Unit 2 Flashcards
Voltage = (equation)
- E/Q ( energy in joules/ Charge passing in coulombs)
EMF equation
- EMF= E/Q
Potential difference equation
- V = W/Q (energy transferred in joules/ charge passing in coulombs)
Voltage is
The amount of energy a component transfers per unit of charge passing through it
Electromotive force
Work done per unit charge moving around a whole circuit
Potential difference
The work done/ energy transferred per unit of charge moving through two points in a circuit
Resistance is
The opposition to the flow of current within a conductor
Resistance = V/I
Electronvolt is
The amount of energy an electron gains by passing through a voltage of 1V
1eV = 1.6 x 10^-19
Ohm’s law
The current through a component is directly proportional to the voltage across it, providing the temperature remains the same
V= IR
Resistivity
For a material, the same value as the resistance between opposite faces of a cubic metre of the material
Resistance = resistivity (ohm meter)xsample length (m) / cross sectional area (m^2)
Value for electric current in a metal
Calculated from the fundamental movement of electrons
- I= DQ/Dt
Principle of charge conservation
Total electric charge in a closed system does not change, total current flowing into a junction = current flowing out of that junction according to Kirchoff’s first law
In a series circuit current is
The same everywhere in the circuit
In a series circuit current =
The sum of currents in each parallel set of branches = total current
Principle of conservation of energy
Energy cannot be created or destroyed only transferred from one form to another so total energy in a closed system stays constant
Kirchhoff’s second law
The sum of all the voltages in a series circuit = battery voltage or the sum of all the voltages in the loop is zero
- So in series: battery p.d is shared across all elements in the circuit so total sum of voltages across all elements = supply p.d
- And in parallel: potential difference across each branch is the same
Resistance equation in a series circuit
RT = R1 + R2 + R3 + …
Resistance equation in a parallel circuit
1/RT = 1/R1 + 1/R2 + 1/R3 + …
- Power is and =
- And energy transferred (W)
- The energy transferred over time or rate of transfer of energy
P=VI
P= I^2 x R
P= V^2/R
- W=Pt
So W= VIt
Progressive waves
A repeated disturbance that transfers energy without transferring matter and can be either transverse or longitudinal
Transverse waves
- Waves that have oscillations perpendicular to the direction of energy transfer
- Eg. Electromagnetic waves, ripples on the surface of water, Seismic S waves
Transverse waves have peaks and troughs, define peaks and troughs
- Peaks: a point in the wave with the highest amplitude
- Troughs: A point in the wave with the lowest amplitude
Longitudinal waves
- Waves that have oscillations parallel to the direction of energy transfer
- E.g.; Sound waves, ultrasound, Seismic P waves
Longitudinal waves have compressions and rarefactions, what are they?
