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AS REVISION UNIT 1 TOPIC : PARTICLES, QUANTUM PHENOMENA AND ELECTRICITY Explain what is meant by an electric current. An electric current consists of a flow of electrons. What is the difference between “conventional current” and the an “electron flow current”? Conventional current flow from positive to negative, electron flow current flows from negative to positive. Define the Volt This is the p.d. when the work done of 1J when 1 coulomb of charge flows. Define the Ohm The Ohm is the resistance when 1 volt is applied when a current of 1 ampere flows. Explain the meaning of resistance. Resistance is the opposition to flow of an electric current. Explain the origin of resistance in a conductor. The atoms in the conductor form the basis of opposition to the flow of current. State the three formulas for power P = I.V , P = V2 /R , P = I2 R. Define the term resistivity Resistivity is the resistance in a unit cross-sectional area per unit length. State two factors that affect the resistance of a material Length, cross-sectional area, material and temperature of the material. State Ohm’s Law The p.d. across a conductor is proportional to the current flowing through it, provided the temperature remains constant. Describe what happens to the resistance of a thermistor when it is heated. As the temperature of the thermistor increases its resistance decreases. What is a kilowatt-hour? A kilowatt-hour is the energy an appliance of 1kW used in one hour. IkWh = 3.6 x 106 J What happens to the resistance of a material if the length doubles? The resistance doubles . What happens to the resistance of a material if the cross-sectional area doubles? The resistance halves. What happens to the resistance if the diameter of a wire doubles? The resistance decreases by a factor of 4. Since doubling the diameter increases the area by a factor of four. Define Resistance Ratio of the p.d. across a conductor to the current through it. Can resistivity change with temperature? Yes, the resistivity increases with temperature. Does a filament bulb obey Ohm’s Law? No, the resistance changes due to a change in temperature. Does a diode obey Ohm’s Law? No, the resistance changes due to a change in charge carriers. State a formula for the work done in a circuit. W = VxIxt V = P.D., I = Current in amp t = time in seconds How is power related to work? Power = Work Done / Time Taken What is the meaning of electric charge? Electric charge is a quantity of electricity on charged particles. Why does an electric current move slowly? An electric current consists of free electrons moving superimposed on random motion of electrons Define the term potential difference. Potential difference is defined as the work done per unit charge in converting electrical energy into other forms Define the term resistivity. Resistivity is defined as the resistance of a conductor in a unit cross-sectional area per unit length. Define Electromotive Force Work done per unit charge in converting the source energy into electrical energy in a circuit. How is emf different from p.d.? Emf is the energy per unit charge in converting energy in a circuit. P.d in energy/charge converted across a component. How is charge conserved in a circuit? In any closed circuit the amount of charge flowing is constant, the rate of flow of charge is constant. Explain how energy is conserved in a circuit. In any circuit the energy supplied by the source is dissipated in the components. State the formulas for the sum of three resistors in parallel. 1/RT = 1/R1 + 1/R2 + 1/R3 For 2 resistors RT = Product/Sum State the formulas for the sum of three resistors in series. . RT = R1 + R2 + R3 What is the purpose of a potential divider? A potential divider gives a proportion of the source potential to another circuit. What is meant by an ‘open circuit’? It is a circuit where a current does not flow. Sketch the Current-Voltage curve for a Resistor. Sketch the Current-Voltage curve for a Lamp. Sketch the Current-Voltage curve for a diode. What is meant by a closed circuit? • It is a circuit where a current flow What is meant by the terminal pd? State a formula in terms of the emf and the internal resistance. Terminal p.d. is the pd across the terminals of a cell. IR = E - Ir Explain why the pd across a cell drops when a resistor is connected to a cell. P.d. is dropped across the internal resistance which reduces the terminal pd. from the formula, IR = E -Ir How is a voltmeter connected in a circuit? Why is the resistance of a voltmeter high? In parallel. The resistance is high so no current flows through it. How is an ammeter connected in a circuit? Why is the resistance of a ammeter low? In series. So that it take all the current and it does not change the current in a circuit. What is the purpose of a potential divider? A potential divider gives a proportion of the source potential to another circuit. State the purpose of a wave To carry and