Electric Fields and Forces. AP Physics B

Transcription

1 lectric ields and orces AP Physics B

2 lectric Charge Charge is a property of subatomic particles. acts about charge: There are types basically, positive (protons and negative (electrons LIK charges RPL and OPPOSIT charges ATTRACT Charges are symbolic of fluids in that they can be in states, STATIC or DYNAMIC.

3 lectric Charge The specifics Some important constants: Particle Charge The symbol for CHARG is q The unit is the COULOMB(C, named after Charles Coulomb If we are talking about a SINGL charged particle such as 1 electron or 1 proton we are referring to an LMNTARY charge and often use, e, to symbolize this. Mass Proton lectron Neutron 1.6x10-19 C 1.6x10-19 C x10-7 kg 9.11 x10-31 kg 1.67 x10-7 kg

4 Charge is CONSRVD Charge cannot be created or destroyed only transferred from one object to another. ven though these charges attract initially, they repel after touching. Notice the NT charge stays the same.

5 Conductors and Insulators The movement of charge is limited by the substance the charge is trying to pass through. There are generally types of substances. Conductors: Allow charge to move readily though it. Insulators: Restrict the movement of the charge Conductor Copper Wire Insulator Plastic sheath

6 Charging and Discharging There are basically ways you can charge something. 1. Charge by friction. Induction BIONIC is the first-ever ionic formula mascara. The primary ingredient in BIONIC is a chain molecule with a positive charge. The friction caused by sweeping the mascara brush across lashes causes a negative charge. Since opposites attract, the positively charged formula adheres to the negatively charged lashes for a dramatic effect that lasts all day.

7 Induction and Grounding The second way to charge something is via INDUCTION, which requires NO PHYSICAL CONTACT. We bring a negatively charged rod near a neutral sphere. The protons in the sphere localize near the rod, while the electrons are repelled to the other side of the sphere. A wire can then be brought in contact with the negative side and allowed to touch the GROUND. The electrons will always move towards a more massive objects to increase separation from other electrons, leaving a NT positive sphere behind.

8 lectric orce The electric force between objects is symbolic of the gravitational force between objects. RCALL: 1 g α Mm g α r 1 q1q α q1q α α r r k constant of proportionality k Coulomb constant 8. 99x10 k q1q r Coulomb' s Law 9 Nm C

9 lectric orces and Newton s Laws lectric orces and ields obey Newton s Laws. e e e mg xample: An electron is released above the surface of the arth. A second electron directly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second? k q q r 1 mg mg 9 r? 8.99 r (1.6 x10 (9.11x10 19 ( 31 k q1q mg ( m

10 lectric orces and Vectors lectric ields and orces are ALL vectors, thus all rules applying to vectors must be followed. Consider three point charges, q x10-9 C (located at the origin,q x10-9 C, and q -.00x10-9 C, located at the corners of a RIGHT triangle. q is located at y 3 m while q 3 is located 4m to the right of q. ind the resultant force on q 3. Which way does q push q3? 4m q q3 Which way does q1 push q3? 3m q1 θ 5m on 3 due to q3 on 3 due to 1 θ 37 θ tan -1 (3/4

11 xample Cont 3, 3, 3,1 3,1 9 9 (5.0x10 (x10 (8.99x x10-9 N 9 (6x10 (8.99x10 1.1x10-8 N 4m q q3 3m 5m θ q1 θ tan -1 (3/4 9 (5x resultant res x x y on 3 due to 3.18x10 3,1 3,1 q3 cos(37 Direction θ tan ( on 3 due to 1 + ( θ 37 3,1 cos37 sin(37 6.6x10 9 x N 7.34x10-9 N 1 ( 3, y y x 9 N 3,1 sin degrees above the +x

12 lectric ields By definition, the are LINS O ORC Some important facts: An electric field is a vector Always is in the direction that a POSITIV test charge would move The amount of force PR test charge If you placed a nd positive charge (test charge, near the positive charge shown above, it would move AWAY. If you placed that same charge near the negative charge shown above it would move TOWARDS.

13 lectric ields and Newton s Laws Once again, the equation for LCTRIC ILD is symbolic of the equation for WIGHT just like coulomb s law is symbolic of Newton s Law of Gravitation. The symbol for lectric ield is,. And since it is defined as a force per unit charge he unit is Newtons per Coulomb, N/C. NOT: the equations above will ONLY help you determine the MAGNITUD of the field or force. Conceptual understanding will help you determine the direction. The q in the equation is that of a test charge.

14 xample An electron and proton are each placed at rest in an external field of 50 N/C. Calculate the speed of each particle after 48 ns r r r What do we know q 1.6x10 m e 9.11 x kg m p 1.67 x10-7 kg q both 1.6 x10-19 C v o 0 m/s 50 N/C t 48 x 10-9 s v v v e p Net v 8.3 x10-19 N o a a m a m ma e p a + at e (48x10 p (48x10 Net (9.11x10 (1.67 x a a 4.38 x10 6 m/s.39 x10 3 m/s 9.13x10 13 m/s/s 4.98 x10 10 m/s/s

15 An lectric Point Charge As we have discussed, all charges exert forces on other charges due to a field around them. Suppose we want to know how strong the field is at a specific point in space near this charge the calculate the effects this charge will have on other charges should they be placed at that point. k Qq r q q Qq q k r point charge kq r POINT CHARG TST CHARG

16 xample A -4x10-1 C charge Q is placed at the origin. What is the magnitude and direction of the electric field produced by Q if a test charge were placed at x -0. m? kq (4x x10 9 r m mag N/C -Q dir Towards Q to the right Remember, our equations will only give us MAGNITUD. And the electric field LAVS POSITIV and NTRS NGATIV.

17 lectric ield of a Conductor A few more things about electric fields, suppose you bring a conductor NAR a charged object. The side closest to which ever charge will be INDUCD the opposite charge. However, the charge will ONLY exist on the surface. There will never be an electric field inside a conductor. Insulators, however, can store the charge inside. There must be a positive charge on this side There must be a negative charge on this side OR this side was induced positive due to the other side being negative.