Physics UNIT 4 Study Design

AOS 1: Electric Power  Describe the shape & direction of fields around magnets, wires, coils & solenoids.  Quantify magnetic forces on a parallel & perpendicular wire: F = nBIℓ.  Describe operation of DC motors.  Quantify the magnetic flux through a parallel & perpendicular loop: =BA.  Explain, quantify & find the direction of induced AC Voltages: =–n/t.  Describe AC Generators including use of commutators & slip rings.  Compare sinusoidal AC generation for; frequency, peak-to-peak & peak voltage.  Quantify rms as the AC supply which delivers the same power as a DC supply.  Compare & contrast DC motors, AC Generators & Alternators.  Describe & quantify ideal transformer action using turn ratio: VP/VS=NP/NS=IS/IP.  Describe & quantify power supply (P=IV) & transmission losses (V=IR, P=I2R).  Explain the use of transformers in an electricity transmission system.  Use safe practices when working with electricity & electrical measurement.

AOS 2: Interactions of Light and Matter  Explain incoherent light from broad spectrum sources: sun, light bulbs & candles.  Describe Young’s double slit experiment as evidence of wave-like nature of light. o Constructive/Destructive interference in terms of path differences. o Qualitative effects of wavelength on inference patterns.  Describe diffraction through a gap or passed an obstacle: /w sin= pd/w≈ W/D  Describe the photoelectric effect as evidence for the particle-like nature of light. o Kinetic energy of photoelectrons in joules & electronvolts: EKmax=hf – W.

 Describe diffraction of electrons as wave-like nature of matter: DeBroglie=h/p.  Compare the momentum & wavelength of photons and electrons: p= h/.  Describe absorption/emission spectra using quantised energy levels: E= hf.  Describe quantised energy levels modelled as electron standing waves.  Use safe practices working with light sources and lasers and related equipment.

DS 3: Sound Recording and Reproduction  Describe sound as energy transmission via longitudinal pressure waves.  Quantify the relationship between frequency, wavelength & speed of sound: v=f.  Describe and quantify differences in sound intensity (Wm-2) & sound level (dB). 2 2 2  Quantify sound intensity differences at different distances: I 1/r ; I1r1 =I2r2 .  Describe resonance as superposition of a travelling sound wave & its reflection.  Quantify for strings & tubes, the fundamental (1st) & subsequent harmonics.  Describe electric & electromagnetic sound recording/reproduction devices; o Electret-condenser, crystal, dynamic & velocity microphones. o Dynamic loudspeakers.  Describe the effects of baffles & enclosures for loudspeakers.  Describe frequency response curves for loudness (phon) of systems & hearing.  Compare fidelity of systems for purpose, frequency response & construction.  Describe the directional spread of frequencies for gap/obstacle widths: sin /w.  Use safe practices when working with sound sources and equipment. AOS 1 (40%): Electric Power  Describe the shape & direction of fields around magnets, wires, coils & solenoids.

- Field lines travel from north to south around a magnet. - Field lines never cross.

- Direction of force at a point is at a tangent to the field lines. - Concentration of filed lines indicates field strength.

- Right-Hand Grip Rule: Thumb=I, Fingers=B (To find directions of Current/Field). - I = Conventional current inducing a Magnetic Field.

- B = Magnetic Field induced by the current (N-to-S = +ve, S-to-N = –ve). - For coils/solenoids: Clockwise I = South Pole (in-to-page), Anticlockwise I = North Pole (out-of-page).  Quantify magnetic forces on a parallel & perpendicular wire: F = BIℓ.

- F = Force on electric charge carrier (current or particles). - B = Magnetic field strength affecting a charge carrier.

- I = Conventional current of a charge carrier (may need to multiply by the number of current carriers). - ℓ = Length of a charge carrier perpendicular to and inside the field.

- Right-Hand Slap Rule: Thumb=I, Fingers=B, Palm = F (To find directions of Current/Field/Force).  Describe operation of DC motors.

- A current through a coil in a magnetic field will rotate due to equal but opposite forces on each of the sides (“right-hand slap”). A commutator changes the direction of the current relative to the wire to keep the current moving in the same direction relative to the magnetic field, allowing it to continue rotating.  Quantify the magnetic flux through a parallel & perpendicular loop: =BA.

-  = Magnetic flux (Strength of magnetic through a cross-sectional area) - B = Magnetic filed strength passing through a coil (N-to-S = +ve, S-to-N = –ve).

- A = Cross-sectional area inside a coil perpendicular to the magnetic field.  Explain, quantify & find the direction of induced AC Voltages: =–n/t.

