Chapter 17. Electric Potential & Energy. I Scalars & Vectors. Giancoli 17-1. Is
there a simpler way of describing the influence of charge on a region of space?
II Electrical Nature of Matter: Matter consists of a variety of particles: l e,
p, n, γ, π, ν, u, d
Each of these has a definite charge: l e-,
p+, n0, γ0, π+, π−, π0, ν0, u+2/3, d-1/3
The massive constituents of atoms are e -, p+, n0 l
10-10 m
integral charges electron
nucleus
II Electrical Nature of Matter: Because of these electric charges, there are electrical forces between particles: Particles:
Electric Force:
p&p
repulsive
e&e
repulsive
p&e
attractive
n & anything
zero
LIKE CHARGES REPEL. UNLIKE CHARGES ATTRACT.
III Charge conservation: Processes in high energy physics can transform particles into other particles
But in all observed cases, l
free charge is quantised in units of 1 e l and
conserved
LAW OF CONSERVATION OF ELECTRIC CHARGE: The net amount of electric charge produced in any process is zero.
V Electrical properties of materials: Three main types: l Conductors l Insulators
(metals)
(glass, plastic, gases)
l Semiconductors
(Si, Ge, diamond)
VIII Explorations of Coulomb’s Law: Coulomb’s Law:
Q1 Q2 F=k 2 r
G16-5
k ≈ 9 × 10 9 Nm2 / C 2 1 = , 0 = 8.85 × 10 −12 C 2 /(Nm 2 ) 4 0
Principle of Superposition Newton’s Third Law: Action & Reaction Force F & vectorial addition
Q1 Q2 ˆ ˆ r F = k 2 r, r = r r
X Field lines & lines of force: Definition:
E = F/ q
G16-7
Around systems of charges Indicating direction & magnitude of E Tangent of line gives direction Strength = number of lines crossing per unit area +
-
XI Fields & conductors: Definition:
E = F/ q
F = qE
G16-9
l
Charges move in response to F. As far away as possible!
l
To the outside of a conductor
Inside a conductor, E = 0. q in a hollow conductor induces charges on surfaces: E exists inside the hollow & outside the conductor but NOT within the conductor itself E is normal to the surface of the conductor. Why?
XIII Hydrogen Bonding E = F/ q
Giancoli 17-6
This attraction is due to the hydrogen bond: • the molecule has localised electric dipoles, like induced charges from a charged comb on pieces of paper, or on a conductor • the electron cloud is distorted around the H-N & binds more strongly to the N-CC • q = δ b (Vab = Vb - Va), is there an absolute potential Va & a zero potential? A: Not much difference?! CONVENTION 1: Potential is zero when charges are separated by an infinite distance. CONVENTION 2: Earth is at zero volts. Examples?
Chapter 17. Electric Potential & Energy IV Dipoles & Dipole Potentials E = Vba/d = - ∆ V / ∆ x Q: What is a dipole?
Chapter 17. Electric Potential & Energy V Equipotential Lines Giancoli 17-3 Lines joining all points of equal potential
Chapter 17. Electric Potential & Energy VI Equipotentials in a conductor Potential differences between regions of conductors: V+ V-
There are no electric fields inside a conductor = the whole conductor is at a single equipotential
Chapter 17. Electric Potential & Energy VII Electron Volt, a unit of energy The SI unit for W, Energy is the joule J. An electron volt, eV, is the energy gained when an electron moves through a ∆ V of 1 Volt. qe = 1.6x10-19 C, so W = qV = 1.6x10-19 J = 1 eV
Chapter 17. Electric Potential & Energy VIII Capacitors Giancoli 17-7 Capacitors store charge. - or separation of charge Q = CV e.g. two parallel conducting plates: Connecting to a battery
-
+
(or Potential Difference) allows electrons to flow from -ve to +ve potentials, accumulating as shown e
-
+
-
+
e
Chapter 17. Electric Potential & Energy VIII Capacitors Giancoli 17-7 E = Q/[A ε0] = constant V = E d = Qd/[A ε0] Q = CV so V = Q/C C = Q/V = A ε0 /d
-
+
Units of Farad (F): 1F = 1C/1V After removing the battery, Q = constant
-
+
V = Qd/[A ε0] 2000 V is required to deflect the electroscope
e
-
+
e
Chapter 17. Electric Potential & Energy IX Dielectrics (insulators) Giancoli 17-8 Q = CV. In air, Q = C0 V0 With an insulator (dielectric) between the plates, the electrons can move a little, creating lots of little dipoles. The dipoles try to reduce the electric field (& Force) inside, so the dielectric becomes POLARIZED. Einside = (1/κ) Eoutside = Efree - Epolarised κ = dielectric constant Then V = Einside d = Eoutside d/κ. C=Q/V= κ C0
-
+
Chapter 17. Electric Potential & Energy X Energy storage Giancoli 17-9 Q = CV
-
+
Chapter 17. Electric Potential & Energy XI Applications of Electrostatics Q = CV
-
+
Chapter 17. Electric Potential & Energy XII Electrocardiogram Giancoli 17-10 Q = CV
-
+
