Charges, Forces, & Fields Mr Finn Honors Physics

ToC Charges Forces Fields

Under the Influence Two uncharged metal balls (X & Y) rest on glass stands. A 3 rd ball (Z) has a positive charge and is brought near the first two. A conducting wire connects X and Y. The wire is removed and then Z is removed. At the end, it is found that A.X and Y are uncharged B.X and Y are positively charged C.X and Y are negatively charged D.X is + but Y is – E.X is – but Y is + Epstein, Thinking Physics, p 381

Diagram

Pithballs Three pithballs are suspended from thin silk threads and each is charged by touching it with a different charged object. Pithballs 1 and 2 repel each other; pithballs 2 and 3 repel each other. What can we conclude? A.only that 1 and 3 have opposite charges B.only that 1 and 3 have like charges C.all three have like charges D.one object has no charge E.only that more information is needed

Electric Force Two uniformly charged spheres are firmly attached to and electrically insulated from frictionless air pucks. The charge on sphere 2 is three times the charge on 1. Which diagram correctly shows the magnitude and direction of the forces? A. G. F.E. D.C. B.

Charles de Coulomb ( ) Force law for electric charges Quantified concept of charge –Unit named for him Describe “amber effect” following Newton’s approach for gravity

Equipment Identical metal/conducting spheres –conducting wire Calibrated torsion balance –calibrated spring balance Meter stick/ruler

Procedure Obtain 2 identically charged spheres –place one at end of torsion balance –place counterweight on other end Place 2 nd charged sphere 10 cm from 1 st –measure angle  repulsive force Repeat at other distances, other charges

Results Force/angle proportional product of charges –F  q and F  Q Force/angle - inverse square law –F  1/d 2 Put together: –F  qQ/d 2

Proportionality Constant Define unit of charge –based on electromagnet –1 C = 6.25  electrons k =  10 9 N m 2 /C 2 Alternative – k = 1 / 4  o –  o = “ permittivity of free space ” = 8.85  C 2 /N m 2

Example Find the force between two charges 12 cm apart –q = +3.0  C –Q = +2.0  CSolution: What happens if distance between charges - doubled to 24 cm? Solution:  Is the force on Q the same as the force on q?

Constants e = charge on electron or proton x C m p = mass of proton x kg m N = mass of neutron x kg k = x 10 9 Nm 2 /C 2

Torsion Balance Light bar suspended by wire Measure force to twist wire by angle –1 N force twist bar 1° –2 N force twist bar 2° Force from spring balance –calibrate spring hang known weights measure amount spring stretched 1 cm stretch per 1 N weight

Charging Spheres QQ/2 Q/4 Q/ q+q

Measure Force Q Q Looking down from above Q d = 10 cm angle  force

Alternative Approach Philosophical problem with Newton ’ s approach to describing “ action-at-distance ” Objects interact with their surroundings = rest of universe? Michael Faraday

Action at a distance? Problem local distant delayed Problem: Should observable effects have only local causes? And distant causes should have only delayed effects q Q Move charge up here … … how long before charge down here “ feels ” the effect?

Space is not empty

Field Lines

Dipole Fields

Rules for Field Lines 1. Arrow on field line = direction of electric field; tangent to curved field line 2. Magnitude of E is proportional to number of field lines 3. Start on Positive charges and end on Negative charges 4. Number of starting/ending lines proportional to magnitude of charge

I still don’t get it?!

Fields & Conductors 1. No electric field inside the conductor – only in space outside of the conductor. 2. Electric field at surface of conductor must be perpendicular to the surface.

Power of Points 3. Electric fields are strongest along “ pointed ” regions of a surface

Connected to Light (Energy) How does the field get from the charge to a point in space Maxwell discovered that light is an electromagnetic wave –ripples of electric & magnetic fields moving through space –created by vibrating/accelerating electric charges Electric fields propagate through space as waves at the speed of light –distantdelayed –distant causes now have delayed effects James Clerk Maxwell

Electric Fields (1) Consider the four field patterns. Assuming there are no electric charges in the regions shown, which of the patterns is a possible electrostatic field? A.only (a) B.only (b) C.only (b) and (d) D.only (a) and (c) E.only (b) and (c) F.none of the above

Electric Fields (2) Suppose the strength of an electric field about an isolated point charge has a certain value (E) at a distance of 1 m. What will the strength of the field be at a distance of 2 m? A.E B.E/2 C.E/4 D.2E E.4E F.none of these values G.impossible to say with the given information Hewitt, Conceptual Physics, p 396

Elbow Room Molecules in a gas resist crowding and get as far apart as possible. Free electrons also resist crowding and get as far apart as possible. When a tank is filled with gas, the molecules uniformly fill the volume of the tank so that each molecule has the maximum distance from its neighbors. When a copper ball is charged with electricity, the free electrons will distribute themselves uniformly throughout the ball ’ s volume for the same reason. A.True B.False Epstein, Thinking Physics, p 377

Solution to “Elbow Room”