El;ectrons waves and photons corrections Flashcards Preview

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Flashcards in El;ectrons waves and photons corrections Deck (20):

Explain how the I-V characteristic show how the resistivity of a semiconductor decreases with increasing temperature

(graph curves so) R changes qualification: I increases faster than V

increased temperature is caused by (larger) current in slice qualification: P = I2R as R decreases ρ decreases


How does the p.d across the resistor vary as the intensity of the light on a LDR increase

total R falls so I in circuit/in R1 increases so V across R1 increases and V across 750 Ω falls


Define the photoelectric effect

emission of electron(s) from a metal (surface) when photon(s)/ light/uv/em radiation are incident (on surface)


The SI unit of energy



Explain why a double slit is used instead of two identical light sources

light from the two sources must be/slits is coherent only possible to produce constant phase difference using a single source


Use of LEDS and positives

torch; car brake/rear light/ traffic light, etc. torch: draws a lower current / light lasts longer before battery discharged/AW or LEDs (much) more efficient (at converting electrical energy into light)/AW or if one LED fails remainder still lit/AW


why do electron transitions between the energy levels produce different wavelengths

(atom releases energy when) electron moves from high to
low level
energy released is in form of a photon
possible transitions are between n = 3 and n = 1, n = 3
and n = 2, n = 2 and n= 1


How to find the separation of the lines on the grating

dsin$= lamda


how to calculate the number of lines per mm on the grating



Properties of em waves

travel in a vacuum
same speed (in vacuum)/at c
caused by accelerating charges
are (oscillating) electric and magnetic fields


How to calculate energy gained by an electron



How does the resistance of the filament lamp change with time

the resistivity/resistance of the (metal) filament increases with temperature the larger the current in the filament the hotter it becomes/AW


Plane polarised wave

oscillations (of particles/e-m fields along the wave) are in one direction only perpendicular to the direction of wave propagation/of travel of the wave/of energy transfer


How are work function and threshold frequency related

a photon with less than the threshold frequency f0 cannot cause electron emission/AW so work function = h (threshold frequency)


Define the photoelectric effect

The emission of electrons from the surface of a metal when electromagnetic waves (of frequency greater than the threshold frequency) are incident on the metal.


Explain how the photoelectric effect cant be explained by the wave model of em waves but it validates the photon model

The wave model cannot explain why there is a threshold frequency for metals.

The new model / photon model proposed one-to-one interaction between photons and electrons and this successfully explained why threshold frequency exists.

Any further one from: Energy of photon (hf) must be greater than or equal to work function of metal. The kinetic energy of emitted electrons was independent of the incident intensity. Correct reference to hf = Φ + KEmax


State the effect intensity has on em radiation

The energy of a photon depends only on wavelength or frequency, so intensity does not change the maximum speed of the photoelectrons.


Determine how time constant can be determined

Connect a voltmeter or data-logger or oscilloscope across the resistor (or capacitor) or an ammeter in series with the resistor.

A stopwatch is started when the switch is opened and stopped when the p.d. or the current to decreases to 37% of its initial value.

The time constant is the time taken for the p.d. or the current to decreases to 37% of its initial value.


Calculating half thickness of lead

Equipment (E) 1. GM tube, counter or rate-meter and lead plates used 2. Micrometer or vernier calliper (to measure thickness of plates).

Description (D) 1. Measure counts for a specific time and hence the count-rate for each thickness of lead 2. Vary the thickness of lead and record the count-rates 3. Plot a graph of count-rate against thickness and determine the half thickness of lead 4. Fig. 23.1 is used to determine the photon energy.

Safety (S) 1. Do not point source at person 2. Keep safe distance between you and source 3. Use tongs to handle source.

Quality of results (Q) 1. The counts are recorded over a long period of time 2. Background radiation taken into account.


Discuss ethical issus the use of PET scanners have

Doctors have to make difficult decisions about who can and cannot have a PET scan.

Some patients will miss out on PET scans because of their location / not all patients will have access to the scans.