Electric Forces and Fields Electric Charge Lets review
- Slides: 40
Electric Forces and Fields
Electric Charge • Let’s review… • Atoms consist of: – Protons, neutrons, electrons • Protons have a charge of: – +e What is e? e=1. 6 x 10 -19 C Unit is the coulomb (C) This is the smallest charge yet discovered. • Electrons have a charge of – -e • Neutrons have a charge of 0
Electric Charge • Intrinsic property of electrons and protons • Most objects have balanced charges- in order to carry a charge, there must be an imbalance btwn p+ and e- (ionization) • Can be positive or negative • Likes repel • Opposites attract
Electric Charge is… • Charge is conserved- net charge cannot be created or destroyed • Charge is quantized - must be an integer multiple of e • q= the charge of an object (unit is C)
Creating Electric Charge • Usually charge is created through transfer of electrons • Gain electrons = gain negative charge • Lose electrons= gain positive charge • You can do this in 3 ways: – Friction – Conduction – Induction
Charging by friction • Different objects have different propensities for gaining or losing electrons • Triboelectric sequence ranks objects-if you rub any 2 objects together, the on top will lose elecrons and the on the bottom will gain • The farther they are apart, the more they will charge Asbestos Fur (rabbit) Glass Mica Wool Quartz Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Copper, Brass Rubber Sulfur Celluloid India rubber
Charging by friction • Using the triboelectric sequence, predict what will happen when a glass rod is rubbed on a piece of silk • Will a charge develop? • How big will it be? • Which object will be negative?
Charging by Conduction • Conduction= TOUCHING • If you touch a charged object to another object, electrons can actually flow from one to the other
Showing Conduction • We can use an electroscope to show charging by conduction • Take a positively charged rod and touch it to the ball on the top of the electroscope Electroscope • Which way would electrons flow? • What would be the residual charge on the shows SAME charge after electroscope? conduction
Charging by Induction • Charged object is brought near but never touches another object • Because electric charge does not require contact (electric field), you can induce a charge just by proximity with a charged object
Showing Induction • If you bring your + rod near the electroscope, electrons will be attracted to it and will gather at the ball- this is temporary- once you move it away, the charge is gone • However, if the electroscope is grounded (attached to the earth which is a pretty good conductor) you could get electrons flowing from the earth and keep a net charge- opposite to charging rod!
Induction in insulators • In an insulator, electrons can’t move freely • You can still induce a charge by polarizing the molecules close to the charged object In this insulator the top surface becomes - and the bottom surface becomes +
Induction in insulators • Try this with a charged rod and tiny scraps of paper (an insulator) • What happens to the paper? Why? • What processes are happening?
Coulomb’s Law • Electrostatic force, Fe depends on • The charge on the objects: q 1 and q 2 • The distance between them, r
Coulomb’s Law • For point charges • F=k q 1 q 2 r 2 • On your green sheet, k is written as: • k= 1/4 0 but in the section on constants, you can find k- look it up now • Easiest to deal with magnitudes of q and then note that likes repel, opposites attract
Coulomb’s Law, visually
Example: Coulomb’s Law • Consider 2 small spheres, one carrying a charge of +1. 5 n. C and the other a charge of -2. 0 n. C, separated by a distance of 1. 5 cm. Find the electric force between them. • (reminder: nano=10 -9)
Solution: Coulomb’s Law • FE=kq 1 q 2/r 2 • =(9 x 109 Nm 2/C 2) (1. 5 x 10 -9 C)(-2. 0 x 10 -9 C) (0. 015 m)2 • =-1. 2 x 10 -4 N • When F is -, it means attraction…but you knew that already by the opposite charges
• Worksheet: Coulomb’s Law
Superposition and Coulomb • If there are more than 2 point charges, the FE is the sum of all FE acting on a point
Worksheet: Coulomb Beyond the Fundamentals and Suspended spheres
The Electric Field
Electric Field E=Fon q/q The Electric field equals the electric force on a small test charge, q, divided by that small test charge. Unit: N/C Direction: same as force • The presence of a charge created an electric field in the space that surrounds it • Other charges will be affected by this field • Directional so it is a vector and can be added/subtracted as a vector The electric field leads to the F
Putting it together: FE and E • FE=kq 1 q 2/r 2 • E=Fon q/q E=kq. Q r 2 q E=k. Q/r 2 Note- not on green sheet. You will need to be able to derive this if you need it!
Electric Field Lines Begin at +, end at -, do not start or stop in midspace, number of lines is proportional to the charge. • Point in for -source • Point out for +source • Density of lines shows strength of field
Distance and Strength of E
Vector Nature of E At any point, you can add the EB+EA
Vector Addition of E
Common E interactions • Note direction of arrows • 2 equal but opposite charges (above right) are called an electric dipole • NOTICE- fields lines NEVER CROSS
Electric Field Example • A charge of q=+3. 0 n. C is placed at a location at which the electric field has a strength of 400 N/C. Find the force felt by the charge q.
Electric Field Solution • Fon q=q. E • F=(3 x 10 -9 C)(400 N/C)=1. 2 x 10 -6 N
• Worksheet: Electrostatic Forces and Fields: Point Charges
Conductors and Insulators • • Conductors permit the flow of excess charge Insulators don’t let electrons flow Semiconductors- kind of in between Superconductors- no resistance to flow of electrons (many metals act this way at low T)
Conductors- a closer look • Any excess charge resides solely on the outer surface of a conductor • The charge inside is zero! • Any LOST fans? Michael Faraday built a “room within a room” to demonstrate this- the man in the picture is safe- no charge inside his cage • We use this to “shield” our sensitive electronics by enclosing in a metal box
Faraday Cage
Conductors- a closer look • For points outside the conductor, the electric field acts as if it is concentrated at the center of the conductor • Electric field is always perpendicular to the surface – No matter what the shape
Special Situations: Parallel Plates • Above you see 2 parallel plates attached to a battery • The symbol on the left is the battery- the longer line is the + terminal and the shorter line is the – • Thus the top plate is + and bottom is – • The electric field then is the direction a + charge would move- thus the arrows
Special Situations: Parallel Plates • This type of field is uniform • In parallel plates, E=V/d where V is voltage supplied by battery and d is distance between plates • (units of V/m= N/C) for E • Thus a test charge would experience the same force regardless of where it is located in the field • F=q. E ANYWHERE between plates
Example: Parallel Plates • V=28 V and d=0. 14 m • Find the F on a 2 n. Ccharge inserted anywhere between the plates
Plates problem solution • • 1 st find E: E=28 V/0. 14 m E=200 V/m Or 200 N/C • • • 2 nd find F F=q. E F=(2 x 10 -9 C)(200 N/C) F=4 x 10 -7 N TOWARD which plate?
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