1. Copyright R. Janow – Fall 2014 1 Physics 121 - Electricity and Magnetism Lecture 3 - Electric Field Y&F Chapter 21 Sec. 4 – 7 Recap & Definition of Electric Field Electric Field Lines Charges in External Electric Fields Field due to a Point Charge Field Lines for Superpositions of Charges Field of an Electric Dipole Electric Dipole in an External Field: Torque and Potential Energy Method for Finding Field due to Charge Distributions –Infinite Line of Charge –Arc of Charge –Ring of Charge –Disc of Charge and Infinite Sheet Motion of a charged paricle in an Electric Field - CRT example

2. Copyright R. Janow – Fall 2014 Fields Scalar Field Examples: Temperature - T(r) Pressure - P(r) Gravitational Potential energy – U(r) Electrostatic potential – V(r) Electrostatic potential energy – U(r) Vector Field Examples: Velocity - v(r) Gravitational field/acceleration - g(r) Electric field – E(r) Magnetic field– B(r) Gravitational Field versus Electrostatic Field force/unit charge q 0 is a positive “test charge” force/unit mass m 0 is a “test mass” “Test” masses or charges map the direction and magnitudes of fields Fields “explain” forces at a distance – space altered by source

3. Copyright R. Janow – Fall 2014 Field due to a charge distribution Test charge q 0 : small and positive does not affect the charge distribution that produces E. A charge distribution creates a field Map E field by moving q 0 around and measuring the force F at each point E field exists whether or not the test charge is present E(r) is a vector parallel to F(r) E varies in direction and magnitude x y z ARBITRARY CHARGE DISTRIBUTION (PRODUCES E) TEST CHARGE q 0 most often… F = Force on test charge q 0 at point r due to the charge distribution E = External electric field at point r = Force/unit charge SI Units: Newtons / Coulomb

4. Copyright R. Janow – Fall 2014 Electrostatic Field Examples Magnitude: E=F/q 0 Direction: same as the force that acts on the positive test charge SI unit: N/C Field LocationValue Inside copper wires in household circuits10 -2 N/C Near a charged comb10 3 N/C Inside a TV picture tube10 5 N/C Near the charged drum of a photocopier10 5 N/C Breakdown voltage across an air gap (arcing)3×10 6 N/C E-field at the electron’s orbit in a hydrogen atom5×10 11 N/C E-field on the surface of a Uranium nucleus3×10 21 N/C

5. Copyright R. Janow – Fall 2014 Electric Field 3-1 A test charge of +3 µ C is at a point P. An external electric field points to the right. Its magnitude is 4 × 10 6 N/C. The test charge is then replaced with another test charge of – 3 µ C. What happens to the external electric field at P and the force on the test charge when the change happens? A. The field and force both reverse direction B. The force reverses direction, the field is unaffected. C. The force is unaffected, the field is reversed D. Both the field and the force are unaffected E. The changes cannot be determined from the information given

6. Copyright R. Janow – Fall 2014 Electric Field due to a point charge Q Q r E E E Coulombs Law test charge q 0 Find the field E due to point charge Q as a function over all of space Magnitude E = KQ/r 2 is constant on any spherical shell Visualize: E field lines are radially out for +|Q|, in for -|Q| Flux through any closed shell enclosing Q is the same (see Lect 01): F = EA = Q.4  r 2 /4  0 r 2 = Q/  0 Radius cancels The closed (Gaussian) surface intercepts all the field lines leaving Q

7. Copyright R. Janow – Fall 2014 Visualization: electric field lines (lines of force) Map direction of an electric field line by moving a positive test charge around. The tangent to a field line at a point shows the field direction there. The density of lines crossing a unit area perpendicular to the lines measures the strength of the field. Where lines are dense the field is strong. Lines begin on positive charges (or infinity and end on negative charges (or infinity). Field lines cannot cross other field lines

8. Copyright R. Janow – Fall 2014 DETAIL NEAR A POINT CHARGE No conductor - just an infinitely large charge sheet E approximately constant in the “near field” region ( d << L ) The field has uniform intensity & direction everywhere NEAR A LARGE, UNIFORM SHEET OF + CHARGE TWO EQUAL + CHARGES (REPEL) EQUAL + AND – CHARGES ATTRACT WEAK STRONG + + + + + + + + + + + + + + + + + + + + L d q F

9. Copyright R. Janow – Fall 2014 Field lines for a spherical shell or solid sphere of charge Outside: Same field as point charge Inside spherical distribution at distance r from center: E = 0 for hollow shell; E = kQ inside /r 2 for solid sphere Shell Theorem

10. Copyright R. Janow – Fall 2014 Use superposition to calculate net electric field at each point due to a group of individual charges + + Do this sum for every test point i Example: point charges

11. Copyright R. Janow – Fall 2014 Example: Find E net at point “O” on the axis of a dipole Use superposition Symmetry  E net parallel to z-axis DIPOLE MOMENT points from – to + Exercise: Do these formulas describe E at the point midway between the charges Ans: E = -4p/2  0 d 3 z = 0 - q +q O z E+E+ E-E- r-r- r+r+ d Exact Fields cancel as d  0 so E  0 E falls off as 1/z 3 not 1/z 2 E is negative when z is negative Does “far field” E look like point charge? For z >> d : point “O” is “far” from center of dipole Limitation: z > d/2 or z < - d/2

12. Copyright R. Janow – Fall 2014 Electric Field 3-2: Put the magnitudes of the electric field values at points A, B, and C shown in the figure in decreasing order. A) E C >E B >E A B) E B >E C >E A C) E A >E C >E B D) E B >E A >E C E) E A >E B >E C.C.C.A.A.B.B

