# OCR Physics A Unit 2 Revision notes

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G482 ­ Electrons, Waves and Photons; Revision Notes
Module 1: Electric Current
Electric Current
· A net flow of charged particles.
· Electrons in a metal
· Ions in an electrolyte
Conventional Current
· A model used to describe the movement of charge in a circuit. Positive to negative
Electron Flow
· The movement of electrons around a circuit. Negative to positive
Coulomb
· Unit of electrical charge (C).
· 1C = 1A * 1s (Q=It)
Ammeter
· An instrument that measures electrical current
· Always connected in series
· Traditionally they used small coils that rotated in magnetic fields to detect current
· To avoid the ammeter affecting the circuit, it must have negligible resistance
Elementary Charge
· The charge on 1 electron (e) = 1.6x10-19C
Kirchhoff's First Law
· The sum of the currents entering any junction is always equal to the sum of the currents leaving the
junction
·
· This is a consequence of the conservation of charge; electrons cannot simply appear or disappear
Mean Drift Velocity
· The average speed of charged particles along the length of a conductor
· I=nAve
Conductors, Semiconductors and insulators
· Semiconductors typically have relatively low number densities; therefore conduction electrons have more
room to move in the material. This means that the electrons are less impeded when travelling (less
collisions); therefore higher mean drift velocity
· Conductors have large number densities; so the conduction electrons collide with each other a lot more,
resulting in a smaller mean drift velocity
· Insulators have such a small number densities, meaning fewer electrons to carry charge. Those that can
must battle their way through a complex structure in the material.

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G482 ­ Electrons, Waves and Photons; Revision Notes
Module 2: Resistance
Potential Difference
· Electrical energy transferred per unit charge, when electrical energy is converted into another form
· Electrical energy to light energy in a light bulb
Electromotive Force
· The energy transferred per unit charge when a type of energy is being converted into electrical energy
· Chemical energy to electrical energy in a cell
Volt
· Unit of potential difference and e.m.…read more

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Experiments to Determine I-V Characteristics
· Set up a circuit diagram as shown:
A
V
Long Thin Wire
· As to keep the current low, so that heating effect is negligible, always use a long thin wire
· Take readings from the voltmeter and ammeter
· Increase the number of cells and take readings for each added cell
· Plot a graph of current against potential difference
o This is the I-V characteristic of the wire used
Uses and benefits of L.E.…read more

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Negative Temperature Coefficient (NTC) Thermistors
· These show a rapid change in resistance over a narrow temperature change
· Resistance decreases as the temperature of a NTC thermistor increases
Power
· The rate at which energy is transferred
Fuses as Safety Devices
· These are devices introduced into circuits to protect the wiring from excessive currents
· These high currents cause wires to get hot, damaging them, and could result in fires
· When a fuse is subjected to higher currents than what they are…read more

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G482 ­ Electrons, Waves and Photons; Revision Notes
Module 3: DC Circuits
Voltage and Current in Series
· Voltage is shared and current is constant
Voltage and Current in Parallel
· Current is shared and voltage is constant
Kirchhoff's Second Law
· The sum of the e.m.f round a loop is equal to the sum of the p.…read more

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Progressive Longitudinal Waves
· The motion of particles are parallel to the direction of propagation
Progressive Transverse Waves
· The motion of particles is perpendicular to the direction of propagation
Displacement of a Wave (x)
· The distance from a given point on the wave to its equilibrium/rest point
Amplitude of a Wave (x0)
· The maximum displacement from the equilibrium/rest point
·
x0
Displacement (m)
x
Time (s)
x0
Wavelength of a Wave ()
· The distance between two identical points on consecutive waves…read more

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Wave Speed
· The speed at which a wave travels
·
· When talking about waves, this becomes
· But since ; it becomes
·
How a Progressive Wave Transfers Energy
· In the case of light, energy is transferred from a source to your eye
· The wave front transfers photons to your eye
· In the case of sound energy is transferred from a source to your ear
· The wave front causes particles in air to vibrate and collide with other particles…read more

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All electromagnetic waves travel at the speed of light (3x108ms-1)
· All posses a magnetic wave and an electrical wave interlocked at right angles to each other
· All are transverse waves
· All can travel through a vacuum
· UV-A: Wavelength 315-400nm
o Causes tanning when skin is exposed to the sun (accounts for 99% of UV radiation)
· UV-B: Wavelength 280-315nm
o Causes damage such as sunburn and skin cancer
· UV-C: Wavelength 100-280nm
o Filtered out by the atmosphere and does not…read more

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Interference
· The formation of points of cancellation and reinforcement, where 2 coherent waves pass through each
other
Coherence
· Two waves with a constant phase relationship
Path Difference
· The proportion of a wavelength by which two waves are `out of synch'
Constructive interference
· When 2 waves reinforce to give increased amplitude
· Occurs when the path difference is a whole number of wavelengths. E.g. , 2
· Occurs when the phase difference is a multiple of 2c. E.g.…read more

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Light of 1st order (n=1)
wavelength 0th order
Grating with slits d 1st order (n=1)
apart
2nd order (n=2)
3rd order (n=3)
· The equation involving , then becomes
· If the grating has X slits per metre, the slit spacing,
G482 ­ Electrons, Waves and Photons; Revision Notes
Module 4: Waves (cont)
Stationary Wave
· A wave where the energy is stored, rather that transferred from place to place
Formation of Stationary Waves
· They are formed when 2 progressive waves, of the same…read more