CS代写 ELECTRIC CHARGE AND ELECTRIC FIELD

ELECTRIC CHARGE AND ELECTRIC FIELD

General Physics

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Dr. Brahmia 1

PARTICLES AND WAVES

• Blackbody Radiation

– At any temperature, any object

emits electromagnetic radiation

called thermal radiation.

– Thermal radiation consists of a

continuous spectrum,

encompassing wavelengths from

the visible, ultraviolet, and

infrared portions of the spectrum.

– From the classical point of view,

electromagnetic radiation is

generated by accelerating

particles.  The distribution of

accelerations produces a

continuous spectrum.

PARTICLES AND WAVES…

• Planck’s Hypothesis

– In 1900, developed a theory that was in complete

agreement with the experimental evidence. Planck asserted that

the molecules in the heated object can vibrate only with discrete

amounts of energy. Thus, the energy of the vibrating atom is

quantized.

– where n, a whole number, is the quantum number, f is the

frequency of the resonator, and h is a constant, now known as

Planck’s constant, given by

General Physics

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PARTICLES AND WAVES…

• The Photoelectric Effect

– When light is incident on certain

metals, electrons are ejected from

the surface of the metal almost

instantaneously. This is called the

photoelectric effect.

– The electrons are ejected from the

surface of the metal only if the

energy of the light is sufficiently

– Notice that photoelectrons, as they

are called, are drawn to the

positive collector, thus producing a

photocurrent.

FunctionWork W max0 KEWhf 

PARTICLES AND WAVES…

• The Photoelectric Effect

– 1) The number of electrons emitted per

second (photocurrent) increases as the

intensity increases. The maximum kinetic

energy of the electrons is not affected by

the intensity of the light.

– 2) The maximum kinetic energy of the

photoelectrons is increased when the

energy of the photons is increased.

– 3) Below a certain frequency, called

threshold frequency, no electrons are

General Physics

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PARTICLES AND WAVES…

• Problem 1

– Electrons are ejected from a metal surface with speeds ranging up

to 3.10×105 m/s when light with a wavelength of λ = 540 nm is

a) What is the work function of the surface?

b) What is the cutoff frequency for this surface?

PARTICLES AND WAVES…

• Pair Production

– A high energy photon known as the gamma ray traveling near the

nucleus of an atom may disappear and an electron and a positron

may appear in its place.

– The electron and the positron have the same mass and carry the

same magnitude of electric charge; however, while the electron is

negatively charged, the positron carries a positive charge.

– The charge, momentum, and mass-energy must all be conserved.

General Physics

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• Pair Annihilation

– The opposite process of pair production is called pair annihilation.

In this case, instead of two antiparticles being produced, two

antiparticles next to each other (and essentially at rest) destroy each

other, producing two identical photons with opposite momentum (a

single photon cannot be created). The following diagram depicts

pair annihilation.

PARTICLES AND WAVES…

PARTICLES AND WAVES…

• Problem 2

– What is the minimum energy of a photon required to produce a

proton-antiproton pair?

– Note: the antiproton has the same mass as a proton but negatively

General Physics

Dr. Brahmia 5

• Wave-Particle Duality

– Louis de Broglie postulated:

– All particles exhibit both wave and particle characteristics.

– The wavelength (), momentum (p) and kinetic energy of a particle

with mass (m) are related by the following equations:

– where m and v are the mass and the velocity of the particle.

– In 1927, Davisson and Germer demonstrated that electrons can be

diffracted, just like electromagnetic waves.

PARTICLES AND WAVES…

• Wave-Particle Duality…

– Since photons do not have mass, the momentum (p) and the total

energy of a photon (E) are related by the following equation:

– where (c) is the speed of light, () is the photon wavelength and h is

the Planck’s constant.

PARTICLES AND WAVES…

General Physics

Dr. Brahmia 6

PARTICLES AND WAVES…

• The Uncertainty Principle

– was also responsible for a concept called the

uncertainty principle.

– The uncertainty principle states that it is fundamentally and

unquestionably impossible to make simultaneous measurements of

a particle’s position and velocity with arbitrarily large accuracy.

– An alternate form of the uncertainty principle applies to the

simultaneous measurement of energy and time.

• Problem 3

– A Nitrogen molecule travels at 515m/s. Suppose the uncertainty in

an experimental measurements of its speed is 5.0%. Compute the

minimum uncertainty in its position.

PARTICLES AND WAVES…

General Physics

Dr. Brahmia 7

PARTICLES AND WAVES…

– In 1923, shot a beam of X-rays at a block of

graphite. He found that the wavelength of the X-rays was longer,

and therefore the energy was smaller. Furthermore, he found that

the amount by which the energy was reduced was dependent on the

angle at which the X-rays were scattered.

PARTICLES AND WAVES…

• Compton scattering…

– Applying the laws of conservation of energy and momentum to the

collision, we can predict the wavelength change by the following

– where m0 is the rest mass of an electron, h is Planck’s constant, and

c the speed of light.

– The quantity (h/ m0 c) is called the Compton wavelength and is

approximately 0.00243 nm.

  cos1

General Physics

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PARTICLES AND WAVES…

• Problem 4

– An X-ray of wavelength 0.1000 nm is scattered by an electron. In

the resulting collision, the scattered photon is reflected directly

backward while the electron travels in the direction of the incident

photon. Determine the

– a) wavelength of the scattered photon and

– b) energy of the recoil electron.

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