Photoelectric Effect and X-Rays Flashcards
Define the photoelectric effect.
The photoelectric effect is the emission of electrons from the surface of a metal using light (electromagnetic radiation) of a suitable frequency
On what does the number of electrons emitted depend?
The number of electrons emitted depends on the intensity of the incident light.
On what does the energy of the electrons emitted depend?
The energy of the electrons depends on the frequency of the incident light.
Justify how x rays production is the opposite of the photoelectric effect.
In x ray production electrons hit a metal target and photons are emitted.
In the photoelectric effect photons hit a metal target and electrons are emitted.
Give three applications of a photocell/the photoelectric effect
Control of central heating burners
Light meters for cameras
Reading bar codes
Define thermionic emission.
Thermionic emission is the emission of electrons from the surface of a hot metal
Explain why the leaves of a gold leaf electroscope collapse when UV light is shined on them.
Ultraviolet radiation causes photoelectric emission. Electrons leave the surface of the zinc which reduces the size of the negative charge on the gold leaves. The reduces the potential difference between the gold leaves and the outer body of the electroscope causing the leaves to collapse.
Explain why the leaves don’t collapse when the zinc is covered with an ordinary piece of glass?
Ultraviolet radiation cannot pass through ordinary glass so photoemission doesn’t occur.
Explain why the leaves don’t collapse when the zinc is illuminated with green light?
The frequency of green light is lower than ultraviolet radiation and below the threshold frequency of zinc so photoemission cannot occur.
Explain why the leaves don’t collapse when the electroscope is charged positively.
Because any electrons emitted will be attracted back to the positively charged electroscope. The charge on the electroscope remains constant and the leaves do not collapse.
Define X-rays.
X-rays are electromagnetic radiation of very short wavelength and very high frequency.
Who discovered X-Rays?
W. Rontgen
In an x-ray tube, how are the electrons emitted from the cathode?
Electrons are emitted from the cathode by thermionic emission.
In an x-ray tube, how are electrons accelerated?
A very high voltage between the cathode and the anode is used to accelerate the electrons.
Write down a formula for the kinetic energy gained by an electron?
E = q x V; q=charge V=Voltage/Potential difference
Or
qV=hf
Why was the quantum theory of light revolutionary?
The theory suggests that light consists of photons. I.e. discrete bundles of energy rather than being a continuous wave.
Define photon.
A photon is a discrete quantity of electromagnetic radiation.
For the photoelectric effect how is the intensity of the light varied?
Vary distance from light source to photocell
Vary the current/voltage to bulb in light source
How do x-rays differ from light rays.
Unlike light rays, x-rays can penetrate matter and can cause ionisation.
Give three uses of x-rays.
Medical analysis of bones in the human body.
Luggage scanners at airports
Astronomy
Irradiation of foods
What happens to the majority of the energy when the electrons hit the target metal? As a result what kind of metal must be used?
The majority of the energy is converted to heat energy.
So the target metal must have a very high melting point. E.g. tungsten
Give two properties of radio waves.
They travel at the speed of light.
They can travel in a vacuum.
Why are radio waves not very penetrating?
They have low frequencies and therefore low energies and low penetration.
Name an electromagnetic wave that may induce cancer.
X-rays; they ionise cells in the human body.
