OCR Physics A Spec (Answers)

By: Jonathan Oloyede, Ray Anuoluwa Williams and Davidson Otobo

Module 1: Charge and Current

(a) Explain that electric current is a net flow of charged particles

(b) Explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the movement of ions;

(c) Explain what is meant by conventional current and electron flow;

Conventional current is where current is considered to be the flow of positive charge from positive to negative due to the negative charge flowing in the opposite direction.

Electron Flow is the movement of electrons carrying a negative charge from negative to positive.

(d) Select and use the equation Q = It;

(e) Define the coulomb;

One coulomb is the total charge supplied by a current of one ampere in a time of one second.

(f) Describe how an ammeter may be used to measure the current in a circuit;

An ammeter is connected in series in a circuit, it's used to measure the current

(g) Recall and use the elementary charge e = 1.6 10-19 C;

(h) Describe Kirchhoffs first law and appreciate that this is a consequence of conservation of charge;

Kirchhoffs first law The sum of currents entering any junction is always equal to the sum of currents leaving the junction.

(i) State what is meant by the term mean drift velocity of charge carriers;

Mean drift velocity is the average displacement of the charge carrier along the conductor per second.

(j) Select and use the equation I = Anev;

I= Current (A), n= number density (m-3), e= elementary charge ( 1.6 10-19 C), v= drift velocity (ms-1)

(k) Describe the difference between conductors, semiconductors and insulators in terms of the number density n.

Conductors contain a huge number of free conduction electrons and have large number densities.

Insulators have very few or no conduction electrons, and their number density is near zero.

Semi-conductors have intermediate properties, e.g Silicon, with number densities almost a million time smaller than conductors but still have free conduction electrons, there small number densities allow the free electrons to travel faster than they do in conductors.

Module 2: Resistance

2.2.2 E.m.f and p.d.

(a) Define potential difference (p.d.);

Potential difference is the electrical energy transferred per unit charge when electrical energy is converted into another form of energy

(b) Select and use the equation W = VQ;

W= Energy (J), V= Potential difference (V), Q= Charge

(c) Define the volt;

1 volt = 1 joule per coulomb

(d) Describe how a voltmeter may be used to determine the p.d. across a component;

Voltmeter connected in parallel to the component being measured and the terminals must connect on either side of the component

(e) Define electromotive force (e.m.f.) of a source such as a cell or a power supply;

Electromotive force is the chemical energy transferred per unit charge when chemical energy is converted into electrical energy.

(f) Describe the difference between e.m.f. and p.d. in terms of energy transfer.

E.m.f is the transfer of energy from chemical to electrical energy per unit charge whereas Potential difference is the transfer of energy from electrical energy to another form of energy per unit charge

2.2.3 Resistance

(a) Define resistance;

Resistance = potential difference / current

(b) Select and use the equation for resistance R = V / I ;

R= Resistance ( ), V= Potential difference (V), I= Current (A)

(c) Define the ohm;

Ohm is the unit of resistance of volts per ampere

SI unit of resistance when a current of one ampere is subjected to a potential difference of one volt.

(d) State and use Ohms law;

Ohms law states that the current through a conductor is proportional to the potential difference across it, provided physical conditions, such as temperature, remain constant.

(e) Describe the IV characteristics of a resistor at constant temperature, filament lamp and light-emitting diode (LED);

Resistor at constant temperature: the current is directly proportional to the potential difference across it. It obeys Ohms law.

Filament Lamp: As the p.d across the lamp increases, its filament becomes hot. This has the effect of increasing the lamps resistance and therefore the lamp doesnt obey Ohms law as it doesnt have a constant temperature.

Light emitting Diode: They have high resistances at low p.ds but when the p.d increase above a certain point the resistance decreases and the current flowing through them increases. They operate on low p.ds.

(f) Describe an experiment to obtain the IV characteristics of a resistor at constant temperature, filament lamp and light-emitting diode (LED);

A circuit consisting of a variable number of batteries connect to a long length of thin wire, an ammeter is placed in the circuit to measure the current and a voltmeter is connected across the wire to measure the p.d.

By using a long length of thin wire the current is kept to a low value so the heating effect is negligible.

