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OCR A physics gce
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Gravity
Terms in this set (53)
Current
The rate of flow of charge
A volt
Is the potential difference across a component when you convert 1 joule of energy moving 1 coulomb of charge through the component
1V = 1J/C
Resistance
A component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it
Resistivity
Is the resistance of a 1m length with a 1 m2 cross sectional area, measured in Ωm
Resistance as heat changes?
The resistance if a metal increases as the temperature increases
Semiconductors
If energy is supplied to a semi conductor more charge carriers are often released
This makes them suitable for detecting changes in environment
Power
Work done / energy transferred per unit time
Kilowatt-hour
A unit of energy equal to 3.6MJ or 1kW for 1h/AW
Ohms law
Voltage is directly proportional to current
As long as temperature and other physical conditions remain constant
The line goes through the origin
Kirchoff's first law
The sum if total current into a junction equals the sum of total current out
Conservation of charge
Kirchoff's second law
Sum of emfs = sum of voltages in a closed loop
Energy is conserved
Coulomb
A unit if charge equal to the quantity if electricity conveyed in one second by a current of one amp
Potential difference
Energy converted from electrical energy to other forms per uni charge
Emf
Energy converted to electrical energy from other forms per unit charge
Progressive wave
Transfers energy without transferring material as a result of oscillations
Intensity
The rate of flow is energy per unit area at right angles to the direction if travel of the wave W/m^2
Energy per unit area per unit time
Malus' law
The intensity of the transmitted light is proportional to the amplitude squared
I = I•cos2Θ
I= intensity of transmitted light
I•= intensity if incident light
Θ= angle the plane had been rotated
Internal resistance
Some energy is transferred into thermal energy in the battery. There is a voltage decrease in voltage from the batter when a current is drawn from it
Principle if superposition of waves
When two or more waves meet at a point the resultant displacement is the vector sum of the individual displacements
Coherent
Constant phase relationship between the waves
Displacement and amplitude
Any distance moved from equilibrium of a point on a wave
Maximum displacement caused by wave motion
Frequency and phase difference
Number if wavelengths passing a point per unit time
Between two points on the same wave of the same frequency, how far through the cycle one point is compared to the other
Polarisation of microwaves experiment
Place transmitter and receiver facing each other
Rotate either transmitter or receiver through 90 degrees about axis joining aerials
Observe signal fall to zero from initial high value on meter monitoring output of receiver
Explanation of observations
Apparatus to detect and measure sound wave
Microphone to transfer mechanical motion to electrical signal
Oscilloscope to display oscillations
Stationary wave on a string
Wave travels to end and is reflected
Reflected wave interferes with incident wave
Interferes destructively at certain points to produce nodes
2 source constructive and destructive interference
Constructive path difference
n
Destructive interference
(2n+1)/2
Standing wave
Energy is stored
Standing wave in an open pipe
f=cn/2L
f= frequency
n= harmonic (n=1 is 1/2 wavelength)
c= wave speed
L= length
Standing wave in a closed pipe
f=[(2n-1)c]/4L
f= frequency
n= harmonic (n=1 is 1/2 wavelength)
c= wave speed
L= length
Measuring the speed of sound using a stationary wave
Hold the tube at the bottom of the beaker
Bang the tuning fork above the tube
Move the tube slowly out the water
When the sound is more intense, a standing wave has been formed
C=2f(L2-L1)
L= the distance from the top of the tube to the water
1= fundamental, 2= 2nd harmonic
De Broglie wavelength
Electrons are observed to behave as waves/show wavelike properties
Electron wavelength depends on its speed/momentum
Electronvolt
an eV is the energy acquired by an electron accelerated through a potential difference of 1 volt
1eV = 1.6 x10^-19 Joules (the charge of 1 electron)
Photoelectric effect
A photon is absorbed by an electron in a metal surface
Causing an electron to be emitted from the surface instantaneously
Energy is conserved in the interaction
Only photons with energy above the work function will cause emission
Energy of photon = work function of metal + max possible kinetic energy of emitted electron
Work function is the minimum energy to release an electron from the surface
Experiment demonstrating photoelectric effect
A clean zinc plate mounted on the cap of a gold leaf electroscope
Plate initially charged negatively
A UV lamp shining on the plate
The gold leaf collapses as the charge leaks away from the plate when UV light is incident on plate
So experiment indicates the emission of electrons
Work function of a metal
The minimum energy required to release a photoelectron from the metal
Photons passing through gas X
Some photons will be absorbed
X atoms become excited
Excited X atoms re-emit photons
So photon energy is equal to the transition n=a to n=b (shown on diagram)
Electron diffraction
Electrons behave as waves/have a wavelength
Diffraction observable because gaps/atoms are similar to the wavelength of electrons
Regular pattern of atoms acts like grating
Allowing constructive interference to produce a pattern on a screen
Rings occur because atomic crystals at all possible orientations to beam
Calculating Plank's constant
Connect an LED of known wavelength to aa current containing a variable resistor
Start with no current, and slowly increase it until the current just begins to flow through (LED lights up)
Record the voltage and the wavelength
Repeat this with different wavelengths
Plot a graph of volts on Y and 1/Wavelength on X
The gradient is hc/E, so h= gradient x 1.6x10^-19 / 3x10^8
Mean drift velocity
The average displacement of the electrons along the wire per second
Over time they move slowly in ine direction through the metal
Because they collide constantly
When designing a circuit which includes an LED, why would a resistor be used in series with the LED
The resistor limits the current in the circuit
Otherwise it could overheat
Why would a variable resistor be more suitable for obtaining the I-V characteristic curve of an LED rather than with a variable resistor
On the potential divider circuit, the voltage range is from zero to maximum possible
On the other circuit, the resistance variation is small
So the voltage variation across the LED is small
Define wave speed
Distance travelled by the wave per unit time
Derive c=fλ
In 1 second f waved are produced each if one wavelength
Distabce tragelled by first wave in one second id fλ=c
Define interference
When two waves interact at a point there is a change in overall intensity
Emission if photons from atoms
Electrons have discrete energies in an atom
Each photo produced by electron moving between levels
Photon energy equal to energy difference between levels
Electron loses energy making transition in correct direction
Why blue laser ought emits fewer photons compared to red laser of same power
Blue light has a higher frequency than red
Energy per photon is higher
Why in an electron gun the cathode is connected to the negative terminal
Electrons Should be repelled by cathode and attracted by anode
What is meant by 230V, 25W for a lamp
When connected to the 230V supply,
The power per second from the supply is 25W
Sunscreen
Filters out/blocks/reflects/absorbs UV(-B)
What is meant by line spectra
Light emitted from (excited isolated) atoms produces a line spectrum
a series of (sharp/bright/coloured) lines
against a dark background
Experiment to obtain I-V curve
vary p.d. (across lamp)/current (in circuit)
by changing voltage supply/moving contact on the potential
divider
take/record set of values of V and I
phase difference
relates to the oscillation of two points on the (same) wave
how far 'out of step' one oscillation is from the other/AW
λ/4 means a phase difference of 90o π/2 (rad)
Principle of superposition of waves
when two(or more) waves meet/cross/interact (at a point)
the (resultant) displacement is the (vector) sum of the
(individual) displacements
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