Module 4 – Electrons, waves and photons Flashcards
What is the definition of current?
Electric current, I, is defined as the rate of flow of charge. The SI base unit for current is Amperes (A).
I = Q/t
Describe how charge in metals is carried.
The current in metals is carried by electrons. In a metal, there is a lattice of positive ions, surrounded by free electrons. The positive metal ions are fixed in place, but the electrons can move around, and so when one side of the metal is made positive, and the other side is made negative, the electrons will be attracted to the positive side, and move through the metal as electric current.
Describe how charge in electrolytes (liquids) is carried.
Some liquids can conduct a charge. These conducting liquids are called electrolytes, and are commonly ionic solutions. This means they contain positive and negative ions. An example of this is water with salt, NaCl, dissolved in it. The salt splits in to Na+ cations and Cl- anions. When a pair of electrodes (the anode is the positive electrode and the cathode is the negative electrode) are placed in the solution, the cations will be attracted to the cathode, and the anions will be attracted to the anode. This produces an electrical current.
What is the difference between conventional current and electron flow?
It is the rate of flow of charge from the positive to the negative terminal, and this is how all electric currents are treated, regardless of the direction the charge carriers are moving in. In metals, the electrons flow from negative to positive, so the electron flow is in the opposite direction to the conventional current.
What does Kirchhoff’s first law state?
Kirchhoff’s first law states for any point in an electrical circuit, the sum of the currents in to that point is equal to the sum of the currents coming out of that point. This law is a consequence of the conservation of charge. Charge is a fundamental physical property, which cannot be created or destroyed, so it must be conserved.
What is the number density?
The number density (n) is the number of free electrons per unit volume. The higher the number density, the greater the number of free electrons per m³, so the better electrical conductor.
Explain how the formula for current could be derived into I = Anev
What does the effect of changing cross-sectional area have on the wire?
The narrower the wire, the greater the drift velocity must be in order for the current to be the same.
What is the circuit symbol for a thermistor?
What is the difference between Potential difference and Electromotive force?
Potential difference, V, is used to measure the work done by charge carriers, whereas Electromotive force, ε, is used to measure the work done to charge carriers.
Explain how an electron gun works.
A small metal filament, which acts as a cathode, is heated by passing a potential difference through it. Some of the electrons in the metal gain enough kinetic energy to escape the metal, in a process known as thermionic emission. The circuit is in a vacuum tube, with a high voltage, between the filament and the anode, so the freed electrons are accelerated towards the anode. If the anode has a small hole in it, a beam of electrons can pass through at a specific kinetic energy.
What does Ohm’s law state?
Ohm’s law states for a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the potential difference across it. This is true for some, but not all components. Where Ohm’s law is true, components are considered ohmic, and their i-v characteristic graph will have a constant linear gradient.
What is the resistivity of a metal?
Resistivity is the measure of how much a particular material opposes electron flow. It is measured in Ohm-meters (Ωm).
What does Kirchoff’s second law state?
Kirchhoff’s second law states in any circuit the sum of the electromotive force is equal to the sum of the potential difference in a closed loop. This is a result of the conservation of energy.
What formula links emf, internal resistance, and resistance?
With e.m.f. = 𝜀, internal resistance = r, and Resistance = R,
𝜺 = 𝑰(𝑹 + 𝒓)
where IR = V, the terminal p.d, and Ir is equal to the lost volts.
What is a Progressive wave?
A progressive wave is an oscillation that travels through matter (or in some cases a vacuum), transferring energy from one place to another, but not transferring any matter.
What is meant by ‘Displacement’ in waves?
The distance from the equilibrium position in a particular direction.
What is meant by ‘Period’ in waves?
The time taken for a full oscillation of one wavelength to pass a given point.
How can Phase difference be calculated?
If particles oscillate out of phase, then the equation 𝑥/𝜆 × 2𝜋, where x is the separation in wavelengths between the two particles, can be used to calculate the phase difference.
When does reflection occur in waves?
Reflection occurs when a wave changes direction at a boundary between two media, remaining in the original medium.
When does refraction occur in waves?
Refraction occurs when a wave changes direction as it changes speed, when it enters a new medium. In the new medium, the frequency of the refracted waves remains constant, but the speed of the wave changes.
When does Diffraction occur in waves?
Diffraction is the spreading out of a wave front as it passes through a gap. The wavelength and frequency of the wave are not altered. Maximum diffraction will occur when the gap the wave passes through is the same size as the wavelength of the incident wave.
When does Polarisation occur in waves?
It occurs when the oscillation of a wave is restricted to one place only – this type of wave is said to be plane polarised. Longitudinal waves cannot experience polarization, as the direction of energy transfer is already in one plane only, whereas in transverse waves, the oscillations occur in many planes, at right angles to the direction of travel.