(g) Describe the uses and benefits of using light-emitting diodes (LEDs).

LEDs:

switch on instantly

Are very robust

Are very versatile

Operate on low p.ds

Have a long working life

2.2.4 Resistivity

(a) Define resistivity of a material;

define the terms

(b) Select and use the equation

Shown above

(c) Describe how the resistivity's of metals and semiconductors are affected by temperature;

The internal energy of atoms within a metal increases with temperature. The volume and thus the spacing of atoms within it will be almost unchanged. Since this means there is no change in potential energy, the internal energy must have increased due to an in kinetic energy caused by the vibrations of individual atoms. Conduction electrons then now have to pass through a more turbulent mass of atoms resulting in an increase in the resistance of the metal. So the resistivity of the metal also increases in proportion to the temperature in Kelvin.

Resistance of many metals is directly proportional to the temperature (T) in kelvin.

(d) Describe how the resistance of a pure metal wire and of a negative temperature coefficient (NTC) thermistor is affected by temperature.

Semiconductors contain impurity atoms, its resistance is highly temperature dependent, and the impurity atoms in the semiconductor help conduction significantly.

An increase in temperature reduces the resistance of this type of thermistor.

2.2.5 Power

(a) Describe power as the rate of energy transfer;

(b) Select and use power equations P = VI, P = IR, P = V / R;

(c) Explain how a fuse works as a safety device;

Fuses prevent overloading by using a thin wire to make sure that the passage of a pre-determined current through its resistance makes it hot enough to melt the wire and thus breaking the circuit.

(d) Determine the correct fuse for an electrical device;

Have a fuse that is higher but not to much higher that would damage the component e.g

If A =4.5 fuse should be around 5-9 A rating

(e) Select and use the equation W = VIt ;

where W= work done v = potential difference I = current t = time

(f) Define the kilowatt-hour (kW h) as a unit of energy;

When a 1 KW appliance is turned on for 1 hour

A kilowatt-hour (kWh) is 1000watts for 3600seconds, it is therefore 3.6MJ

(g) Calculate energy in kW h and the cost of this energy when solving problems

Cost of energy = Energy in kWh x Cost per KWh

2.3.1 Series and parallel circuits

(a) State Kirchhoffs second law and appreciate that this is a consequence of conservation of energy;

Kirchhoffs Second Law: In any closed loop in a circuit the sum of e.m.fs is equal to the sum of the p.d.s.

This is because e.m.f is the energy transfer per unit charge transferred into electricity and p.d. is the energy transferred from electrical energy. Charge cannot return back to the supply with surplus energy if it did the more times the charge travelled round the circuit the more energy would be gained.

(b) Apply Kirchhoffs first and second laws to circuits;

(c) Select and use the equation for the total resistance of two or more resistors in series;

(d) Select and use the equation for the total resistance of two or more resistors in parallel;

(e) Solve circuit problems involving series and parallel circuits with one or more sources of e.m.f.;

(f) Explain that all sources of e.m.f. have an internal resistance;

Internal resistance is the resistance of the material in which the e.m.f. source is made of and this resistance increase over the life of the source.

(g) Explain the meaning of the term terminal p.d.;

Terminal p.d. is the potential difference measured when a voltmeter is connect to each terminal of an e.m.f. source such as a battery.

(h) Select and use the equations

e.m.f. = I (R + r), and e.m.f. = V + Ir .

2.3.2 Practical circuits

(a) Draw a simple potential divider circuit;

Circuits which share potential difference

(b) Explain how a potential divider circuit can be used to produce a variable p.d. ;

The same current passes through both resistors, so the p.d. across each is proportional to their resistance

(c) Select and use the potential divider equation

(d) Describe how the resistance of a light-dependent resistor (LDR) depends on the intensity of light;

An LDR has a large resistance when there is light, as the light intensity falling on it increases, the resistance of the LDR decreases.

(e) Describe and explain the use of thermistors and light-dependent resistors in potential divider circuits;

An LDR can be used in a potential divider control circuit to switch lights on as it gets dark.

A thermistor could be used to control the output from a heater, for example when the thermistor has an output of 6.76V the heater switches on.