- Compressions: Regions of high pressure due to particles being close together
- Rarefactions: Regions of low pressure due to particles being spread apart
Wavelength definition
The distance between a point on one wave and the same point on the next wave
Amplitude definition
The maximum displacement of the wave from equilibrium position
What can be obtained from displacement time graphs
The amplitude and time period/ frequency
Frequency definition
The number of complete oscillations passing through a point per second
Waves repeat each cycle and each cycle has a path of _____ degrees or _____ radians
- 360•
- 2 pi radians
The phase of a point in a wave
The phase of a point in a wave is the fraction of a complete wave cycle it has completed
Phase difference is
Phase difference is the difference in phase, in radians, between two waves or between two points in a wave
Path difference of waves
The path difference is how far a wave is ahead of another wave in wavelengths or how far a point in a wave is ahead of another point
What is meant by wave interference and superposition
When two or more waves meet while traveling across the same medium and superpose to form a resultant wave of greater, smaller or the same amplitude
Constructive interference of waves
Maximum constructive interference occurs when two waves are completely in phase and the maxima of the waves add together to form a resultant wave with maximum amplitude
Destructive interference
Maximum destructive interference occurs when two waves are pi radians out of phase, the maxima of the waves cancel out to form a resultant wave with minimal amplitude
Standing/ stationary waves
Wave is formed when two waves which travel in opposite directions have the same frequency and speed and approximately equal amplitudes so they superpose to form a standing wave
Standing waves on a string
A standing waste is formed when the wave traveling along the string reflects after striking a boundary, the incident and reflected waves superpose to form a stationary wave
Node definition
A point along a stationary wave with minimum to no disturbance and the amplitude of the wave at this point is around zero
Antinode definition
A point along a stationary wave with maximum disturbance
The fundamental first harmonic
First possible stationary wave with two nodes and one anti node
How many nodes and antinodes are in each harmonic
(n+1) nodes and n antinodes
Equation for the wavelength of the nth harmonic
2L/n
The wavelength of stationary waves is
2/3 L
Where do nodes and antinodes form on open ended tubes
- Nodes form at closed ends
- Antinodes form at open ends
The distance between nodes in a standing wave is equal to
Half the wavelength of the wave
One coulomb
Charge transported by one ampere in one second
Current is
The rate of flow of charged particles
A current of 4A means
Four coulombs of charge flowing through a circuit every second
The two circuit rules derived from charge conservation
- The current is the same at all points in a series circuit
- The total current entering a junction must = total current leaving the junction
The two circuit rules derived from charge conservation
- Sum of potential differences across the components = the potential difference across the power supply
- The potential difference is the same across every component in a parallel circuit
Why is a voltmeter placed parallel to a component to measure potential difference
Potential difference is a measure of energy transfer per unit charge so two measurements need to be taken, one before charge enters the component and one after and the difference of energy per unit of charge flowing through it is the pd
Why does resistance of a wire increase as temp of metal wire increases
The amplitude of the lattice vibrations increases, increasing the resistance due to more energy, number of electrons stays constant and charge stays constant I=nqva, increasing frequency of collisions between electrons
The four factors that impact the resistance of a material
- Length of the material
- The cross sectional area the current passes through
- Temperature of the material
- The type of material
Uses of variable resistors
- Dimmable lights
- Loudspeakers
- Monitor brightness
How does temp impact the resistance for an NTC thermistor
Latrice vibrations increase increasing resistance but electrons that gain enough energy to escape electrostatic attraction and delocalize decrease resistance so
- As temperature of an NTC thermistor increases its resistance decreases
Points in the sine wave
- 360•= 2 pi radians
- 270•= 3pi/2 radians
- 180•= pi radians
- 90•= pi/2 radians
- 0•= 0 pi radians
The phase point in a wave is
The fraction of a complete wave cycle it has completed
Path difference
How far a wave is ahead of another wave in wavelengths, or how far a point in a wave is ahead of another point
Intensity is
The amount of energy passing through a unit area per unit time
Intensity equation
=E/A x t
= P/A
For spherical waves
= P/4πd^2
What is meant by 1 Wm^-2
1 joule of energy passing through an area of 1m^2 every 1 second
Five properties of light
- Reflection
- Refraction
- Diffraction
- Interference
- Polarisation
What quantity of light never changes during refraction
Speed and wavelength both increase or decrease
As a light meets a medium of higher optical density it
