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This course is designed for engineers. This subject is compiled of physical applications and concepts. This lecture includes: Electric Charge, Static Electricity, Properties of Charges, Law of Conservation of Charge, Charge of an Electron, Charge of a Proton, Coulomb's Law, Electrical Force, Electric Field, Electric Field Due to Point Charges
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like charges repel unlike charges attract charges can move but charge is conserved
Law of conservation of charge : the net amount of electric charge produced in any process is zero. (Not on your starting equation sheet, but a fact that you can use any time.)
Static Electricity
Properties of charges
Although there are two kinds of charged particles in an atom, electrons are the charges that usually move around.
A proton is roughly 2000 times more massive than an electron.
The charge of an electron is – e = – 1.6x10-19^ coulombs.
The charge of a proton is +e = +1.6x10-19^ coulombs.
Charges are quantized (come in units of e= 1.6x10-19^ C).
1 2 2 12
Coulomb’s law gives the force (in newtons) between charges q 1 and q 2 (in coulombs), where r 12 is the distance in meters between the charges, and k=9x10^9 N·m^2 /C^2.
Coulomb’s law quantifies the magnitude of the electrostatic force.
1 2 2 12
In general, you need to begin* solutions with starting equations.
You may begin with any correct variant of a starting equation.
For example, A^2 B is “legal” and may be used.
Don’t get hung up about starting a problem with an equation which is an exact copy of one from the OSE sheet.
*“Begin” does not mean that a starting equation has to be the first thing that appears on your paper. It might be several lines before you use a starting equation.
Coulomb’s Law is valid for point charges. If the charged objects are spherical and the charge is uniformly distributed, r 12 is the distance between the centers of the spheres.
If more than one charge is involved, the net force is the vector sum of all forces (superposition). For objects with complex shapes, you must add up all the forces acting on each separate charge (calculus!!).
r 12
I just told you it’s OK to use Coulomb’s Law for spherically-symmetric charge distributions.
1 2 2 12
On your homework diagrams, show both the magnitudes and signs of q 1 and q 2.
Your starting equation sheet has this version of Coulomb’s Law:
which gives you the magnitude F 12 and tells you that you need to figure out the direction separately.
I want this class to make you hear littlevoices in your head.
If Q 3 were free to move, what direction would its initial acceleration be? How would I calculate the acceleration?
Would the acceleration remain constant as Q 3 moved? Could I use the equations of kinematics (remember them from Physics 23?) to describe the motion of Q 3?
Today’s agenda:
Electric Charge. Just a reminder of some things you learned back in grade school.
Coulomb’s Law (electrical force between charged particles). You must be able to calculate the electrical forces between one or more charged particles.
The electric field. You must be able to calculate the force on a charged particle in an electric field.
Electric field due to point charges. You must be able to calculate electric field of one or more point charges.
Motion of a charged particle in a uniform electric field. You must be able to solve for the trajectory of a charged particle in a uniform electric field.
A charged particle propagates (sends out) a "field" into all space.
Other charged particles sense the field, and “know” that the first one is there.
like - charges repel
unlike charges attract
The idea of an electric field is good for a number of reasons:
It makes us feel good, like we’ve actually explained something.
We can develop a theory based on this idea. From this theory may spring unimagined inventions.
like charges repel
unlike charges attract
OK, that was a flippant remark. There are serious reasons why the idea is “good.”
If the theory explains past observations and leads to new predictions, the idea was “good.”
The electric field is real! Trust me.
This version of the electric field equation is on your equation sheet. Use it for problems involving electric fields and forces:
This is your second starting equation. The equation tells you the direction of the electric field is the direction of the force exerted on a POSITIVE test charge. The absence of absolute value signs around q means you MUST include the sign of q in your work.
The units of electric field are
0
0
In chapter 23, you will learn that the units of electric field can also be expressed as volts/meter:
The electric field exists independent of whether there is a charged particle around to “feel” it.
The idea of a field is not new to you. You experienced fields (gravitational) in Physics 23.
Gravitational Fields
1 2 G (^2) 12
is the local gravitational field. On earth, it is about 9. N/kg, directed towards the center of the earth.
Units of g are actually N/kg!
If the last equationlooks like this, you have missing fonts. docsity.com
Today’s agenda:
Electric Charge. Just a reminder of some things you learned back in grade school.
Coulomb’s Law (electrical force between charged particles). You must be able to calculate the electrical forces between one or more charged particles.
The electric field. You must be able to calculate the force on a charged particle in an electric field.
Electric field due to point charges. You must be able to calculate electric field of one or more point charges.
Motion of a charged particle in a uniform electric field. You must be able to solve for the trajectory of a charged particle in a uniform electric field.