- Faraday’s Law: av=/t.

-  = induced emf (‘electromagnetic force’ = generated voltage source). - av= average emf (usually measured for a ¼ turn of a generator).

- n = number of turns of the generating coil.

- = change in flux = F – I (final minus initial flux).

- t = the time interval of the change in flux= period = 1/frequency: T=1/f. - Lenz’s Law: the induced current is in the direction that induces a field in a direction that is opposite to the direction of the change in flux that induced the current in accordance with Conservation of Energy.  Describe AC Generators including use of commutators & slip rings.

- Rotation of a coil through a magnetic field will induce a current in the coil.

- Slip rings allow the current to alternate directions (AC generation) as the direction of the change in flux changes every half rotation.

- A commutator allows the current to maintain its direction (DC generation) by changing which end is connected to the terminals every half rotation.  Compare sinusoidal AC generation for; frequency, peak-to-peak & peak voltage.

- If either the frequency of rotation, the maximum flux (strength of magnetic field or area of generating coil), or number of turns in the coil changes, use: =–n/t.

- Use a ¼ turn of the coil to estimate the average emf by calculating the maximum n/t, times 4.  Quantify rms as the AC supply which delivers the same power as a DC supply. 2 - Vp = peak voltage, Vp-p = peak-to-peak voltage, Vrms = rms voltage: Vp= ½Vp-p = Vrms.

- An AC power supply will deliver the same power as a DC power supply if Vrms=VDC.  Compare & contrast DC motors, AC Generators & Alternators.

- DC Motors transform electrical energy into rotational kinetic energy – uses a commutator.

- AC Generators transform rotational kinetic energy into electrical energy – uses slip rings. - Alternators are Generators with a stationary generating coil and a rotating electromagnet.

 Describe & quantify ideal transformer action using turn ratio: VP/VS=nP/nS=IS/IP.

- A changing current applied to a primary coil around an iron ring will induce a changing magnetic field in a secondary coil around the same iron ring inducing a current through the secondary coil.

- Assuming that no power is lost in an ideal transformer: VP/nP = –/t= VS/nS

- In real transformers, insulating layers inside the iron reduce the power lost to Eddy currents.  Describe & quantify power supply (P=IV) & transmission losses (V=IR, P=I2R).

- Power, P, is delivered by a generator with a voltage, V, across the generator and a current, I, through it. - Potential difference, V, is lost in transmission due the current, I, through the resistance, R, of the wire. 2 - Power, P, is lost in transmission, with the square of the current, I , through the resistance, R, of the wire.  Explain the use of transformers in an electricity transmission system.

- A step-up transformer (nP < nS) near a generator reduces the current through the wires & so power lost.

- A step-down transformer (nP > nS) near a load reduces the voltage for safe use of the transmitted power. AOS 2 (30%): Interactions of Light and Matter  Explain incoherent light from broad spectrum sources: sun, light bulbs & candles.

- Light is electromagnetic energy: perpendicular electric & magnetic fields produced by electron motion. - At high temperatures collisions between electrons increase and release a range of ‘random’ light.  Describe Young’s double slit experiment as evidence of wave-like nature of light.

- A wave is a continuous disturbance that transfers energy without a net transfer of matter: v= f.

- Each point on a wavefront is a source of an expanding wavelet & the new wavefront is the envelope of all wavelets. Therefore, waves diffract around an obstacle as the edge wavelets expand out around it. - Passing light through two slits produces the dark and bright bands of an interference pattern. 8 -1 - v = velocity of wave; v of light in a vacuum = c = 3.0 ×10 ms . -  = (‘lambda’) wavelength [m] = distance between corresponding successive points of a periodic wave. -1 - f = frequency [Hz] = cycles per second [s ] = 1/T, T = period [s] = time for a single cycle of a wave. - The phase of a wave is the degree of misalignment between corresponding points on another wave. o Constructive/Destructive interference in terms of path differences.

- Constructive interference: amplitude doubling when two waves meet in-phase: pdC= nC, th st nd - nC= 0, 1, 2…, order of bright bands from the central bright band (0 , 1 ,2 , etc..).

- Destructive interference: amplitude cancelling when two waves meet out-of-phase: pdD= (nD- ½), st nd rd - nD= 1, 2, 3…, order of dark bands from the central bright band (1 ,2 , 3 , etc..). - pd = path difference= the difference in the distance travelled to a point by two equal wavelength waves. o Qualitative effects of wavelength on inference patterns.

- As the wavelength increases the constructive & destructive path differences increase, so bands widen.

