Download Electrostatics: Electric Charges at Rest All matter is electrical in

An electron orbits a proton at a speed of 5.1 x 104 m/s at a radial distance of 5.3 x 10­11 m. What is the centripetal acceleration of the electron?
Electrostatics: Electric Charges at Rest
All matter is electrical in nature
What is the kinetic energy of the electron if its mass is 1.7 x 10­27 kg?
Atoms ­ Neutral atom
Objects are charged when they have an excess or deficiency of electrons
Ion
Positive ion
Conductor ­ Negative ion
Insulator ­ The smallest charge in nature is the charge found on
Does it make sense for an atom or group of atoms to have half an electron? No!
Charge is quantized. Charge on any object is an integer multiple of the elementary charge 'e'
q = net charge = 1
Conservation of Charge
Example:Example:
q = ­8 x 10­19 C
q = ­1.92 x 10­19 C
In a closed system
Charges cannot be created or destroyed, only transferred
Example:
2 identical conducting
spheres
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Example:
2 identical conducting
spheres
0
+3
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Why conducting and
not insulating for
sharing charges?
+6
?
Problems:
1. Which of the following net charges could be found on an object?
a) 7 elementary charges
What is the net charge of an object with b) 1.6 x 10­19 elementary charges
5,000 excess electrons?
c) ­1.8 x 10­19 C
d) +3.2 x 10­19 C
2. A conducting sphere with net charge q = ­16 elementary charges is touched by an identical conducting sphere with 0 charge. a) What is the net charge on the conducting sphere in elementary charges?
b) What is the net charge on the conducting sphere in Coulombs?
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Transferring Charge – 3 methods for charging an object
Only electrons move when transferring charge. Positively charged objects lose electrons, they do not gain protons
2 – Charging by contact
A charged object touches another object. The objects then share the charge.
A conducting sphere with charge +8 is brought in contact with an identical sphere which is neutral.
What is the net charge on each sphere after contact?
Meet the Electroscope – The electroscope was an early invention for detecting the presence of charge. There is a conductor at the top which connects to leaf(s) which will move depending on the charge of the electroscope. 1 – Charging by Friction
Both objects begin neutral.
When one object is rubbed against the other (friction) electrons are transferred.
This occurs because one object has a higher affinity for electrons than the other.
Examples:
Rubber & Fur
Glass & Silk
Meet the Electroscope – The electroscope was an early invention for detecting the presence of charge. There is a conductor at the top which connects to leaf(s) which will move depending on the charge of the electroscope. Neutral Electroscope
Meet the Electroscope – The electroscope was an early invention for detecting the presence of charge. There is a conductor at the top which connects to leaf(s) which will move depending on the charge of the electroscope. Negatively charged electroscope
What will happen to the 2 leafs at the bottom when they have the same negative charge?
Positively charged electroscope
What will happen to the 2 leafs at the bottom when they have the same negative charge?
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Meet the Electroscope – The electroscope was an early invention for detecting the presence of charge. There is a conductor at the top which connects to leaf(s) which will move depending on the charge of the electroscope. Grounding – A ground is any large object that can accept or donate electrons. Best ground is the Earth. Grounding an object makes the object neutral
3 – Charging by Induction
3 – Charging by Induction
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3 – Charging by Induction
3 – Charging by Induction
4. What is the force of gravity between an electron and a proton separated by 0.6 x 10­10 m?
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Electrostatic Force ­ Coulomb’s Law
Four Fundamental Forces
1. Strong (nuclear) force – keeps protons in atomic nuclei together
2. Electromagnetism
3. Weak (nuclear) force – responsible for radioactive decay
4. Gravity
Electrostatic force studied by Charles Coulomb (France, 1785)
Electrostatic force – responsible for:
Both gravity and electrostatic force follow an inverse square law
5. What is the electrostatic force between two 1 Coulomb charges separated by 1 m?
Unlike gravity, 6. What is the electrostatic force between an electron and a proton, separated by 0.6 x 10­10 m?
7. What is the electrostatic force exerted between two charged spheres at a distance of 5 nanometers if the first sphere has a charge of ­5 micro­coulombs and the second has a charge of ­2 micro­coulombs?
In what direction is the force? How does the magnitude of this force compare to Fg (from the Do Now)?
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8. When two spheres, each with charge Q, are positioned a distance R apart, they are attracted to one another by a force of magnitude F. By what factor of F would this force change if…
a) …one of the charges was tripled, while the other was doubled?
9. Sphere A has an excess of 5 x 1016 electrons. Sphere B has an excess of 6 x 1017 electrons.
a)Determine the charge on each sphere in coulombs.
b)Calculate the electrostatic force that sphere A exerts on sphere B when they are separated by a distance of 2 x 103 meter.
b) …one of the charges was doubled, while the separation distance was tripled?
c) …both of the charges were halved, while the separation distance was doubled?
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d) …one of the charged was doubled, while the separation distance was halved?
10. What is the electrostatic force between a 3 nC object and a 6 nC object separated by 0.5 m?
Review: Gravitational field
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Electric fields:
The region in which electrical forces act on a charge
The direction of the field is the direction a force acts on a positive test charge (test charge = infinitely small positive charge)
Electric field lines
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Electric Field
Example: What is the electric field strength for a 0.1 C charge experiencing a 100 N electrostatic force?
2 Parallel Plates
Electric field lines never cross
11. Sketch the electric field lines that surround each of the following arrangements of charged objects. 8
12. An electron is placed within a uniform electric field with strength of 5.0 x 10­8 newtons per coulomb. a. Calculate the electrostatic force that is experienced by the electron. 13. An object with a charge of ­4 x 10­3 coulombs is placed between a set of ­6 oppositely charged, parallel plates. The object experiences a force of 5 x 10
newtons while between the plates. a. Sketch a set of vectors showing the direction in which the electric field lines point between the plates. b. Sketch a vector showing the direction in which the object will travel when placed between the plates. c. Calculate the strength of the electric field between the plates.
b. Calculate the acceleration of the electron. [electron mass = 9.11 x 10­31 kg] 9