(f) Describe the advantages of using data-loggers to monitor physical changes

Data loggers can produce a continuous record of physical changes and this data can easily be used to form graphs.

2.4.1 Wave motion

(a) describe and distinguish between progressive longitudinal and transverse waves;

Traverse wave: a wave where the oscillations are perpendicular to the direction of motion

Longitudinal wave: a wave where the oscillations are parallel to the direction of motion

(b) define and use the terms displacement, amplitude, wavelength, period, phase difference, frequency and speed of a wave;

displacement: displacement of a wave from its rest position

amplitude: maximum displacement of a wave

wavelength: Distance between 1 point on a wave to a similar point on the next.

period: time taken for one complete wave cycle to occur

Phase difference: How much 1 wave lags behind another.

frequency: number of wave cycles per unit time.

(c) derive from the definitions of speed, frequency and wavelength, the wave equation v = f;

s = d/t v = /t f = 1/t v = f

(d) select and use the wave equation v = f;

(e) explain what is meant by reflection, refraction and diffraction of waves such as sound and light.

reflection: when waves rebound from a barrier, changing direction but remain in the same medium.

refraction: When waves change direction when changing medium because of difference in wave speed.

diffraction: When a wave spreads out after passing around an obstacle or moving through a gap.

2.4.2 Electromagnetic waves

(a) State typical values for the wavelengths of the different regions of the electromagnetic spectrum from radio waves to -rays;

Wavelength

Radio Waves: 10-1 104 m

Microwaves: 10-4 10-1 m

Infrared: 7.4x10-7 10-3 m

Visible light: 3.7x10-7 7.4x10-7m

Ultraviolet: 10-9 3.7x10-7m

X-rays: 10-12 10-7m

Gamma rays: 10-16 10-9m

(b) State that all electromagnetic waves travel at the same speed in a vacuum;

Speed of light = 3x108

(c) describe differences and similarities between different regions of the electromagnetic spectrum;

Similarities:

All are made up from oscillating electrical and magnetic waves at right angles to each other.

Can all travel through a vacuum (at 3x10^8)

Are all transverse

Differences

Frequencies wavelengths and other properties

(d) Describe some of the practical uses of electromagnetic waves;

Radio: Communication and television

Microwave: Satellite, Microwave ovens, radar

Infrared: remote control, heaters, night vision

Visible light: Sight

Ultraviolet: tanning, Counterfeit/ 419 detection

X-ray: CT scans /diagnosis

Gamma ray: Cancer diagnostics and treatment

(e) Describe the characteristics and dangers of UV-A, UV-B and UV-C radiations and explain the role of sunscreen (HSW 6a);

UV a = Tanning

UV B = causes sunburn and skin cancer sunscreen prevents this problem by reflecting the UV light away from the skin

UV C = most dangerous but is filtered out by our atmosphere.

(f) Explain what is meant by plane polarised waves and understand the polarisation of electromagnetic waves;

Plane polarised wave: a transverse wave that only oscillates in 1 plane

(g) Explain that polarisation is a phenomenon associated with transverse waves only;

As longitudinal waves only oscillate parallel to the direction of motion so cannot be polarised

(h) State that light is partially polarised on reflection;

(i) Recall and apply Maluss law for transmitted intensity of light from a polarising filter.

I = Imax cos(-)

2.4.3 Interference

(a) State and use the principle of superposition of waves;

When 2 or more waves pass through each other the resultant wave is the sum of the displacements of the waves

(b) Apply graphical methods to illustrate the principle of superposition;

(c) Explain the terms interference, coherence, path difference and phase difference;

Coherence: two sources are coherent if they have the same wavelength and frequency and a constant phase difference

Path difference: the amount by which the path travelled by one wave is longer than

Interference: The resultant displacement of waves

(d) State what is meant by constructive interference and destructive interference;

Constructive: the resultant the crests of 2 waves at a single point in time

Destructive: The resultant of a crest and trough of 2 waves at a point in time giving 0 resultant displacement

(e) Describe experiments that demonstrate two-source interference in terms of path difference and phase difference;

Young's double slit experiment:

Two waves of same source in phase are shone through 2 slits a distance apart the resultant pattern is seen Dm away from the slits where x is the distance between similar fringes. If the path difference is n where n is an integer constructive interference occurs.

