Physics P2

Physics P2

1. Motion

   • acceleration=V-U/T
   • F=Mxa
   • Speed=distance travelled in metres/time taken in seconds
   • Stopping distance-driver
     • Braking distance-road,weather conditions

2. Work is done on an object when a force makes the object move

3. Electrons have a negative charge, when taken off the material it is left positive, but the resisting material is negative

4. Ohm's law states that the current through a resistor at constant tempreture is directly proportional to the potenial dfference across the resistor

5. Potential difference-a measure of the work done or energy transferred to the lamp by each coulomb of chrge that passes through it.

   • Unit of potential difference is volt (V)

6. Filament bulb: resistance increases with increase of the filament tempreture

   • Diode: 'forward' resistance low; 'reverse' resitance high
     • Thermistor: resistance decreases if its temptreture increases
       • LDR: resistance decreases if the light intensity on it increases

7. Mains electricty

   • Alternating currents repeatedly reverse their direction
   • A fuse contains a thin wire that heats up and lets if too much current passes through it.
     • A circuit breaker is an electromagnetic switch that opens (i.e 'trips') and cuts the current off if too much current passes through it

8. Radioactivity

   • Gamma
     • Stopped by thick lead and has an unlimited range in air
     • Electromagnetic
     • Weak ionising
     • Not deflected by electromagnetic and magnetic fields
     • Tracers in medicine- injected, the source needs a half life of a few hours, so the patient isn't exposed
   • Beta
     • A neutron in the nucleus changes into a proton and an electron
     • The electron created in the nucleus is instantly emitted
     • The proton stays in the nucleus so the atomic number goes up by one and the mass number is unchanged. The electron is instantly emitted.
     • Stopped by thin metal, a metre of air
     • Thickness monitoring-in the manufacture of things, the source needs a half life of many years
   • Alpha
     • Positive charge, because some of the alpha particles rebound, they must be repelled by another positive charge
     • The nucleus loses 2 protons and 2 neutrons
     • 2 protons and 2 neutrons are emitted as alpha particle
       • The atomic number then goes down by 2
     • Stopped by a few centimetres of air
     • Smoke alarms-half life of several years
   • Radioactive decay is a random event-we cannot predict or influence when it happens
   • Isotopes-atoms with the same number of protons but different numbers of neutrons. Therefore they have the same atomic numbers.
   • Half life
     • The average time it takes for the number of nuclei of the isoptopes in a sample to halve
     • The activity of a radioactive source is number of nuclei that decay per second
     • The number of atoms of a radioactive isotope and the activity both dercease by half every half-life
     • Radioactive dating-we need a radioactive isotope that is present in the sample which has a half-life about the same as the age of the sample

9. Energy from the nucleus

   • Nuclear fission is the splitting of an atomic nucleus
     • Release of 2-3 neutrons
     • Happens when a neutron hits a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits
     • A chain reaction occurs
     • Nuclear reactors: control rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission
   • Nuclear fusion is the process of forcing two nuclei close enough together so they form a single larger nucleus
     • Energy is released when two light nuclei are used together
   • Nuclear issues
     • Radon gas is an alpha emitting isotope that seeps into houses.
   • The universe was formed by hydrogen and helium
     • Distance between stars is millions times greater than between planets
       • The distance between galaxies is millions times greater than between stars within galaxies.
   • A protostar is a gas and dust cloud in space that can go on to form a star
     • Low mass star:Protostar, mainsequence star, red giant, white dwarf, black dwarf
     • High mass star: protostar, main sequence star, red super giant, supernova, black hole if sufficient mass
     • The sun will become a black dwarf
     • A supernova is the explosion of a supergiant after it collapses
     • Elements as heavy as iron are formed inside stars as a result of nuclear fusion
       • Elements heavier than iron are formed in supernovas, along with lighter elements
         • The sun and rest of the solar system were formed from the debris of a supernova