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# lec21 - Physics I Class 21 Thinking About Electric Fields...

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21-1 Physics I Class 21 Thinking About Electric Fields and Electric Potential Rev. 02-Apr-04 GB

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21-2 Class #19 Take-Away Concepts (Review) 1. Electric field from point charge sources: (Total field is the superposition of point source fields.) ) ( r q 4 1 E i 2 i i 0 - ε π = 2. Force on a charge in an electric field: E q F = 3. Electric field points away from + source charges. 4. Electric field points toward – source charges.
21-3 - + The Electric Field of a Point Charge (as a Source ) The electric field is a vector field, meaning at each point in space the electric field has a magnitude and a direction . We show that by drawing arrows at representative points in the correct directions with lengths proportional to the magnitudes. Away from positive Toward negative Just because we don’t draw an electric field vector at a point doesn’t mean there is no electric field there.

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21-4 Class #20 Take-Away Concepts (Review) Physicist General’s Health Warning: Confusing electric potential (V) with electric potential energy (U) will be hazardous to your score on the next exam. 1. Electric potential (V) and electric potential energy (U): V q U = 2. Electric potential from point source charges: ε π = i i 0 r q 4 1 V 3. Relationship of electric field and electric potential. 4. Electric field lines and equipotential lines. 5. Calculating change in kinetic energy using electric potential.
21-5 Why do we solve problems like: “Where is the Net E Field = 0?” There are several problems in electromagnetic design where the objective is to find a location where the electric (or magnetic) field has specified properties, like being zero, constant, or similar. Examples range from electrostatic traps (for atoms, ions, or anti- matter) to magnetic resonance imaging systems. We pose problems like this in Physics I to give you the chance to exercise your knowledge of electric fields and electric potential.

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