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P3 Notes What you should know: Making electricity: Electricity is made to move in a wire if there is movement between it and magnetic field lines. This can be because the wire itself moves OR because a magnetic field moves through the wire: Many separate coils of wire set at angles to each other A radial magnetic field – so that coils are always spinning around and cutting magnetic field lines – so making electricity Output from an ac generator: To increase the amount of electricity made: Use more coils of wire Move the coils of wire faster Use a stronger magnetic field. *Reversing the direction of movement of the wire reverses the direction that the electricity flows. An ac generator consists of a coil of wire moving in a magnetic field. The direction of the flow of the electricity made is given by Flemmings Right hand rule: Point the first finger of your right hand out, put your thumb upwards at 900 to your first finger and lastly put your middle finger at 900 to both of these. In a commercial generator ( one used in power stations) – they will use: At A: The spinning coil of wire is moving upwards at 900 to the direction of the magnetic field. This is cutting magnetic field lines at the fastest rate – so producing maximum voltage and current. At B and D: the wire is moving parallel to the magnetic field, so no lines are being cut. This means no voltage or current is being made. At D: The spinning coil of wire is moving downwards at 900 to the direction of the magnetic field. This is cutting magnetic field lines in the opposite direction at the fastest rate – so producing maximum negative voltage and current. Output Voltage: One spin of coil: Either doubling coil or 2x strength magnet Spinning at twice speed (2x voltage; 2 frequency) Radial magnetic field: The magnetic field lines radiate out from centre North Pole to Outer South pole. Np Ns = Vp Vs Np = number of coils on primary Ns = number of coils on secondary Vp= primary voltage Vs = secondary voltage This equation holds for 100% efficient transformers ( transformers that lose no energy as heat or sound). At 100% efficiency: Power in = Power out Vp x Ip = Vs x Is where I is the current flowing Transformers: These are used to change the size of an AC voltage WAVES There are two types of waves: 1. Transverse, where the disturbance is at 900 to the direction in which they travel. Iron core 1. 2. 3. 4. 5. The ac voltage in the primary circuit produces a magnetic field in the iron core. This produces a moving magnetic field in the iron core, which is transferred to the secondary coil. As the secondary coils are ‘cut’ by the moving magnetic field, a voltage is induced in the secondary coil. The size of the secondary voltage is determined by the number of primary and secondary coils . Step up transformers have more coils on the secondary than on the primary. Step down transformers have less coils on the secondary than the primary. On HIGHER tier: You have to calculate values using the equation: Examples of transverse waves: water waves, electromagnetic waves including light, secondary earthquake waves (S waves) 2. Longitudinal waves, where the disturbance is parallel to the direction of motion. Wave front diagrams: As water waves and light waves are transverse waves, their direction of motion is at 900 to their wavefronts. Direction of motion. You have to either complete, or interpret, wave front diagrams for reflection and refraction Reflection Refraction Examples of longitudinal waves:sound, ultrasound, Primary earthquake waves (P waves) Refraction of waves: This occurs because the speed of the waves change as they travel from one material to another eg fast in air, slow in transparent material. Ultrasonic waves: These are longitudinal waves of frequencies above human hearing i.e. frequencies above 20 000Hz. They need particles to travel though, just like sound – going fastest through solids, next fastest through liquids and , with difficulty, slowly through air. If used for foetal scanning or finding faults in solid materials, then a layer of gel has to be used on the surface in order to transmit the waves. At each density change some of the ultrasound is reflected to a receiver. This reflected wave is transferred to a display eg computer screen, so an image of the foetus is seen. Faults: The ultrasound enters the block, some of the wave is reflected (R1), the rest travels into the block. Some reflects off the top of the crack (R2) the rest travels