Slows down and its wavelength therefore decreases and the ray bends towards the normal (and vice versa)
Refraction occurs because
The speed of light changes
Why does a wave refract as it travels to a different optical density
- Part of the wavefront meets the boundary before the rest leading to a change in direction of the wave
- As the incoming wave strikes the boundary at an angle
- And refraction occurs when the speed of a wave changes as it enters a medium with a different refractive index
Snells law
N1sin(Ø1) = N2sin(Ø2)
Define the critical angle
The angle of incidence for which the angle of refraction is 90 degrees
Sin(C)=
1/n
Total internal reflection is
When the angle of incidence is greater than the critical angle, the light ray reflects at the boundary
Conditions required for total internal reflection
- From a more optically dense to a less optically dense medium
- Angle of incidence must be greater than the critical angle
Unpolarised light
Light oscillations are in all planes perpendicular to the direction of wave travel
Plane-polarised light
Light oscillations are in one plane perpendicular to the direction of wave travel
When unpolarised light passes through a polarizer like a Polaroid filter the intensity of the light:
The intensity of the light is reduced by a factor of 1/2
When unpolarised light reflects from a non metallic surface like a road the waves will be ____, and the reflected lights orientation will be _____
- Partially plane polarised
- In an orientation parallel to the surface it is reflecting from
Difference between unpolarised and plane polarised light
- Unpolarised light oscillates in all planes
- Plane polarised light oscillates in one plane including the direction of wave travel
How can areas of a material with different amounts of stress be identified with polarization
- Polarising filters at 90° to each other don’t allow light to pass through
- Rotation of plant of polarization due to stress allows light to pass through
- So dark areas represent less stress and brighter areas represent more stress
Diffraction
The process by which waves spread out when they go through a gap or past the edge of a barrier
Hugens’s principle
Every point in a wavefront acts as a source of secondary spherical wavelets, the new wavefront is a live tangent to all the wavelets
Explain using huygens’ construction how diffraction occurs as waves pass through a gap
- Waves spread out as they pass through the gap
- Each point on the wavefront act as a source of secondary wavelets
When the path difference between light rays arriving on the screen is nλ
The rays are in phase and will constructively interfere forming bright regions
When the path difference between light rays arriving on a screen is (n+1/2) λ
The rays are in anti phase so will destructively interfere and form dark regions
dsinø=
n λ
Photoelectrons
Electrons that gained energy photoelectrically from the electrons
Threshold frequency
Minimum frequency required to cause any photoelectric emissions
Photons
Particles that electromagnetic waves consist of, a packet of electromagnetic energy also known as quanta
E=
hf
hf=
Ek + Ø
C=
f x λ
Particle nature of light photoelectric effect any 6 mark question on photoelectric effect/ wave model/ particle model:
- Light consists of particle like photons
- There are one to one interactions between the photons and electrons
- Photons are released instantaneously, as opposed to gradually which is expected from the wave model of light
- The energy of photons depends on the frequency of light (E=hf)
- Photons are only released when the frequency of light is above the threshold frequency/ the energy of the photon is above the work function
- The wave model suggests photons can be released at all frequencies
- The intensity of light impacts the number of photoelectrons emitted, whereas the wave model suggests that the intensity impacts the energy of the photoelectron emitted
Fundamental frequency of the nth harmonic =
Fn= n/2L • √(T/u)
If the wave model were true
- Photoelectrons would be emitted at all frequencies of light
- Higher intensity would mean higher K.E. Of emitted electrons
- Emissions would be gradual as energy is absorbed over time
- Higher frequency would increase the number of photoelectrons emitted
- Because light consists of photons (particles of energy and each photon interacts with one electron (1 to 1 interaction)
If the particle model is true
- A minimum frequency (threshold frequency) is required for emission
- Work function is the minimum energy needed to emit photoelectrons
- Max K.E. Is proportional to frequency (E=hf)
- Higher intensity increases the number of electrons emitted not their K.E.
- Photoelectrons are emitted instantly
- Because light consists of photons (particles of energy and each photon interacts with one electron (1 to 1 interaction)
In a displacement distance graph for a longitudinal wave what is the displacement of particles at the point of a compression or rarefaction
Displacement is zero
I= in a conductor
nqvA
Internal resistance equation
E= I(R+r)
Increasing the intensity of a light incident on a LDR
- As Light intensity increases number of photons per second incident on LDR increase
- Resistance of the LDR decreases and internal resistance of the cell is constant
- As current increases the potential difference across the internal resistance increases
- Pd across LDR decreases