What is meant by ‘Intensity’ in waves?
The intensity of a progressive wave is defined as the radiant power passing at right angles through a surface per unit area, P/A. It has units of watts per metre squared, wm⁻².
The intensity of the light is therefore inversely proportional to the square of the radius of it’s sphere, and the amplitude of the wave, Intensity ∝ Amplitude²
List all wave types of the EM spectrum, starting from largest wavelength to smallest.
Radio - 10³ m
Microwave - 10⁻² m
Infrared - 10⁻⁵ m
Visible - 0.5 x 10⁻⁶ m
Ultraviolet - 10⁻⁸ m
X-ray - 10⁻¹⁰ m
Gamma ray - 10⁻¹² m
Describe how the refractive index, n, can be used to determine the angle of refraction after entering a new medium..
When does Total internal reflection occur?
Total internal reflection occurs only when two conditions are met. Firstly, the light must be travelling from a material with a higher refractive index, to a material of lower refractive index. Secondly, the angle of incidence of the ray to the normal must be above the critical angle.
The formula sinC = 1/n can be used, where C is the critical angle to the normal, and n is the refractive index of the material the ray is travelling in to. This formula is only true for refraction occurring where the original material is air.
What does the The principle of superposition state?
The principle of superposition states when two waves meet at a point, the resultant displacement of the wave at that point is equal to the sum of the displacements of the individual waves.
Explain how two coherent waves cancel each other due to interference.
Two waves are coherent when they are emitted with a constant and unchanging phase difference. When two coherent waves interfere, the maximum resultant displacement occurs when the phase difference is an even multiple of π, so the two crests of the wave combine. The minimum resultant displacement occurs when the phase difference is an odd multiple of π, so one crest and one trough act to cancel each other out.
Describe how the double-slit experiment can be used to determine the wavelength of monochromatic light.
A laser which produces monochromatic light is placed behind a sheet with two small slits in it, a distance ‘a’ apart. The two coherent waves produced by the slits overlap and superpose, creating alternating bright (maxima) and dark (minima) fringes on a screen. The distance between two adjacent maxima is ‘x’, and the distance between the double slits and the screen is ‘d’.
The equation 𝜆 = ax/d can then be used to determine the wavelength of the light used.
Explain Stationary waves.
Stationary waves are formed when two progressive waves with the same frequency (and ideally the same amplitude), travelling in opposite directions, superpose. Nodes are points which always have zero amplitude and zero frequency, and antinodes are points which always have maximum displacement. Two adjacent nodes are half a wavelength apart. The phase difference between two points on the wave is given by 180°n, where n is the number of nodes between the two points. Stationary waves do not transfer energy, they store it.
Describe how stationary waves can be produced with sound in an air filled tube.
Stationary waves can be produced with sound in an air filled tube. A tuning fork is used to produce a loud sound, and is then held over the end of the tube. The length of the tube can be adjusted (e.g. by placing one end in water) until a stationary wave is produced, the structure of which will vary depending on the air column.
What are ‘photons’?
Photons are discrete energy quanta (‘packets’) produced when electromagnetic radiation interacts with matter.
What is 1 electronvolt in joules?
1.60×10⁻¹⁹ joules
Explain how LEDs can be used to determine the Planck constant experimentally.
- An LED will only allow current to pass after a minimum voltage has been put across it.
- At this voltage all the electrons will have the same energy as a photon emitted by the LED (which the frequency of will be known)
- Finding the threshold voltage by seeing when the current flows in the circuit can then be used to find h from h = E/f.
Explain how multiple LEDs can be used to improve the accuracy of a method used to determine the Planck constant experimentally.
- To improve the accuracy of this estimate, the experiment can be repeated with a variety of different coloured LEDs, which all emit different wavelengths of light.
- The values and wavelength and threshold p.d. for each can be recorded, and a graph drawn of V against 1/λ.
- The gradient of this graph will be equal to hc/e, from which h can be calculated.
What is the the photoelectric effect?
When electromagnetic radiation is shone on to a metal, photoelectrons are released from the surface of the metal.
What is the de Broglie equation?
All matter can exhibit both wave and particle properties (wave-particle duality), and that the wavelength associated with a particle is inversely proportional it its momentum, 𝑝.
This relationship is given by the de Broglie equation 𝜆 = h/𝑝 = h/mv
How does the photoelectric effect prove light is not a wave?
If light was a wave, then the energy of the electrons would increase proportional to intensity, however the energy of electrons depends on frequency, with no electrons released below the threshold frequency.
What is the work function?
The minimum amount of energy an electron is required to have to leave the surface of a metal.
What affects the rate of electron emission with the photoelectric effect?
Intensity is proportional to rate of electron emission (provided the light is above the threshold frequency).