transfer energy. What is the photoelectric effect? Emission of electrons off the surface of a metal using light. How can the the photoelectric effect be demonstrated? Using a gold leaf electroscope which is negatively charged, a clean piece of zinc and a uv lamp. The leaf slowly collapses as uv light is shown onto the zinc plate. What is meant by the threshold frequency? Smallest frequency of light required for electron emission. What is the work function? Smallest energy of light photons required for electron emission. What is Einstein’s Photoelectric equation. hf = + ½ mv2 max hf = incident photon energy = work function ½ mv2 max = Max ke of photoelectrons Explain why the photoelectric current is proportional to the intensity of the electromagnetic radiation. The greater intensity of the incident radiation (i.e more photon hitting the metal surface) the number of electrons emitted per second. Explain what is meant by a photon of electromagnetic radiation . This is a packet or quantum of energy. State the formula that relates the energy of a photon to the frequency of electromagnetic radiation. Identify each symbol. E = h.f h = Planck’s Constant f = frequency of photon Why is the energy of each electron different? Each electron could originate from different depths within the metal surface. What does the photoelectric effect show? It shows that light has a particle nature. State two effects that show light has a wave nature? Diffraction and interference. Define the electron-volt. Energy acquired by an electron when accelerated by a pd of 1V. State the order of the Electromagnetic Spectrum starting from the smallest wavelength. Gamma Rays, X-Rays, Ultraviolet, Visible Light, Infra Red Microwaves, Radio Waves State three common features of the e-m spectrum. All transfer energy. All travel by transverse wave motion. All move at the speed of light in a vacuum. State two difference of the em spectrum Different frequencies/wavelengths Different methods of production State the evidence that electrons can behave as waves. Electrons can show a diffraction pattern when accelerates through a fine hole in an evacuated tube. What is meant by the terms Excitation and Ionisation? Excitation occurs when an electron or atom gains energy and moves to a higher excited state. Ionisation occurs when an electron gains sufficient energy to escape the atom. State what a line spectrum in an atom shows. A line spectra is evidence for the discrete energy levels in atoms. The energy emitted when an electron moves between levels is given by: E1 – E2 = h.f What is meant by proton number, nucleon number and specific charge? Proton Number Z is the number of protons in the nucleus. The Nucleon or Mass Number A is the total number of protons and neutron in the nucleus. The Specific Charge is the ratio of charge to mass for an particle or ion in C/kg Explain the reason for the Nuclear strong Force and given some properties The nuclear strong force is the force which is greater than the electrostatic repulsion force in the nucleus. It is an attractive force of short range 3fm. It is repulsive below 0.5fm. It is independent of charge. State the general equation for alpha decay State the equation for beta decay What is meant by an antiparticle? An antiparticle is a particle which has the same mass as its particle but with opposite properties e.g. spin What is an annihilation reaction? This a reaction where a particle and its antiparticle combine with the emission of energy.E.g. Electron and positron. + - What is the meaning of pair production? Pair production occurs when a photon interacts with matter to produce a particle and antiparticle. What is the exchange particle between elementary particles e.g. electrons? Photons. What are the exchange particles in beta decay and in electron capture reactions? W+ , W- bosons. What are Hadrons? Hadrons are particles which are subjected to the strong nuclear force. There are two types, Baryons e.g. protons and neutrons and Mesons. What are Leptons? Leptons are particles which are subjected to weak interactions. Examples of leptons are electrons and neutrinos. What is the baryon number for protons, antiprotons, neutrons, antineutrons and mesons? Protons and neutrons 1 Antiprotons and antineutrons -1 Mesons e.g. pions 0 Draw the Feynman diagram for electron-proton collision. Draw the Feynman diagram for electron-proton collision. Draw the Feynman diagram for beta-minus decay. Draw the Feynman diagram for beta plus decay. State three types of quarks Up, down, and strange. (Top, bottom, and charm) State quarks are in a proton. Up, up and down quarks State the quarks in a neutron Down, down and up quarks What is the change in quark in beta minus decay? In β- decay a down quark changes to an up quark. What is the change in quark in beta plus decay? In β+ decay an up quark in a proton changes to a down quark. In any collision or decay what laws should be obeyed? Lepton number Baryon number Charge Strangeness.