LECTURE Topic 4 POTENTIAL September 19, 2005
Alternate Lecture Titles • Back to Physics 2048 • You can run but you can’t hide!
h B m A The PHY 2048 Brain Partition To move the mass m from the ground to a point a distance h above the ground requires that work be done on the particle. W is the work done by an external force. mgh represents this amount of work and is the POTENTIAL ENERGY of the mass at position h above the ground. The reference level, in this case, was chosen as the ground but since we only deal with differences between Potential Energy Values, we could have chosen another reference. Reference “0”
h B m A Let’s Recall Some more PHY2048 A mass is dropped from a height h above the ground. What is it’s velocity when it strikes the ground? We use conservation of energy to compute the answer. Result is independent of the mass m.
Using a different reference. y y=h B m y=b (reference level) y=0 Same answer! A Still falls to here.
Energy Methods • Often easier to apply than to solve directly Newton’s law equations. • Only works for conservative forces. • One has to be careful with SIGNS. • VERY CAREFUL!
I need some help. Push vs Pull Mrs. FIELDS vs Mr. External
THINK ABOUT THIS!!! • When an object is moved from one point to another in an Electric Field, • It takes energy (work) to move it. • This work can be done by an external force (you). • You can also think of this as the FIELD doing the negative of this amount of work on the particle.
Let’s look at it:move a mass from yi to yf Change in potential energy due to external force: yf yi Same Answer Negative of the work done BY THE FIELD. External Field Got It ?? Keep it!
Move It! • Move the charge at constant velocity so it is in mechanical equilibrium all the time. • Ignore the acceleration at the beginning because you have to do the same amount of negative work to stop it when you get there.
And also remember: The net work done by a conservative (field) force on a particle moving around a closed path is ZERO! What the *&@^ does that mean???
A nice landscape Work done by external force = mgh How much work here by gravitational field? h mg
IMPORTANT • The work necessary for an external agent to move a charge from an initial point to a final point is INDEPENDENT OF THE PATH CHOSEN!
The Electric Field • Is a conservative field. • No frictional losses, etc. • Is created by charges. • When one (external agent) moves a test charge from one point in a field to another, the external agent must do work. • This work is equal to the increase in potential energy of the charge. • It is also the NEGATIVE of the work done BY THE FIELD in moving the charge from the same points.
A few things to remember… • A conservative force is NOT a Republican. • An External Agent is NOT 007.
Electric Potential Energy • When an electrostatic force acts between two or more charged particles, we can assign an ELECTRIC POTENTIAL ENERGY U to the system.
Example: NOTATION U=PE HIGH U LOWER U E q F A B d d Work done by FIELD is Fd Negative of the work done by the FIELD is -Fd Change in Potential Energy is also –Fd. The charge sort-of “fell” to lower potential energy.
Gravity Negative of the work done by the FIELD is –mg D h = D U Bottom Line: Things tend to fall down and lower their potential energy. The change, Uf – Ui is NEGATIVE! mg OOPS !
Electrons have those *&#^ negative signs. • Electrons sometimes seem to be more difficult to deal with because of their negative charge. • They “seem” to go from low potential energy to high. • They DO! • They always fall AGAINST the field! • Strange little things. But if YOU were negative, you would be a little strange too!
An Important ExampleDesigned to Create Confusionor Understanding … Your Choice! The change in potential energy of the electron is the negative of the work done by the field in moving the electron from the initial position to the final position. Final position FORCE d E Initial position Force against The direction of E negative charge e A sad and confused Electron.
An important point • In calculating the change in potential energy, we do not allow the charge to gain any kinetic energy. • We do this by holding it back. • That is why we do EXTERNAL work. • When we just release a charge in an electric field, it WILL gain kinetic energy … as you will find out in the problems! • Remember the demo!
AN IMPORTANT DEFINITION • Just as the ELECTRIC FIELD was defined as the FORCE per UNIT CHARGE: We define ELECTRICAL POTENTIAL as the POTENTIAL ENERGY PER UNIT CHARGE: VECTOR SCALAR
Watch those #&@% (-) signs!! The electric potential difference DV between two points I and f in the electric field is equal to the energy PER UNIT CHARGE between the points: Where W is the work done BY THE FIELD in moving the charge from One point to the other.
Let’s move a charge from one point to another via an external force. • The external force does work on the particle. • The ELECTRIC FIELD also does work on the particle. • We move the particle from point i to point f. • The change in kinetic energy is equal to the work done by the applied forces.
Furthermore… If we move a particle through a potential difference of DV, the work from an external “person” necessary to do this is qDV
Electric Field = 2 N/C d= 100 meters 1 mC Example
The Equipotential SurfaceDEFINED BY It takes NO work to move a charged particle between two points at the same potential. The locus of all possible points that require NO WORK to move the charge to is actually a surface.
Field Lines and Equipotentials Electric Field Equipotential Surface
Components Enormal Electric Field Dx Eparallel Work to move a charge a distance Dx along the equipotential surface Is Q x Eparallel X Dx Equipotential Surface
BUT • This an EQUIPOTENTIAL Surface • No work is needed since DV=0 for such a surface. • Consequently Eparallel=0 • E must be perpendicular to the equipotential surface
Therefore E E E V=constant
ds V+dV V Consider Two EquipotentialSurfaces – Close together Work to move a charge q from a to b: b a E Note the (-) sign!
A Brief Review of Concept • The creation of a charged particle distribution creates ELECTRICAL POTENTIAL ENERGY = U. • If a system changes from an initial state i to a final state f, the electrostatic forces do work W on the system • This is the NEGATIVE of the work done by the field.
Calculation E • An external force F is necessary to move the charge q from i to f. The work done by this external force is also equal to the change in potential energy of the charged particle. Note the (-) sign is because F and E are in opposite directions. • Continuous case: i f q
For Convenience • It is often convenient to set up a particular reference potential. • For charged particles interacting with each other, we take U=0 when the particles are infinitely apart. • Consequently DU=(-) of the work done by the field in moving a particle from infinity to the point in question.
Keep in Mind • Force and Displacement are VECTORS! • Potential is a SCALAR.
UNITS • 1 VOLT = 1 Joule/Coulomb • For the electric field, the units of N/C can be converted to: • 1 (N/C) = 1 (N/C) x 1(V/(J/C)) x 1J/(1 NM) • Or 1 N/C = 1 V/m • So an acceptable unit for the electric field is now Volts/meter. N/C is still correct as well.
In Atomic Physics • It is useful to define an energy in eV or electron volts. • One eV is the additional energy that an proton charge would get if it were accelerated through a potential difference of one volt. • 1 eV = e x 1V = (1.6 x 10-19C) x 1(J/C) = 1.6 x 10-19 Joules.
Coulomb Stuff Consider a unit charge (+) being brought from infinity to a distance r from a Charge q: x r q To move a unit test charge from infinity to the point at a distance r from the charge q, the external force must do an amount of work that we now can calculate.
The math…. Remember This !!!