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Electrons , Photons and Waves Module 1: Electric current Module 2: Resistance Module 3: DC circuits Module 4: Waves Module 5: Quantum physics define the coulomb • The SI unit of electrical charge. One Coulomb is defined as the amount of charge that passes in 1 second when the current is 1 ampere. define potential difference (p.d.) • Energy transfer per unit charge from electrical to other forms define the volt • 1 Volt is equal to 1 Joule per Coulomb (JC-1) The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component. • Or • A pd of 1V is the voltage required to produce a current of 1A in a resistor of resistance 1 Ohm define electromotive force (e.m.f.) • Energy transfer per unit charge from chemical/other to electrical form. Measured in V or JC-1 define resistance • Resistance = Potential difference/current . • ratio of voltage to current; voltage per (unit) current (VA-1) • resistance = p.d./current define the ohm • A component has a resistance of 1 ohm if a potential difference of 1 volt makes a current of 1 amp flow through it. define resistivity of a material • ρ=RA/l define the kilowatt-hour (kW h) • A unit of energy equal to 36 MJ or the energy used by a 1kW for 1h Define displacement, amplitude, wavelength, period, phase difference, frequency & speed of a wave Displacement-how far a point on the wave has moved from its equilibrium position Amplitude- Maximum displacement Wavelength-Distance between neighbouring identical points Period-Time taking for one complete oscillation of a particle Define displacement, amplitude, wavelength, period, phase difference, frequency & speed of a wave Phase Difference- The fraction of a cycle between the oscillations of two particles Frequency-Number of waves passing a point per unit time Speed-Distance travelled by the wave per unit time define the terms nodes and antinodes • Node-When the amplitude is always zero • Antinode-When the amplitude is always at its maximum possible value define fundamental mode of vibration • Simplest pattern of movement and has the lowest possible frequency band and the longest wavelength define harmonics • Harmonics are different modes of vibration of a wave with increasing frequency and decreasing wavelength define the electronvolt (eV) • The Electronvolt is defined as the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt. define work function • Work function- the minimum energy required to release an electron from the surface of a material define threshold frequency • Threshold frequency- the minimum frequency of a photon that will cause and electron to be emitted from the material. Define the term intensity • intensity is the incident energy per unit area per second Remember I α Amplitude2 State what is meant by the term mean drift velocity of charge carriers • The average distance travelled by the charge carriers along the wire per second State Ohm’s law • Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it. State Kirchhoff’s second law • sum of e.m.f’s = sum of p.d.s around a closed loop in a circuit Energy is conserved State Kirchhoff’s First law • The sum of the currents into a junction is equal to the sum of the currents out of the junction Charge is conserved State the principle of superposition of waves • When two or more waves meet at a point and interfere, The resultant displacement equals the vector sum of the displacements of each wave. Q=It 1mA = 1 x 10-3 A I = nAve Electric Current is the rate of flow of charge. 1μA = 1 x 10-6 A Kirchoff’s first law states that the sum of the current into a junction is equal to the sum of the currents flowing out of the junction. An example of conservation of charge Conventional current n conductors >> n semiconductors >> n insulators v insulators >> v semiconductors >> v conductors Electrons , Photons and Waves Module 1: Electric current Module 2: Resistance Module 3: DC circuits Module 4: Waves Module 5: Quantum physics I-V Characteristics Interpreting I-V Graphs I V Interpreting I-V Graphs I V Interpreting I-V Graphs I V Drawing Graphs Work out where it starts Get the shape right Consider if it has to go through any particular point ( often calculated in a previous part of a question) Resistivity A Calculate the area carefully ( use m) . Is it a circle ? B Which of these two wires has the greatest resistance ? Resistivity Remember if you are asked what happens if l doubles and r halves you can try out an answer with numbers. The Fuse Explain how a fuse works? When a current bigger than the fuse rating flows through, the fuse gets hot, melts and cuts off the power. The fuse prevents fires .. 