However if the path difference is (n+ 1/2) then destructive interference occurs.

Phase (n radians = constructive and n+ 1/2 radians = destructive)

Determine the wavelength of light from different LEDs using a diffraction grating and the equation = ax/D

(f) describe constructive interference and destructive interference in terms of path difference and phase difference;

if the path difference is n where n is an integer constructive interference occurs.

However if the path difference is (n+ 1/2) then destructive interference occurs.

Phase ( n radians = constructive and n+ 1/2 radians = destructive)

(g) use the relationships

intensity = power/cross-sectional area

intensity amplitude2;

For instance if I(1) was 2Wm^-2 , I(2) was 4Wm^-2 and a(1) was 2m then (4/2)^2 = k =4 then a(2) = 4*2 = 8

(h) describe the Young double-slit experiment and explain how it is a classical confirmation of the wave-nature of light;Young's Double slit experiment:

two waves of same source in phase are shone through 2 slits a distance apart the resultant pattern is seen Dm away from the slits where x is the distance between similar fringes. This shows that light is a wave because the waves diffraction and inteference are properties of waves

(i) Select and use the equation

for electromagnetic waves;

a = slit spacing x= distance between similar fringes = wave length and D = distance between slits and screen

(j) describe an experiment to determine the wavelength of monochromatic light using a laser and a double slit;

a laser light is shone through a double slit of known spacing, after the light passes through the double slit the 2 waves show an interference pattern Dm away from the slits on a screen

using a ruler measure the distance x between the 2 nearest similar fringes. and use the equation = ax/D to find the wavelength

(k) describe the use of a diffraction grating to determine the wavelength of light (the structure and use of a spectrometer are not required); repeat young's double slit experiment with more slits and use equation (dsin)/n = to find wavelength

(l) select and use the equation dsin = n;

d= spaces between slits theta = angle between 0 and 1st order n = order of spectrum

(m) explain the advantages of using multiple slits in an experiment to find the wavelength of light.

The fringe pattern is shaper with more slits so it is easier to measure the distances between fringes.

2.4.4 Stationary waves

(a) explain the formation of stationary (standing) waves using graphical methods;

A standing wave is the superposition of two progressive waves with the same wavelength and frequency, moving in opposite directions.

Formed when a progressive wave is reflected back at a boundary:

(b) Describe the similarities and differences between progressive and stationary waves;

Similarities: same wavelength, frequency

Differences: in anti-phase

(c) define the terms nodes and antinodes;

node: where there is no interference between the stationary wave.

antinode: where there is maximum interference in a stationary wave.

(d) describe experiments to demonstrate stationary waves using microwaves, stretched strings

and air columns;

Microwave: is transmitted from a transmitter to a metal plate the wave is reflected back towards the transmitter and a probe/receiver is moved between the transmitter and reflective plate to detect nodes and antinodes (the probe is connected to a receiver.

Air Column: If a source of sound is place at the open end of the tube there will be some frequencies for which resonance occurs and a standing wave is set up. (resonant frequencies are when a half number of wavelengths fit into the tube)

2) if the instrument has a closed end the node will form there you get the lowest resonant frequency at a quarter wavelength

3) antinodes form at open ends of the pipe.

sound waves are used in instruments which are longitudinal.

String: oscillator and string

1)resonant frequencies are a half number of wavelengths that fit onto string

2) shortest resonant frequencies is half a wavelength and the distance between a node and antinode is wavelength /4

(e) determine the standing wave patterns for stretched string and air columns in closed and open pipes; string: strings: standing waves can occur at whole numbers of half wavelengths in strings

open tube: standings waves can occur at whole number of half wavelengths

Closed tube: standing waves lowest can occur at l = wavelength/4

(f) use the equation:

separation between adjacent nodes (or antinodes) = /2;

also distance between adjacent node and antinode is / 4

(g) define and use the terms fundamental mode of vibration and harmonics;

Harmonics: Whole number multiples of the fundamental frequency of a stationary wave

Fundamental frequency: lowest frequency of in harmonic series where a stationary wave is formed

(h) determine the speed of sound in air from measurements on stationary waves in a pipe closed at one end.

