Electric Fields and Capacitors The basis of Electric storage

Two objects are just resolved when the central diffraction maximum of one object is at the first minimum of the other. (Rayleigh’s criterion)  As before,  is approximately tan  = y/L  Circular aperture has diameter d A Learning Summary

What type of forces could act on e-? Gravity, if... another mass is around. Electricity, if... another charged object is around. Magnetism, if... the e- is in a magnetic field The weak nuclear force, if... any other fermion is around

Gravity’s force between e- and p+ Force between two objects due to gravity: m 1 = m e = 9.11 E-31 kg m 2 = m proton = 1.67 E-27 kg r = 1 nm = m F = 1.01 E-49 N

Electrical force between e- and p+ Force between two objects due to Coulomb (electric) attraction: q 1 = q e = E-19 C q 2 = q proton = E-19 C r = 1 nm = m F = 2.31 E-10 N

Nuclear force between e- and p+ Force between two objects due to the weak nuclear force: s = 2mc 2 E = 1 GeV 2 = 2.5 E-20 J 2 m W = 1.42 E-25 kg r = 1 nm = m G F = 4.52 E 14 J -2 F < 1 E -100,000 N

Comparing forces between e- and p+ Electric: F= 2.31 E-10 N Gravity: F= 1.01 E-49 N Nuclear:F < 1 E -100,000 N If I had 1% more electrons than protons, and I stood an arm’s length away from a similarly imbalanced person, the force between us would be great enough to lift a weight the size of... The earth!

How can a force act at a distance? If I took my electron away from the proton and brought a positron (positive e) near the proton, the positron would... accelerate away from the proton So, does my proton exert a force if no one is around to feel it? Force, no. But we can define an electric field which describes the force a charge would feel if it came near the proton

How can a force act at a distance? A charge creates an electric field that fills space, whether or not any other charge is around to feel its effects!

Make Predictions for the Activity, then Answer Questions 1-7 of the Activity

What’s a Field? Electric field E = F/q, where q is the charge feeling the force Since forces obey the law of linear superposition (i.e., they add), electric fields add too!

What does a “field” look like?

Finish the Activity, working as far as you can in the time allowed

Charges in Conductors  Electric fields are created when positive charges and negative charges are separated  A uniform electric field existing over a region sets up a potential difference between points in that region:  V=E  x, where  x is the distance along a field line.  If I apply a potential difference across a conducting object (including semi-conductors), charges experience a force, and charge carriers will flow until the potential difference is removed.

What if charge can’t flow?  Consider charge separated by two metal plates –A potential difference exists between the plates –An electric field exists between the plates, pointing from positive plate to negative plate –No current can flow

Introducing,... The Capacitor

Introducing,... Capacitance The battery provides the work needed to move the charges and increase their potential energy

What determines capacitance? C =  0 A/d

More about capacitors

U = Vq = Ve 1 eV = x J = energy to move electron through 1 volt