- For diffraction to be noticeable, the ratio of wavelength, , to slit-width, w, must be between 1 and 0.1.  Describe diffraction through a gap or passed an obstacle: sin=pd/w≈W/D /w.

- Light diffracts through gaps & around obstacles in ways that are similar to the double slit experiment.

- The extent of diffraction, sin, is dependant on the wavelength-to-width ratio: /w.  Describe the photoelectric effect as evidence for the particle-like nature of light.

- A particle transfers energy in discrete amounts as it moves through space & collides with objects.

- The photoelectric effect involves the emission of electrons (photoelectrons) due to incident light. - A photon is a particle-like ‘packet’ of electromagnet (light) energy proportional to frequency: EP=hf.

- EP= energy associated with a single photon. -34 - h = Plank’s constant = 6.6 ×10 Js = proportionality between photon energy & photon frequency.

- A wave model would predict that the energy is proportional to the amplitude of light, not its frequency! - Electrons are only emitted if a photon carries enough energy to overcome the atom’s attraction.

- A photoelectron gains kinetic energy, EK, from a photon, EP, less the ionisation energy, EI: EK= EP – EI. o Kinetic energy of photoelectrons in joules & electronvolts: EKmax=hf – W.

- Photoelectric experiment: a photocell in series with an ammeter connected to a variable voltage source. - A photocell is an evacuated tube containing separated metal plates connected to terminals.

- If light of sufficient energy is incident on one of the plates photoelectrons are emitted. -19 - Electronvolt: 1 eV=1.6 x10 J= work done passing an electron through a potential difference of 1 volt.

- As the voltage across the photocell is made more positive the photoelectric current increases. - Beyond a critical voltage, Vmax, the photoelectric current reaches a maximum value, Imax .

- As the voltage across the photocell is made more negative the photoelectric current decreases. - Beyond a critical voltage, the stoping voltage, V0, the photoelectric current becomes zero, I = 0.

- Increasing the intensity of light increases the maximum photoelectric current. - Increasing the frequency of light increases the magnitude of the stopping voltage, V0..

- V0 = EKmax, because the negative voltage just pushes emitted electrons back to the original metal plate. - Plotting the stoping voltage against the frequency results in a straight line: y = mx + c::EKmax= hf – W.

- y = V0 [V] = the stopping voltage for different frequencies of light. -15 -34 - x = f [Hz] = frequency of incident light; m = h [eVs] = Plank’s constant= 4.1 ×10 eVs = 6.6×10 Js.

- c = W = the work function of the metal = the minimum ionisation energy of a particular type of atom. - –c/m (x-intercept) = –W/h = f0 [Hz] = the critical frequency below which no electrons are emitted.

 Describe diffraction of electrons as wave-like nature of matter: De Broglie=h/p.

- Photons have a momentum, p = h/: and therefore interact with forces: F= p/t.

- Electrons diffract with a ‘De Broglie wavelength’ associated with their momentum: DeBroglie=h/mev.  Compare the momentum & wavelength of photons and electrons: p= h/.

- Photons and electrons with similar wavelengths show similar patterns of diffraction.  Describe absorption/emission spectra using quantised energy levels: E= hf.

- Electrons can only move up or down discrete energy levels, E, when absorbing or emitting photons.

- Emitted photons show up as coloured lines in the spectrum of light at points where f = E/h.

- Absorbed photons show up as dark lines missing from the spectrum with a dark band below f0.  Describe quantised energy levels modelled as electron standing waves.

- An Electron is a circular wave that must meet itself in-phase or else destructively interfere with itself. - Electron energy levels have a circumference equal to an integer multiple of the electron’s wavelength. DS 3 (30%): Sound Recording and Reproduction  Describe sound as energy transmission via longitudinal pressure waves.

- Sound is the mechanical vibration of air molecules under compression and rarefaction (expansion).

- Sound is therefore a longitudinal wave with the disturbances parallel to the direction of propagation. - Sound travels as a wave with peaks of maximum pressure and troughs of minimum pressure.

- Pressure is force per area, & work is done (energy is transferred) by a force in the direction of motion. - Points of maximum pressure variation are points of minimum particle motion & vice versa.  Quantify the relationship between frequency, wavelength & speed of sound: v=f.

- Sound waves can be modelled by the wave equation: v=f, v= speed,  =wavelength, f= frequency.

- The frequency depends on the source of vibration; the speed & wavelength depend on the medium.  Describe and quantify differences in sound intensity (Wm-2) & sound level (dB). -2 -2 - Sound intensity, I [Wm ], is the power, P [W], delivered to a cross-sectional area, A [m ]: I= P/A. - Sound intensity level is a log rhythmic measure of sound intensity scaled to our experience of loudness.