1)place a hollow tube in a measuring cylinder full of water

2) use a tuning fork of known frequency to tap the hollow tube.

3)hold tuning fork above tube. the sound wave go down the tube and are reflected at the surface of the water to create a node

4)move the tube up and down and repeat step 1-3 until you find the shortest distance at which the fork resonates at.

5) just like any other close tube this distance = /4

6) the antinode forms slightly above the tube so you need to add a constant called an error correction to the distance from water to the top of the tube.

7) you now have and f so use v = f to find the speed of sound :)

Module 3: Quantum Physics

2.5.1 Energy of a photon

(a) describe the particulate nature (photon model) of electromagnetic radiation

The energy of a photon is proportional to the frequency of the radiation.

(b)state that a photon is a quantum of energy of electromagnetic radiation

(c) select and use the equations for the energy of a photon

E = Energy of a photon

h= Planck constant - 6.63 * 10-34

c= speed of light = 3 x108

= wavelength

(d) define and use the electronvolt (eV) as a unit of energy

Electronvolts (eV) are used to measure very small amount of energy.

One Electronvolt is the energy of charge in an electron when it moves through a potential difference of 1 volt.

W = V Q

1eV = 1JC-1 1.60 10-19C = 1.60 10-19 J

(e) use the transfer equation for electrons and other charged particles;

(f) describe an experiment using LEDs to estimate the Planck constant h using the equation (no knowledge of semiconductor theory is expected).

LEDs come in different colours so can be used to determine Planck constant.

Each Electron that passes through the LED loses a fixed amount of energy. Energy lost by electron = charge on electron p.d. across LED = eV so

Or eV = hf.

1. A variable p.d. is connected to the LED

2. The p.d. is increased from 0 until it begins to glow (this p.d. is recorded).

3. The experiment is repeated using different coloured LEDs (which emit light with different frequencies).

4. An Energy lost by an electron (J) - frequency of light emission (Hz) graph is plotted

5. The gradient is the Planck constant (h)

2.5.2 The photoelectric effect

(a) describe and explain the phenomenon of the photoelectric effect;

The clean metal surface emits electrons when Ultraviolet light is shone on it.

The electrons are called photoelectrons.

The gold leaf electroscope measures the charge/change in charge.

1. When the zinc is placed on the cap of the electroscope it becomes negatively charged.

2. The stem of the electroscope also become negatively charged, making it repel from the gold leaf

3. Bringing the UV light closer, the gold leaf starts to fall back to the stem which shows that the UV causes emission of electrons from the zinc.

4. Electrometers can be used to measure the kinetic energy emitted from the electrons.

The energy of photoelectrons

The photoelectrons in the UV radiation have negative charge so are repelled from the negative terminal and attracted to the positive terminal. If the photoelectrons have enough kinetic energy they reach the negative plate and a current is recorded. If the p.d. in the negative plate is high it will be repelled , simply.

When the p.d. is positive, the photoelectrons are collected and the current is independent of the p.d. When the p.d. is negative some photoelectrons don't reach the terminal as they don't have enough kinetic energy and the photocurrent falls, when the negative charge is really high no photoelectron reaches the negative terminal.

Photoelectron current - p.d. graph

Stopping potential - Voltage required to stop the outward movement of electrons emitted by photoelectric, enables the maximum energy of photoelectrons to be found.

(b) explain that the photoelectric effect provides evidence for a particulate nature of electromagnetic radiation while phenomena such as interference and diffraction provide evidence for a wave nature;

In the photoelectric effect:

The intensity had no effect on the maximum energy (but the number of photoelectrons increase), which contradicts light is a wave as in waves intensity if proportional to kinetic energy.

Below a certain frequency the photoelectric effect doesn't occur, in waves it should occur regardless of light

There is no delay time between activation and emission which contradicts wave behaviour.

UV shows particle behaviour in the photoelectric effect.

