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Chapter 21 – Electric Fields – Discrete Distributions. Electric charge Coulomb Force Law Electric field of discrete charges Field along axis Off-axis electric fields – field lines Motions of charges in fields Dipoles. Why? Chapter 21 – Electric Fields – Discrete Distributions.
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Chapter 21 – Electric Fields – Discrete Distributions • Electric charge • Coulomb Force Law • Electric field of discrete charges • Field along axis • Off-axis electric fields – field lines • Motions of charges in fields • Dipoles
Why? Chapter 21 – Electric Fields – Discrete Distributions • Electric charge • Coulomb Force Law • Electric field of discrete charges • Field along axis • Off-axis electric fields – field lines • Motions of charges in fields • Dipoles
Electric charge Opposites attract … Like charges repel .. Composition of matter Mostly neutral (can have ions) Conducts or “insulates”
Pop can physics? • Q: What happens if I bring the charged rod closer to the neutral metal pop can on the table? • Pop can attracted to charged rod. • Pop can repelled by rod. • Pop can not affected by rod at all. 4
Lightning rods Do they actually attract lightning? Yes, but they also provide a safe path to the ground!
Trying to find “zero force” location for Q0 Hint: “outside” the system, look close – look far .. • Q: Is there a location of “zero force” for charge q0 that exists somewhere to the right of charge q2? • Yes, a location can be found. • No, not possible with these charges. • It depends on the sign of the charge of q0.
We’ll see these problems again when we get to Electric Fields! • Steps to find zero force (or E-field): • Pick an origin! • One distance is chosen as X • Other distance will be combination of X and distance between (Make it a POSITIVE distance construction) blank next
Q: If a zero electric field value on the axis is possible, it will be: • Zero electric field point in this situation is not possible. • Between q1 and q2 • To the right of q2 • To the left of q1
Graphical method for finding E field along axis (Ex !) Ex pointing right – positive values Ex pointing left – negative values “progenitor” graph for positive charge Multiple charges on axis? Add graphs! “progenitor” graph for negative charge
Finding “zeros” of Ex graphs – closer to weaker charge! In fact, all “artifacts” (zeros, peaks, minimums) will be closer to the “weaker” charge in the system (relative magnitude). • Investigate the system by looking CLOSE to each charge and FAR from the system (on both sides): • Close to a charge – that charge dominates • Far from a charge – “net charge” of system dominates • If there is a “change” from close to far – crossing point = zero! Efield Physlet
Off axis Electric Field … VECTORS!! What happens if you increase the magnitude of q2? Decrease the magnitude? Reverse the sign?
Electric Field Lines Electric field lines radiate outward from a positive charge - pretend there is a small positive test charge – which way would it want to go – that’s the direction of the field! If that is positive charge picture … what about negative charge picture? Efield Physlet
Two charges – combine Electric Field Lines! Can you find any “zeros” along the x axis in either figure? How might you change one of the scenarios to create zeros?
Two unequal charges Close up – can see how field lines affected Far away – larger charge dominates! (“net charge”)
TV !! Why study Motion of Point Charges inElectric Fields?
Motion of a charged particle in an Electric Field – Scenario 1 • Q: The path of the electron will be a: • Horizontally East B) Horizontally East then West • C) Diagonal line to the North D) Diagonal line to the South Motion of a charged particle in an Electric Field – Scenario 2 Q: The path of the electron will be a: A) Horizontally East then West B) Curve to the NorthEast C) Curve to the SouthEast D) Diagonal line to the NE E) Diagonal line to the SE blank next
Horizontally launched charge in vertical electric field (projectile!)
Electric Dipoles – two equal magnitude, opposite sign charges – short distance apart We consider the dipole charges as acting together (thus, think of an imaginary “bar” and the charges are on the ends – the bar can rotate if necessary, keeping the charges separated by a distance L)
Dipole in an Electric Field – in Equilibrium? Q: The dipole will be in unstable equilibrium if : A) p is parallel to E B) p is perpendicular to E C) p is anti-parallel to E (opposite direction) D) a dipole can never be in equilibrium in an Electric field
Dipole in Electric Field scenarios (assume equal vector lengths for p) 1) Which ones (if any) are in stable equilibrium? 2) Which ones (if any) are in unstable equilibrium? 3) Which ones (if any) have torques pointing out of the board? 4) Which ones (if any) have torques pointing into the board? 5) Which one(s) has(have) the largest magnitude torque? 6) Which figures (if any) are “identical”