forward. When the ultrasound hits the bottom the crack reflection R3 occurs, and the final refection comes from the bottom of the block (R4). This is seen on the screen. If the speed of the wave is known in the metal, and the time of reflection is calculated from the display, then the equation; Speed = distance / time Can be used to find the depth of the crack on the metal block. Eg If ultrasound travels at 600m/s in the metal and R2 is received after 0.0002 seconds. Distance travelled by ultrasound = speed x time = 600 x 0.0002 = 0.12m This is NOT the depth of the crack – it is the time for the ultrasound to travel to the crack and back!!! So the depth of the crack is 0.12/2 = 0.06m Cleaning: ultrasonic waves through a liquid vibrate dirt particles trapped in delicate jewellery etc until it is dislodged, so leaving the dirt in the liquid and the jewellery clean. Wave equation: (Higher tier only) You will have to use: wave speed = frequency x wavelength In the context of ultrasound. Example: ultrasound of frequency 25 000 Hz travels at 750m/s. What is the wavelength of the wave? Speed = frequency x wavelength 750 = 25 000 x wavelength 750 / 25000 = wavelength 0.03 m = wavelength Seismic waves: When an earthquake occurs some of its energy travel as waves around the world. There are three waves you need to know about: P waves: these are longitudinal, travel very fast and can travel through solids and liquids S waves: these are transverse, travel more slowly and can only travel through solids. Surface waves: cause of damage and destruction. The path of P waves through the mantle and core, together with the fact that S Waves only travel through the mantle – suggests the outer core as being liquid. Refraction of waves is shown in drawing. The time lag between a P wave and an S wave arriving at a seismology station indicates the distance between the station and the earthquake. b) Velocity time graphs Speed C A O If only the P wave arrives then the earthquake was on the opposite side of the earth and unable to travel through the liquid of the outer core. Motion Speed is the distance travelled each second in ANY DIRECTION. Velocity is also the distance travelled each second IN A GIVEN DIRECTION. (In collision questions velocity is a positive value in one direction and a negative value in the opposite direction) Graphs of motion: a) Distance time graphs Distance C A O B Time The graph shows: O-A steady fast speed A-B stopped B-C slower steady speed **the greater the gradient of the line the faster the speed (Gradient means steepness of line). OR Mean Speed = distance time B D Time The distance travelled Is equal to the area under the graph. O-A shows slow rate of acceleration (Because the gradient is small) A-B shows a steady speed B-C shows a fast rate of Acceleration (steeper line) C-D shows a deceleration ** the gradient of the line gives the acceleration OR Acceleration = change in speed Time taken Equations of motion: These can be used for any object moving at a steady rate of acceleration. If it’s not accelerating just use equation for mean speed! Symbol What it represents U Starting speed (m/s) V Final speed (m/s) A Acceleration (m/s2) T Time (Seconds) x Distance travelled (m) Foundation paper: you will need to be able to use V= u + at x= u+v T 2 Also remember that a mean speed can be calculated as any other mean – add two speeds together and divide by 2!! Higher paper: you will also be required to use x= ut +1/2 at2 V2 = u2 + 2 ax Momentum This is another way to measure motion. It is useful when explaining collisions between objects. The bigger the mass of object and the faster it is moving, the more momentum it has. You will have to be able to use the equation: Momentum = mass x velocity (kgm/s) (kg) (m/s) Because it is velocity in the equation, momentum also has direction. So it is positive in one direction and negative in the opposite direction. When something moving is brought to a stop, it has to have its momentum taken away from it. The more momentum it has, the bigger the force needs to be OR its momentum could be taken away by a smaller force acting for a longer length of time. The latter is achieved by crumple zones in vehicles OR by air bags for passengers. HIGHER Tier only: Force, energy loss in collisions, circular motion In any collision or explosion, momentum is conserved, This means: Momentum before = momentum after Collision collision Explosions: momentum before = 0 kgm/s as nothing is moving. After the explosion fragments move in all different