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## ELECTRIC POTENTIAL

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### ELECTRIC POTENTIAL

September 13, 2006

Goings On For the Next Few Days

- Quiz Today – Gauss/Electric Field
- Today – Begin Chapter 25 – Potential
- Monday - Potential
- Wednesday – EXAMINATION #1
- Friday – More Potential

Picture a Region ofspace Where there is an Electric Field

- Imagine there is a particle of charge q at some location.
- Imagine that the particle moves to another spot within the field.
- Work must be done in order to accomplish this.

Electric Potential

- We will be dealing with
- Work
- Energy
- We do work if we try to move a charge in an electric field.
- Does the FIELD do work?
- Does the FIELD have a brain? Muscles?

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!

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.

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!

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 –Fd.

The charge sort-of “fell” to lower potential energy.

Another way to look at it:

- Consider an external agent.
- The external agent moves a charge from one point to the other.
- The work done by the external agent in moving the charge between these points is the change in potential energy.
- This is the negative of the work that the FIELD does!

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 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

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. Assume this is zero for now.

Furthermore…

If we move a particle through a potential difference of DV, the work from an external “person” necessary to do this is qDV

Consider Two Plates

OOPS …

The difference in potential between the accelerating plates in the electron gun of a TV picture tube is about 25 000 V. If the distance between these plates is 1.50 cm, what is the magnitude of the uniform electric field in this region?

An ion accelerated through a potential difference of 115 V experiences an increase in kinetic energy of 7.37 × 10–17 J. Calculate the charge on the ion.

Important

- We can define an absolute level of potential.
- To do this, we need to define a REFERENCE or ZERO level for potential.
- For a uniform field, it doesn’t matter where we place the reference.
- For POINT CHARGES, we will see shortly that we must place the level at infinity or the math gets very messy!

An Equipotential Surface is defined as a surface on which the potential is constant.

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.

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

V+dV

V

Consider Two EquipotentialSurfaces – Close togetherWork to move a charge q from a to b:

b

a

E

Note the (-) sign!

Over a certain region of space, the electric potential is V = 5x – 3x2y +2yz2. Find the expressions for the x, y, and z components of the electric field over this region. What is the magnitude of the field at the point P that has coordinates (1, 0, –2) m?

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 sometimes 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.
- Nothing mysterious.

Coulomb Stuff: A NEW REFERENCE: INFINITY

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.

Ponder –What is the potential a distance d from an infinite plane whose charge per unit area is s?

Given two 2.00-μC charges, as shown in Figure P25.16, and a positive test charge q = 1.28 × 10–18 C at the origin, (a) what is the net force exerted by the two 2.00-μC charges on the test charge q? (b) What is the electric field at the origin due to the two 2.00-μC charges? (c) What is the electrical potential at the origin due to the two 2.00-μC charges?

The three charges in Figure P25.19 are at the vertices of an isosceles triangle. Calculate the electric potential at the midpoint of the base, taking q = 7.00 μC.

A disk of radius R has a non-uniform surface charge density σ = Cr, where C is a constant and r is measured from the center of the disk. Find (by direct integration) the potential at P.

Example: Find potential at P

q1 q2

d

P

r

q3 q4

q1=12nC q2=-24nC q3=31nC q4=17nC Sq=36 x 10-9C

V=350 Volts (check the arithmetic!!)

dx

r

d

An Examplefinite line of chargeP

What about a rod

that goes from –L to +L??

At P

Using table of integrals

In the figure, point P is at the center of the rectangle. With V = 0 at infinity, what is the net electric potential in terms of q/d at P due to the six charged particles?

4

1

2

Derive an expression in terms of q2/a for the work required to set up the four-charge configuration in the figure, assuming the charges are initially infinitely far apart.

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