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Parallel Plates

Parallel Plates. When two parallel plates are connected across a battery, the plates will become charged and an electric field will be established between them.

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Parallel Plates

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  1. Parallel Plates • When two parallel plates are connected across a battery, the plates will become charged and an electric field will be established between them.

  2. The direction of an electric field is defined as the direction that a positive test charge would move. So in this case, the electric field would point from the positive plate to the negative plate. The field lines are parallel to each other and hence this type of electric field is uniform and is calculated with the equation E = V / d.

  3. V = Volts E = N/C d = m Therefore 1 N/C = 1 V/m

  4. Since the field lines are parallel and the electric field is uniform between two parallel plates, a test charge would experience the same force of attraction or repulsion no matter where it was located. That force is calculated with the equation F = q E.

  5. In the diagram above, the distance between the plates is 0.14 meters and the voltage across the plates is 28V. If a positive 2 nC charge were inserted anywhere between the plates, it would experience a force in the direction of the negative, bottom plate, no MATTER where it is placed in the region between the plates.

  6. Millikan Oil Drop Experiment • Millikan found the charge of an electron. • Fine oil drops were sprayed from an atomizer in the air. Gravity acting on the drops caused them to fall. A potential difference was placed across the plates. The resulting electric field between the plates exerted a force on the charged drops. The resulting electric field between the plates was adjusted to suspend a charged drop between the plates

  7. Each electron always carries the same charge. • Charges are quantized • Changes in charges are caused by one or more electrons being added or removed.

  8. Example An oil drop weighs 1.9 x 10-15 N. It is suspended in an electric field of 6 x 103 N/C. What is the charge of the drop? How many excess electrons does it carry?

  9. Problem #15 A positively charged oil drop weighs 6.4 x 10-13 N. An electric field of 4 x 106 N/C suspends the drop. A) What is the charge on the drop? B) How many electrons are in the drop?

  10. Problem #16 A charged particle of mass 2 x 10-9 kg and a charge of 2 x 10-6 C is placed next to one of the two parallel plates 10 cm apart with a 2000 N/C electric field intensity. A) What is the acceleration that the particle gets because of the force due to the electric field between the plates? B) What is the speed of the particle at the other plate?

  11. Problem #17 Two parallel plates, separated by a distance of 10 cm have an electric field of 5000 N/C. If a 4 x 10-6 C charge with a mass of 5 x 10-15 kg is placed next to one plate, A) What will be the acceleration of the charge? B) If it starts from rest, what will be the speed of the charge at the other plate?

  12. Capacitor • Capacitor is a device designed to store electric charge. A typical design of a capacitor consists of two parallel plates (metal) separated by a distance d. Plates are connected to a battery. Electrons leave one plate giving it a +Q charge, transferred through the battery and to the other plate giving it a –Q charge. This charge transfer stops when the potential difference across the plates equals the potential difference of the battery. Thus the charged capacitor acts as a storehouse of charge and energy that can be reclaimed when needed for a specific application.

  13. The capacitanceC of a capacitor is defined as the ratio of the magnitude of the charge on either conductor to the magnitude of the potential difference between the conductors.

  14. Problem #18 Both 3.3 x 10-6 F and 6.8 x 10-6 F capacitor are connected across a 15 V battery. Which capacitor has a greater charge? Thus6.8 x 10-6 F has a greater charge.

  15. BATTERIES • Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect some type of load to the battery using the wire. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

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