directions, with equal sized momentums (which then all added together total zero). This means large masses have to have small velocities in order to have the same momentum as lighter masses with high velocities . This explains why smaller particles travel further after an explosion. In explosions, momentum is conserved ( same before and after), but energy can be lost (or changed into other non-useful forms). You have to use: Kinetic energy = 1/2mv2 To calculate the energy before the collision and then again calculate it after collision. Subtract values to find the change or loss in Kinetic energy. To calculate the force of impact in a collision, or the time taken for a collision, you will be expected to use Force x time = change in momentum during collision (N) (s) Kgm/s A force is needed to make an object change direction. If that force stays at right angles to the direction of motion of the object, the object will move in a circle. Direction of Motion Force force acts toward centre of circle Higher tier: Sling shots This led to the atomic structure we now accept. If the speed of the probe is u m/s, and the speed of the planet is v m/s The speed of the probe leaving the planet = Original speed + 2xspeed of planet U + 2v Foundation tier& higher: Atomic Structure: The original model of what was in the atom was put forward by Thomson. A solid sphere with electrons embedded in it. This became known as the Plum Pudding model of the atom. Rutherford tested this model in his alpha scattering experiment: This atom has: 3 electrons (negative charges) 3 proton (positives – atomic number) 4 neutrons (zero charge) Symbol: 7 Li Atomic mass = 7 3 Proton/Atomic Number = 3 (Atomic mass = 4 neutrons + 3 protons) Isotopes: these are atoms of the same element that have the same number of protons but different number of neutrons Eg 12 C 14 C 6 6 Both have 6 protons The first has 12 particles in the nucleus The second has 14 particles in the nucleus – due to 2 more neutrons. These are isotopes of carbon. Higher tier: atomic spectra When energy is added to an atom, the electrons in the outer shell become ‘excited’ – this extra energy allows them to move into s higher energy electron shell. When they return to their original energy shell, the atom emits the extra energy as light. As every atom has a different configuration of electrons, each atom emits its own specific spectrum of light. Foundation & higher: The chemical symbols below need to be known; Alpha: 4α Beta: 0β 2 -1 Electron: 0 e Helium: 4 He -1 2 Higher tier: Balanced decay equations; using radioactive decay to age the earth/solar system. You will be asked to complete equations like the one here: 226 Ra ..... Rn + 4 α 88 ..... 2 Top line: 226 particles in Radium = ? + 4 removed by alpha So Radon has 226-4 = 222 articles in its nucleus Bottom line: Radium has 88 protons, 2 of which are removed by the alpha emission, so radon has 88-2=86 protons. So the answer is: 226 Ra .222. Rn + 4 α 88 ...86 2 Nuclear fission: 235 U + 1 n 144 Ba + ....... Kr +1 n +1 n +energy 92 0 56 ...... 0 0 Top line: 235 +1 = 144 + ? +1+1 so ? = 90 Bottom line: 92 +0 = 56 +? =0: so ? = 36 235 U + 1 n 144 Ba + .90.... Kr +1 n +1 n +energy 92 0 56 ..36. 0 0 Nuclear fusion: 2H + 3 H 4 He + 1 n + energy 1 1 2 0 These are isotopes of hydrogen fusing together at high temperatures. Age of earth: Some radioactive elements that have been present in the earth since it was formed, have very long halflives. For example Uranium 235 has a half life of 700 000 000years (7x108) By examination of how much of the original material remains, and using its half life, we can work out how many half lives it has undergone and so find the age of the earth. (Just like carbon dating in P2) Foundation and Higher: Nuclear fission: When the uranium or plutonium atoms split and release energy, they leave behind other radioactive elements, some of which have a long half life. Making disposal a problem. Nuclear fusion: This diagram shows a chain reaction of neutrons splitting uranium atoms releasing huge quantities of energy. If the process is controlled you have a nuclear reactor. An uncontrolled chain reaction leads to a bomb. As we have so much hydrogen on earth – most of which is combined with oxygen in water. We have a potential source of energy. Unfortunately we have to raise its temperature so high that we, at present, put more energy into fusing hydrogen than we get out of the reaction. The Sun however has the high temperatures needed. Reaction: H+ 3H→ 4He+ 1n 2 1 1 2 0