13. Copyright R. Janow – Fall 2014 A Dipole in a Uniform EXTERNAL Electric Field Feels torque - Stores potential energy (See Sec 21.7) ASSSUME THE DIPOLE IS RIGID Torque = Force x moment arm = - 2 q E x (d/2) sin() = - p E sin() (CW, into paper as shown) |torque| = 0 at q = 0 or q =  |torque| = pE at q = +/-  /2 RESTORING TORQUE: t(-q) = t(+q) OSCILLATOR Potential Energy U = -W U = - pE for  = 0 minimum U = 0 for  = +/-  /2 U = + pE for  =   maximum Dipole Moment Vector

14. Copyright R. Janow – Fall 2014 3-3: In the sketch, a dipole is free to rotate in a uniform external electric field. Which configuration has the smallest potential energy? E A B C DE 3-4: Which configuration has the largest potential energy?

15. Copyright R. Janow – Fall 2014 Method for finding the electric field at point P - - given a known continuous charge distribution 1. Find an expression for dq, the charge within a differentially “small” chunk of the distribution 2. Represent field contributions at P due to point charges dq located in the distribution. Use symmetry 3. Add up (integrate) the contributions dE over the whole distribution, varying the displacement and direction as needed This process is just superposition

16. Copyright R. Janow – Fall 2014 Example: Find electric field on the axis of a charged rod Rod has length L, uniform positive charge per unit length λ, total charge Q. Q/L. Calculate electric field at point P on the axis of the rod a distance a from one end. Field points along x-axis. Interpret Limiting cases: L => 0 ? a >> L ? L >> a ? Add up contributions to the field from all locations of dq along the rod (x  [a, L + a]).

17. Copyright R. Janow – Fall 2014 Electric field due to a straight LINE of charge P oint P on symmetry axis, a distance y off the line solve for line segment, then let y << L point “P” is at y on symmetry axis uniform linear charge density: = Q/L by symmetry, total E is along y-axis x-components of dE pairs cancel Integrate from –  0 to +  0 Falls off as 1/y As  0   /2 (y<<L) LONG wire looks infinite Along –y direction “1 + tan 2 (  )” cancels in numerator and denominator Find dx in terms of  j ˆ r )cos( dq k dE r ˆ r kdq Ed 2 yP, 2 P    SEE Y&F Example 21.10 x y P L dq = dx r     

18. Copyright R. Janow – Fall 2014 Electric field due to an ARC of charge, at center of arc Uniform linear charge density  = Q/L dq = ds = Rd       R dq P dEdE L P on symmetry axis at center of arc  Net E is along y axis: E y only Angle  is between –  0 and +  0 For a semi-circle,  0 =  /2 For a full circle,  0 =  Integrate: In the plane of the arc

19. Copyright R. Janow – Fall 2014 Electric field due to a RING of charge at point P on the symmetry (z) axis Uniform linear charge density along circumference:  = Q/2  R dq = charge on arc segment of length ds = Rd  P on symmetry axis  xy components of E cancel Net E field is along z only, normal to plane of ring Integrate on azimuthal angle f from 0 to 2p integral = 2p E z  0 as z  0 (see result for arc) Exercise: Where is E z a maximum? Set dE z /dz = 0 Ans: z = R/sqrt(2) Limit: For P “far away” use z >> R Ring looks like a point charge if point P is very far away! dfdf SEE Y&F Example 21.9

20. Copyright R. Janow – Fall 2014 Electric field due to a DISK of charge for point P on z (symmetry) axis dEdE  R x z  P r dA=rd  dr s Uniform surface charge density on disc in x-y plane  = Q/  R 2 Disc is a set of rings, each of them dr wide in radius P on symmetry axis  net E field only along z dq = charge on arc segment rd  with radial extent dr Integrate twice: first on azimuthal angle f from 0 to 2p which yields a factor of 2p then on ring radius r from 0 to R Note Anti- derivative “Near field” is constant – disc approximates an infinite sheet of charge For z<< R : disc looks infinitely large P is close to the surface See Y&F Ex 21.11

21. Copyright R. Janow – Fall 2014 Infinite (i.e.”large”) uniformly charged sheet Non-conductor, fixed surface charge density  L d Infinite sheet  d<<L  “near field”  uniform field Method: solve non-conducting disc of charge for point on z-axis then approximate z << R

22. Copyright R. Janow – Fall 2014 Motion of a Charged Particle in a Uniform Electric Field Stationary charges produce E field at location of charge q Acceleration a is parallel or anti-parallel to E. Acceleration is F/m not F/q = E Acceleration is the same everywhere in uniform field Example: Early CRT tube with electron gun and electrostatic deflector ELECTROSTATIC DEFLECTOR PLATES electrons are negative so acceleration a and electric force F are in the direction opposite the electric field E. heated cathode (- pole) “boils off” electrons from the metal (thermionic emission) FACE OF CRT TUBE ELECTROSTATIC ACCELERATOR PLATES (electron gun controls intensity)

23. Copyright R. Janow – Fall 2014 Motion of a Charged Particle in a Uniform Electric Field electrons are negative so acceleration a and electric force F are in the direction opposite the electric field E. x DyDy L vxvx D y is the DEFLECTION of the electron as it crosses the field Acceleration has only a y component v x is constant Measure deflection, find  t via kinematics. Evaluate v y & v x Time of flight  t yields v x