Chapter 17: Electric Potential

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As in earlier chapters on mechanics we learned that energy is conserved; it is neither created nor destroyed but is transferred from one object to another or transformed into another type of energy

Energy and its interactions can help us understand nature

Work performed on a charged particle in an electric field can result in the particle gaining electric potential energy (PE), kinetic energy (KE) or both

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The electric field does work when it moves the charged particle from location a to b

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Electric potential is defined as the electric potential energy per unit charge and is measured with a voltmeter

Va= Electric Potential: Units=volt (V). Named after Alessandro Volta, inventor of the electric battery

PEa= Electric Potential Energy: Unit= joule (J)

q = Charge on particle: Unit=Coulomb (C)4

Also called voltage Electric potential difference is the

difference in electric potential (V) between the final and initial position

Δ Also the ratio of work needed to move a

charge between two points divided by the magnitude of the charge

ΔV=W q

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17.2 Relation between Electric Potential and Electric Field

A uniform electric field can made by placing two large flat conducting plates of opposite charge parallel to each other The electric field can be calculated by dividing the potential

difference between the plates by the distance between the plates (in meters)

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17.3 Equipotential Lines Electric potential can be

represented by drawing equipotential lines (green)

An equipotential is a line over which the potential is constant

Equipotential lines are perpendicular to the Electric field (red)

Conductors are equipotential surfaces

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The electric field is strongest where the equipotential lines are closest together.

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17.4 The Electron Volt, a Unit of Energy

The joule is a large unit to deal with energies of electrons, atoms or molecules

The electron volt (eV) is used An eV is the energy gained by an electron

moving through a potential difference of one volt.

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17.7 Capacitance A capacitor consists of two conductors

that are close but not touching. A capacitor has the ability to store electric charge

In general capacitance increases as the plates become larger and decreases as the separation between plates increases

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Capacitors are used widely in electronic

circuitspower failure

back upsBlocking

surges of charge and energy

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(a) Parallel-plate capacitor connected to battery. When connected to a battery the plates become charged; one + and one –

(b) In a circuit diagram the capacitor is represented as seen here

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• C is the capacitance and depends on the size, shape, position and separating material of the capacitor

•Unit of capacitance: farad (F) 1 F = 1 C/V (coulomb/volt)

When a capacitor is connected to a battery, the amount of charge (Q) on its plates is proportional to the potential difference (voltage) between them

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17.8 Dielectrics Most capacitors have an insulating sheet of between the plates

This insulator is a dielectric

Do not break down and allow charge to flow as easily as air, allowing higher voltages

Allow plates to be closer together

Increase the capacitance by a factor of K; a dielectric constant

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If the electric field in a dielectric becomes too large, it can tear the electrons off the atoms, thereby enabling the material to conduct. This iscalled dielectric breakdown; the field at which this happens is called the dielectric strength

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17.9 Storage of Electric Energy

A charged capacitor stores electric energy by separating + and – charges

The energy stored is equal to the work done to charge it

Stored energy in a capacitor can cause burns or shocks, even when the external power is off!

There are many uses for capacitors; a camera flash, a cardiac defibrillator, etc.

An essential part of most electrical devices used today 17

A defibrillator is a capacitor charged to a high voltage. Once charged it sends a brief charge through the heart. This can stop the heart and (hopefully!) allow it to resume normal beating

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Zitewitz. Physics: Principles and Problems. 2004

Giancoli, Douglas. Physics: Principles with Applications 6th Edition. 2009.

Walker, James. AP Physics, 4th Edition. 2010

http://commons.wikimedia.org/wiki/File:Capacitor_schematic_with_dielectric.svg

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