq3iscalculatedinmuchthesamewayqqfffrr??312223132313??23?r?013234qr???????23?r?cossinij???????substitutingtheunitvectors13?r?j??99994810?4810?1831018310ffiijj????????45?ymq3223f?ff?3??22933318910nxyfff??????magnitude13183tan75348???????direction
Same sign charges
repulsion
Hale Waihona Puke Opposite sign charges
attraction
Coulomb’s Law and Newton’s Law of Gravitation
k
G
F 1/r2 q1, q2 can be or F: attractive or repulsive m1, m2 can only be F: attractive
It shows that on the atomic scale, the electric force is much greater than the gravitational force, and justifies ignoring gravitation at that level.
3. Superposition principle of electrostatic force The net electric force on a body is the vector sum of the forces due to the individual point charges. ★for Discrete Charges ★for Continuous Charges
1-3 Charge Conservation and Quantization
Law of conservation of charge: In a closed or isolated system the algebraic sum of the positive and negative charges is constant. If you rub a glass rod with silk, a positive charge appears on the rod and a negative charge of equal magnitude appears on the silk.
1-1. Electromagnetism
Before Christ 600 Ancient Greece Thales recorded electricity for the first time In 1820 Oersted discovered electricity acted on magnetic needle In 1831
1. Point charge (Charged particle)-----Ideal model When the scale of a charged body is much less than the distance between that charged body and the other charged body, the charged body can be regarded as a point charge. 2. Coulomb’s law deals with interaction between two rest point charges in free space--- electrostatic force.