Electric Fields
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Electric Field
- E = F / qtest ← if there is a test charge
- E is due ONLY to SOURCE charges (exists regardless of test charge)
- If there is a test charge, F is parallel to E if qtest > 0...
- E points away from +, towards -
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Electric Field Lines
- A schematic for the electric field
- Drawing:
- Lines may start on + charges, end on - charges
- Lines may also come from or end at infinity
- Lines tend to spread out as far from each other as possible, and may never cross
- Interpreting:
- At a point on a line, the direction of E is parallel to the line
- At a point between two lines, the direction of E is the average of the directions of the two nearest lines
- [Schematic: Single point charge, E field lines]
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Electric Potential
- V = PEelectric / qtest
- NOTE: there does not need to be a test!
- V is high near +, low near -
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Equipotentials
- Lines on which electric potential is constant
- Example: single point charge → equipotentials are concentric spheres
- [Add equipotentials to schematic]
- NOTE: E field lines are perpendicular to equipotentials!
- Lab instructions:
- Setup (do DOT-DOT together)
- Cork, blank sheet, carbon paper, conducting paper
- Add pins, connect to power supply (give them a picture)
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Using the voltmeter
- Plugs:..., Dial setting:..., Signs of a problem: ...
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Finding equipotentials
- Connect black probe to pin connected to - on power supply
- TRACE CONDUCTOR using retracted pen as a stylus
- Push red probe gently against conducting paper.
- Find and mark ~ 10 points with ΔV = 1.0V
- Find and mark ~ 10 points with ΔV = 2V, ...
- Field lines from equipotentials
- [Schematic: ++, with equipotentials]
- pick a point on an equipotential, draw short normal to tangent
- look to adjacent equipotential, find the normal which would meet the first normal halfway between the two equipot.s
- Connect the two short normals using smooth curve
- DO
- DOT-DOT, DOT-LINE, DOT-CIRCLE
- ↑Indicate where electric field is strongest, weakest.
- Answer experimental statements 2 and 3, referring to one or more of the diagrams above.
- Answer questions 2 and 3.