Physics 3204 Electrical Fields

Transcription

1 Physics 3204 Electrical Fields Definition: An electric field is a region of influence in the space surrounding a charged object which will exert a force on another charged object that is brought into the region.

2 Drawing electric field: The direction of any field, whether it be a gravitational field, electric field, or magnetic field, can be determined by testing the field. What was used to test gravitational fields? A test mass.

3 What is used to test electric fields? A small test charge. By convention electric fields are drawn based on the direction that a small POSITIVE test charge will move when placed in that field

4 The arrows drawn representing the electric field show the electrostatic force that acts on the positive test charge based on the charged object(s) that are generating the field. Bonus: What is used to determine the direction of a magnetic field? A compass

5 Draw the electric field around the following point charges. A) B)

6 Notes on electric fields 1. The direction of the lines of force is the direction in which a small positive test charge will move when placed in that field. Lines of force BEGIN on a positively charged object Lines of force END on a negatively charged object

7 2.The lines of force meet the charged object at right angles. 3.The lines of force never touch or cross. 4.The density of the lines of force indicates the strength of the field. The electric field strength is greatest near the charged object and diminishes as the distance from the charged object increases.

8 5. Electric fields are 3-dimensional

9 Steps for drawing interacting electric fields 1. To determine the direction of the overall field at a particular point, ask yourself which way a small positive test charge would move if placed at that point. 2. Remember that lines of electric force start on a positive object and end on a negative object.

10 3. Draw the lines so that they are normal to the charged objects Perpendicular to the surface of the charged object 4. Make sure that the lines never touch or cross each other

11 1. Draw the electric fields around the following charged objects - +

12 2. - -

13 3. + +

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15 5. Two separate charges. The left charge is 1 C and the right charge is 0.5 C + + There should be proportionally less lines on the smaller charge. Half as many in this case

16 6. + There should be proportionally more lines on the pointy (higher curvature) end.

17 The electric field bulges out on the ends because there is not an equal amount of charge on either side of the test charges at the ends

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19 9. Conductor with a negative charge Note: At the centre of the conductor the field strength is zero. At the centre, the forces on the test charge would balance out.

20 Coaxial cable Coaxial cable is the cable that is used to connect cable, or VHS players to TVs Coaxial cable has a shielding capability that reduces electromagnetic interference of the television signal. Radio-grade flexible coaxial cable. A: outer plastic sheath B: copper screen C: inner dielectric insulator D: copper core

21 The diagram to the right shows how the shielding works from a field strength point of view. Field lines are shown ending on a circle of negative charges. You should be able to see that, inside the circle, for every line in one direction there is an opposite canceling line in the other direction. Consequently, the electric field strength inside the circle is 0 N/C. Thus the weak television signal at the centre of the cable is protected from external electric fields

22 Test Questions Why a car is a safe place to be in an lightning storm? A) The rubber in the tires is a good conductor. B) The rubber in the tires is NOT a good conductor. C) There is no electrical field inside a hollow, charged conducting body. D) Cars rarely get struck by lighting, because of their reflective paint.

23 Where is the safest place to be in a lightning storm? A) at the center of a large metal drum B) in a rowboat on a lake C) standing on a rubber mat D) under a tree

24 Electric Field Strength