EIT-review.pdfFundamental Exam review ppt of elect

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详细说明：Fundamental Exam review ppt of electrical engineering in USElectric field Charge 1 large 12 Net force (or ficld) Chargc 3 If there are more charges, the fields and forces add as vectors Electric Field and voltage Charge 1 Charge E1 r It takes work to move a charge in a field For example, the work required to move Charge 2 to the left is 元E Electric Field and Voltage Charge 1 Charge 2 Because it takes work to move a charge in a field charge possesses electrical potential energy when in a field Voltage is the electrical potential energy per unit charge and is always measured as the difference in potential energy between two points-if a second point is not explicitly stated, one has been assumed(e.g, ground) Note that potential only makes sense for a conservative field! Electric fields We saw that the field from a point Charge 1 charge is given by The field from an infinitely long line of charge is The field from an infinite sheet of charge is Summary: Consider Test Charge Consider a spatial distribution of charges Now consider a test charge Negative Charge Positive Charge Test Charge Force There is a force on this test charge it is attracted by the opposite charge it is repelled by the charge of the same sign Negative Charge Positive charge Force Test Charge Electric field We can visualize this force by saying that there is an Electric field in the region then if the electric field is E, the force on the test charge is F=gE Negative Charge Electric ield Positive Charge ●●●● ↓F Work energy Work is equal to force times distance for a constant force(for a variable force we integrate) The work done in moving some particle is equal to the change in the particles energy Consider lifting an object off of the floor; the gravitational force is approximately constant(mg) so the work done in lifting to a height h is mgh Equating the work done with the gravitational potential energy of the particle, we say the potential energy is Um=mgh. We can think of h as the gravitational potential energy per unit weight (weight =mg) Electrical Potential Energy If a particle has electric charge, then it may also have an electrical potential energy, Uel We find ue by equating it with the work required to move the charge in a static electric field If we move in opposition to the field Uo increases and the inner product is negative Allowing for the possibility of a field that varies with position, we have (in one dimenSion, assuming the charge moves fromo to x) ·久 · oltage It is convenient to define the voltage between two points in space, V21, as the integral of the electric field, E(V2i>0= the potential at point 2 is higher 几·元 If we define x, to beo and assume v(,=0, then we see from our previous result that the voltage is the electrical potential energy per unit charge, 1.e 1· Vis, therefore analogous to h The atom Start with a model of the atom - the bohr model protons and neutrons make up the nucleus, protons have positive charge electrons have negative charge and move around the nucleus in different orbits electrons in the outermost orbit can easily be removed if that shell is not full These electrons then become mobile charge(e. g, the are free to move under the influence of an electric field) Electrical conductor a conductor is a material that contains a substantial number of mobile charges -for now let's just assume that they are always electrons Conductor 8 Conductor in a field Now suppose we apply an electric field to the conductor According to Newtons second law(F=ma), the electrons will continuously accelerate while in the field Conductor Electric Field Conductor in a field But, what do we actually observe when the mobile electrons in a conductor are exposed to an electric field? Conductor Electric Field Conductor in a field We find that the electrons move with a constant average velocity! Why? Conductor Electric Field Electrical resistance We find that the electrons move with a constant average velocity! Why? Our simplest model of this situation is that the electrons occasionally collide with atoms in the conductor and loose energy in each collision These collisions are the cause of resistance which is the electrical analog of friction 10

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