Ch 17: Electric Potential and Electric Potential Energy

Electric Potential Energy: Parallel Plates • • Gravitational PE is similar to Electric PE Height*Gravity is similar to Electric Potential

Electric Potential: Parallel Plates

• • The electric field between two charged parallel plates is uniform.

Work has to be done on a charge to move it.

Electric Potential: Parallel Plates

Electric Potential

• • Electric potential, like electric field, is a property of the source charges.

Electric potential is the ability of charges to have an interaction if other charges show up.

Electric Potential Difference

• • • • The potential energy of a particle is determined by knowing the charge…and the electrical potential.

We only care about potential difference.

Charged particles change energy (speed up or slow down) when they move through a potential difference.

Unit is Volts.

Sample Problem

• Two parallel plates, connected to a 200V power supply, are separated by an air gap. How small can the gap be if the air is not to become conducting by exceeding its breakdown value of E = 3 x 10 6 V/m?

Conservation of Energy

• • • • Electric potential links electricity to conservation of energy.

A (+) potential difference will slow down a positive charge.

– Will do the opposite for a (–) charge.

A (–) potential difference will speed up a positive charge.

– Will do the opposite for a (–) charge.

WE ALREADY KNOW HOW TO DO THIS.

Sample Problem

• A proton with a speed of 2.0 x 10 5 m/s enters a region of space in which there is an electric potential. What is the proton’s speed after it moves through a potential difference of +100V?

Sample Problem

• An electron with a speed of 2.0 x 10 5 m/s enters a region of space in which there is an electric potential. What is the electron’s speed after it moves through a potential difference of +100V?

Electron Volt

• • • If an electron "falls" through a potential difference of it gains a kinetic energy equal to electron volt).

Because elementary particles such as electrons and protons are pretty small, the SI unit of the Joule is generally too large to easily deal with these particles.

The electron volt is a UNIT OF ENERGY.

Conceptests

Equipotential Surfaces

• Places where electric potential is equal.

Equipotential Surfaces

• • • These contour lines are analogous to equipotential lines.

If you move between contour lines, you will be changing your gravitational potential.

The closer together the lines are, the steeper the slope you are climbing up.

Electric Potential: Point Charges

• The electric field is not uniform near a point charge.

Electric Potential: Point Charges

• • Electric potential is a SCALAR!

That means in order to find the total potential at a given point….add ‘em up!

• • Forces and Field = vectors Potential and PE = scalars

Conceptests

Sample Problem

• Three point charges are arranged in the following manner. – What is the potential at the fourth corner taking V=0 at a great distance?

– How much work must be done to move an electron from infinity to point A?

Things Due

• • • • • Electric Forces (Friday @ start of class) – Ch 16: 1, 6, 7, 12, 23, 27, 28 – *Think everyone is almost done with this.* Electric Fields (Friday @ start of class) – Ch 16: 13, 14, 17, 34, 37, 67 Electric Potential and (Friday @ start of class) – Ch 17: 11, 15, 16, 21, 24 AP: 1993 B2, 1996 B6, 1999 B2 Tuesday: – – Quiz (Chapter 16 and 17) Equations and problems I am free every day this week 7:40-3:15 – I am afterschool Friday and Monday

ConcepTests 14-27

Capacitors

• • A capacitor is a device which can store electric charge via two conducting objects placed very near each other, but not touching.

Examples of capacitors: – Camera flash – Key on a keyboard

Charging a Capacitor

• • • While going through a battery, work is done on charges to increase their potential.

The plates are initially neutral and of area E. A battery removes charge Q; from one plate and place it on the other. One plate has a positive charge, the other has a negative charge.

Once our capacitor is fully charged each terminal of our battery is at the same electrical potential (voltage) as our respectively charged plates on our capacitor.

• Camera flash and keyboard key

Simulations

Capacitors

• Capacitance: how “good” something is at storing charge.

– Measured in Farads (usually picofarad-millifarad) – The capacitance of a capacitor is only based on the geometry.

• Capacitors store energy in the separation of charge.

Sample Problem

• Two circular plates of radius r are separated by an air gap of width d. What is the magnitude of the charge on each plate when connected to a battery of magnitude V ?

Sample Problem

• • Calculate the capacitance of a parallel-plate capacitor whose plates are 20 cm x 3.0 cm and are separated by a 1.0 mm air gap.

– What is the charge on each plate if a 12V battery is connected across the two plates.

– What is the electric field between the plates?

Estimate the area of the plates needed to achieve a capacitance of 1F, given the same air gap.

Dielectrics

• • A dielectric is made of an insulating material.

When placed in a capacitor….the capacitance goes up because of the rearranging of charge in the insulator.

Capacitors

• Capacitors are useful because they store energy by the separation of + and - charge.

Sample Problem

• A camera flash unit stores energy in a 150 microfarad capacitor at 200 V. How much electrical energy can be stored?

Why do we Physics?

• • To save lives.

Defibrillators work using the charging and discharging of capacitors.

Why do we Physics?

• • To watch TV (when we were younger) Cathode Ray Tubes (CRT). The electrons are "aimed" by placing a certain potential difference on each of the deflection plates.

Why do we Physics?

• • A standard issue TV works by making the electron beam sweep across 525 lines filling the entire screen in 1/30 sec In an HDTV, the electrons move rapidly through a greater number of lines to create a better looking picture.

Why do we Physics?

• • Again…to save lives.

Electrocardiogram (EKG) records potential changes of a person’s heart.

Concept Recap

• • Electric potential is the ability for there to be potential energy.

– Potential and Potential Energy are scalars (add or subtract them) Capacitors store energy by separating charge.

Practice

• • • Electric Potential and Capacitors (Tuesday @ start of class) – Ch 17: 11, 15, 16, 21, 24, 37, 39 {can skip one} – I’d suggest about 2 hours – Graded AP: 1993 B2, 1996 B6, 1999 B2, 2010 B3 (Tuesday @ start of class) – I’d suggest about 1 hour – To study for quiz Read Chapter 16 + 17 to see if there’s anything you missed.

– I’d suggest about 1 hour