33 The Kilowatt Hour An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time? Number of kWhrs: number of units of electricity = number of kilowatt hours = 3 kW x 4 h = 12 kWh Cost of electricity: cost = number of kWhrs x cost per unit = 12 kWhrs x 8 p = 96p Electrons , Photons and Waves Module 1: Electric current Module 2: Resistance Module 3: DC circuits Module 4: Waves Module 5: Quantum physics Circuit Circuit Analysis What is the current flowing in this circuit and the voltage across each resistor ? Circuit Circuit Analysis What is the current flowing in this circuit and the voltage across each resistor ? Using a thermistor Resistance of Thermistor decreases as temperature increases. V across thermistor decreases so V increases across fixed resistor since voltages must add up to the supply voltage. Internal Resistance Electrons , Photons and Waves Module 1: Electric current Module 2: Resistance Module 3: DC circuits Module 4: Waves Module 5: Quantum physics Polarisation of Radio Waves Path Difference Light Interference Use the formula to work out what is happening if a, x, D or λ changes. The Diffraction Grating The spectral pattern seen from a monochromatic source such as a sodium lamp Stationary Waves The apparatus used for setting up a stationary wave in a closed tube Quantum Physics • Energy of a photon • The photoelectric effect • Wave–particle duality • Energy levels in atoms Energy of a Photon What is a Photon ? a quantum/packet/of electromagnetic energy Energy of a Photon How do you calculate the energy of a photon ? The Photoelectric Effect Describe the Photoelectric Effect A photon is absorbed by an electron in a metal surface causing an electron to be emitted. Energy of photon = hf Energy is conserved. Only electrons with energy above the work function energy will be emitted. Only light with a frequency greater than the threshold frequency will cause emission Energy=work function + Max KE of electron. The work function is the minimum energy required to release an electron from the surface. Number of electrons emitted depends on light intensity. ( Energy of Electron does not) The Photoelectric Effect Describe the Photoelectric Effect Experiment A clean zinc plate is mounted on the cap of a gold leaf electroscope where the plate is initially charged negatively . Shine a UV light on the plate and watch the gold leaf collapse as charge leaves the plate, indicating the emission of electrons The Photoelectric Effect Photoelectric Effect Graphical Analysis The Photoelectric Effect Photoelectric Effect Graphical Analysis As intensity goes up it does not affect the energy of the electrons. As frequency goes up it does not affect the number of electrons emitted As intensity goes up more electrons emitted The Photoelectric Effect Photoelectric Effect Numerical Analysis To convert from eV to J multiply by 1.6 x 10 -19 The Photoelectric Effect Describe the LED Experiment to find h Adjust the potential divider to low/zero voltage Connect flying lead to one LED Increase voltage until LED just lights/strikes Repeat several times and average to find Vmin Repeat for each LED Shield LED inside opaque tube to judge light more accurately Wave Particle Duality When do waves behave like particles ? In the photoelectric effect – the energy of the light does not depend on the amplitude of the wave but on its frequency. Wave Particle Duality When do particles behave like waves ? Travelling electrons are diffracted by graphite producing a series of diffraction rings. Maximum diffraction occurs when the de Broglie wavelength of the electrons is similar to the gap between layers of carbon atoms. Wave Particle Duality What is the de Broglie wavelength for an electron ? Travelling electrons are observed to behave as waves/show wavelike properties Where the electron wavelength depends on its speed/momentum Energy Levels in Atoms What is an Emission Line Spectrum? light emitted from excited isolated atoms produces a line spectrum a series of sharp/bright/coloured lines against a dark background Energy Levels in Atoms What is an Absorption Line Spectrum? in an absorption spectrum a series of dark lines appears against a bright background/within a continuous spectrum Energy Levels in Atoms Why are spectra important? They reveal that electrons within atoms can only occupy certain energy levels. Explaining Line Spectra Energy Levels in Atoms An energy-level diagram for the hydrogen atom Numerical and Graphical Analysis Electrons have discrete energy levels within atoms - they cannot have any energy . hf or hc/λ = E1 – E2