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Electric Charge
Electric charge is an intrinsic characteristic of the fundamental particles that make up objects.
Positive Charge Negative Charge
Electric Charge
Net charge of a system: algebraic sum of all the charges Law: Conservation of charge The net charge of a closed system never changes
Electric Charge
Electric charge is quantized
- Elementary charge: e = 1.60602176462(63) x 10–19^ C Coulomb (C): one coulomb is the amount of charge that is transferred through the cross section of a wire in 1 second when there is a current of 1 ampere in the wire.
$ = n
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Charge of Particles
+ Ze
neutral atom
electron orbits nucleus
e –
e –
e –
Electron
Positron
Proton
Anti-Proton
Neutron
Photon 0
Particles Charge Nucleus charge= + Ze , atom with Z electrons is neutral.
e −
e −
e +
e +
Proton charge: | e+^ | = 1.60 x 10–19^ C
Electron charge: | e-^ | = 1.60 x 10–19^ C
Interaction of Charges
Charged objects interact by exerting forces on one another. DEMO: Pith Balls
Conductors versus Insulators
- Conductors: material in which electric charges can move around “freely.
- Insulators: material in which electric charges are “frozen” in place.
- Semi-conductor: material in which electric charges can move around but not as freely as in conductors.
- Super-conductor: no resistance to the movement of charge.
Interaction of Charges: Insulators
*Insulators: material in which electric charges are “frozen” in
place.
Interaction of Charges: Insulators
Force of Repulsion Force of Attraction Charges with opposite electrical signs attract each other. Charges with the same electrical sign repel each other
Charge Induction
• Demo: Chimes
Conducting thread Insulating thread Grounded Charged
Coulomb’s Law of Electro-static Force
q 1 q 2 r The electro-static force of attraction/repulsion has a magnitude: Coulomb’s Law
k =
4 πε o
= 8.99 x 109 Nm^2 / C^2
where:
and the permittivity constant is^ ε o =^ 8.55 x^10
− 12
C
2
/ Nm
2 Charles-Augustin de Coulomb (1736 - 1806)
Coulomb’s Law of Electro-static Force
*Each particle exerts a force of this magnitude on the other particle. *The two forces form an action-reaction pair.
F =
4 πε o
Q 1 Q 2
r^2
ˆ r
1
2 1
Coulomb’s Law of Electro-static Force
Force exerted by q 1 on q 2 at a distance r 12
F 12 =
kq 1 q 2
r 1 , 2
2 r ˆ^1 ,^2
q 1 , q 2 in coulombs (C)
r 12 in meters (m)
F 12 in newtons (N)
F 12
Coulomb’s Law Analogous to Newton’s Equation
of Gravitation
F = k
q 1 q 2
r
2
F = G
m 1 m 2
r
2
- k electro-static constant
- Inverse Square Law
- Charge *Attractive/repulsive depending on sign of charges *Two kinds of charges *Dominates on small scale
- G gravitational constant
- Inverse Square Law
- Mass *Always attractive *One kind of mass *Dominates on large scales
DIFFERS
Analogous
Electro-Static Force versus Newton’s Force of
Gravitational Attraction
DEMO: 2 x 4
Principle of Superposition
- When several point charges are put together, the total force on any one charge is the vector sum of the each of the separate forces acting on that charge.
- Exercise: Q 2 Determine force on Q 1
Q 3 Q 1 =Q 2 =Q 3 =1μC
R=1m 600
Q 1
y Determine force on Q 1^ x