- Sound intensity level, L, is measured in decibels [dB], where 10 bels = 1 decibel: L = 10log10(I/I0). −12 −2 - I0 = is a reference sound intensity, usually the threshold of hearing = 1.0 ×10 Wm at 1000 Hz. -2 L/10 (L/10) – 12 - I = sound intensity that is measured or reported [Wm ]: I = I010 = 10 .

- A similar formula can be used to find a difference in sound intensity level: L = L2 – L1 =10log10(I2/I1).

- Each addition of 10 dB to the sound intensity level, increases the sound intensity by a factor of 10. 2 2 2  Quantify sound intensity differences at different distances: I 1/r ; I1r1 =I2r2 . 2 - Sound intensity drops with distance by an inverse square law due to its spreading-out: I  1/r .

- I= I2 – I1, is the difference in intensity, r= r2 – r1 is the difference in radius from a source (distance). 2 2 - The intensity of a sound wave times the square of its radius from a source is constant, so: I1r1 =I2r2 .  Describe resonance as superposition of a travelling sound wave & its reflection.

- Superposition is the vector sum of the displacements of component waves to produce a resulting wave. - Resonance in an object occurs when the forcing frequency equals the natural frequency of the object.

- The natural frequency of an object is the frequency at which it oscillates freely once disturbed. - The forcing frequency is the frequency at which an object is forced to oscillate by an oscillating force.

- The amplitude of the oscillations within a resonating object will increase dramatically. - The maximum possible energy from the forced vibration is transferred to a resonating object.

- Reflection occurs when a wave arrives at a barrier, returning into the medium from which it came. - If a wave meets an open end of tube or fixed end of string, destructive interference causes a node.

- If a wave meets a closed end of tube or free end of string, constructive interference causes an antinode.  Quantify for strings & tubes, the fundamental (1st) & subsequent harmonics.

- A tube open at both ends & a string fixed at both ends, are even oscillators with nodes at each end.

- A tube closed at one end & a string free at one end, are odd oscillators with an antinode at that end.

- The longest wavelength, 0, that an oscillator can resonate at is at the fundamental frequency, ‘f0=v/0’.

- The frequencies at which an oscillator can resonate, or ‘harmonics’, are multiples of the fundamental. -1 - v = speed of sound (generally in sound travelling through air= 340 ms ) = nfn.

- The harmonics above the fundamental are called overtones & are named after their order. - For all oscillators, successive harmonics are integer multiples of the fundamental frequency: fn= Nf0.

- For an even oscillator, the fundamental wavelength, 0, is twice the length, L, of the oscillator: 0 = 2L. th th - For an even oscillator, fn= the n overtone = (n+1) = N harmonic (all harmonics are possible).

- For an odd oscillator, the fundamental wavelength is four times the length of the oscillator: 0 = 4L. th th - For an odd oscillator, fn= the n overtone = (2n+1) = N harmonic (only odd harmonics are possible).  Describe electric & electromagnetic sound recording/reproduction devices;

- Loudspeakers transform electrical energy of AC signals into mechanical energy of pressure waves. - Dynamic (moving-coil) loudspeakers: AC current induces a magnetic field in a coil in a magnetic field connected to diaphragm which vibrates with the magnetic attraction-repulsion, changing air pressure. - Microphones transform mechanical energy of pressure waves into electrical energy of AC signals.

- Electret-condenser (capacitor) microphones: motion of one side of a capacitor changes its voltage. - Crystal (piezoelectric) microphones: compression of a crystal induces a voltage across the crystal.

- Dynamic (moving-coil) microphones: motion of a coil through a magnetic field induces a current. - Velocity (ribbon) microphones: motion of a metallic ribbon in a magnetic field induces a current.  Describe the effects of baffles & enclosures for loudspeakers.

- A baffle is a board for mounting speakers that stops sound waves from the front & back from interfering.  Describe frequency response curves for loudness (phon) of systems & hearing.

- A frequency response curve is a plot of intensity (dB) against frequency (Hz): usually an ‘inverted-U’.

- The ‘phon’ is a measure of intensity required to hear the equivalent loudness of different sounds.  Compare fidelity of systems for purpose, frequency response & construction.

- Select system components that can reproduce the frequencies required as shown by their response curve.

- 3-way speaker systems: Woofer for low frequency, Tweeter for high frequency & Mid-range for between.  Describe the directional spread of frequencies for gap/obstacle widths: sin /w.

- High frequencies diffract less than low frequencies & are softer at angels/around corners from a source.