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Lecture

# Field model, field of single charge

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School
Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
29 January The field model In the force model, a particle A exerts a force directly on particle B. In the field model, A alters the space around it. Particle B then responds to the altered space. The altered space is the agent that exerts the force on B. ⃗E F Magnitude of electric field = q Unit: N/C Field Magnitudes Field Field strength (N/C) Inside a current-carrying wire 10 -2 Near the earth’s surface 10 (typical field: 100 N/C, directed down) Near objects charged by rubbing 10 to 10 6 Needed to cause a spark in the air 10 6 Inside a cell membrane 10 (enough to cause spark in air!) Inside an atom 10 11 The field of a single charge The field vectors point away from a positive charge Electric field of charge q at a distance r K∨q∨ ¿ r2 , [away from q if q > 0, toward q if q < 0]) ¿ E=¿ The field lines are closest together near the charge, where the field strength is greatest. Field lines Start on + End on – Tangent to field Close = strong, far = weak Magnetic field is uniform in a parallel plate capacitor The electric field of the heart is a dipole Two parallel plates have charges of equal magnitude but opposite sign. Which of the following changes would not increase the field strength between the plates? Increasing the magnitude of the charge on both plates Decreasing the distance between the plates Decreasing the area of the plates (while keeping the magnitude of the charges the same) Field strength depends on density of charge. A small sphere is suspended from a string in a uniform electric field. Several different cases of sphere mass and sphere charge are presented in the following table. In which case is the angle at which th
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