(c) define and use the terms work function and threshold frequency;

Threshold frequency - the lowest frequency required to cause emission of electrons in a metal surface, most metals emit electrons at frequencies of UV.

work function - energy required to release electron

(d) state that energy is conserved when a photon interacts with an electron;

(e) select, explain and use Einsteins photoelectric equation

Einstein's equation is derived from the principle of conservation of energy. He believed that the energy in the UV (hf) hitting the metal plate release electrons from its atoms and any remaining energy is converted to kinetic energy.

So:

photon energy = energy to release electron (work function) + kinetic energy of electron

Some kinetic energy gained by the electrons are lost through collisions with other electrons, some electrons don't collide and end up with maximum kinetic energy.

(1/2 mv2) max= hf - graph

(f) explain why the maximum kinetic energy of the electrons is independent of intensity and why the photoelectric current in a photocell circuit is proportional to intensity of the incident radiation.

The intensity doesnt effect the kinetic energy as it only increases the number of photoelectrons however the photoelectrons still have the same amount of kinetic energy in lower intensities.

The photoelectric current in a photocell is proportional to intensity of the incident radiation as there's more photons so more electrons are emitted which increases the current.

2.5.3 Wave-particle duality

(a) explain electron diffraction as evidence for the wave nature of particles like electrons.

Particles can be diffracted like waves, particle diffraction is called De Brogile Diffraction.

Electrons are accelerated from an electron gun through a vacuum towards polycrystaline graphite.

(b) explain that electrons travelling through polycrystaline graphite will be diffracted by the atoms and the spacing between the atoms.

Polycrystalline graphite is used instead of a diffraction grating as the wave lengths of electrons are much smaller than light so the atomic spacing in graphite is used. The atomic spacing in graphite is not all lined in the same direction (like in a diffraction grating) so a circular pattern is produced from diffraction rather than lines when diffracting light.

(c)select and apply the de Broglie equation

(d) explain that the diffraction of electrons by matter can be used to determine the arrangement of atoms and size of nuclei.

De Brogile diffraction shows atomic spacing, it can be also used to determine the structure of matter. The speed of the electrons could be increased which decreases the wavelength so smaller values of d could be taken. High speed electrons can determine the arrangement of atoms in crystalline structures and measure the diameter from the nucleus.

2.5.4 Energy levels in atoms

(a) explain how spectral lines are evidence for the existence of discrete energy levels in isolated atoms, ie in a gas discharge lamp.

There are dark lines in the Sun's spectrum called spectra lines, they are missing frequencies. Discharge lamps contain hot gases like hydrogen and also have spectra lines. Spectra lines show absorption and emission of photons. Each element has its unique line spectra.

(b) describe the origin of emission and absorption line spectra.

Emission

Niels Bohr believed that in atom structures there were specific energy levels. As the electrons moved closer to the nucleus they emitted radiation. Electrons moved from higher energy levels to lower ones, returning the ground state.

and

E1= Energy level electron has left.

E2= Energy level electron moves to

Emission spectra from hot solids

Solid's don't have spectra lines and there is less emission than the sun at the violet end of the spectrum (as the violet end correlates to energy and the sun is quite hotter and has more energy).

Sun Intensity - Wavelength graph

Sunlight has maximum intensity in the green part of the spectrum and radiates UV.

Filament lamp Intensity - Wavelength graph

Solids maximum intensity occurs towards the red end of the spectrum and does not radiate UV.

Hot solids produce continuous spectra while gases produce line spectra. As atoms are close together in solids and they interfere changing the energy levels. This creates energy bands so there's a variety of energy which means many different wavelengths and a continuous range of colour.

Absorption spectra

In the sun there are many elements. Some elements absorb some wavelengths of light which makes the electrons move from a lower energy level to a higher one. The opposite of emission.

Absorption spectra from stars

The absorption spectrum of a star contains information about the elements in the star (including the sun). For stars which produce wavelengths longer than wavelengths observed on earth are red-shifted. Edwin Hubble recognised some stars had spectra slightly out of balance and concluded stars are moving away. So the universe is expanding this supports the 'Big Bang Theory' matter and energy were once concentrated in a dense state and expanded for billions of years.