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# Physics A2

### Fields, particles and frontiers of physics Definitions

Question | Answer |
---|---|

electric field strength (E) | Force per unit positive charge, given by E = F/q, measured in N C–1. |

electric field | A region of space where a charged particle experiences a force. It goes from + to – . |

Coulomb’s law | The force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance between the charges. |

magnetic flux density (B) | A measure of the strength of the magnetic field as given by B = F/ILsinθ. Measured in tesla (T). |

magnetic flux (Φ) | The product of magnetic flux density and the area at right angles to the flux. Given by Φ = BA cos θ, measured in weber (Wb). |

magnetic flux linkage (NΦ) | The product of the magnetic flux and the number of turns on the coil it passes through. Given by magnetic flux linkage = NΦ, often measured in weber turns. |

tesla | Unit of magnetic flux density (T), e.g. 2 T. 1 T is the magnetic flux density that will produce a force of 1 N on 1 m of wire carrying a current of 1 A perpendicular to the direction of the magnetic field. |

weber | Unit of magnetic flux (Wb), e.g. 4 Wb. 1 Wb is the flux when a magnetic flux density of 1 T passes through an area of 1 m2 at right angles. |

Faraday’s law | The magnitude of the induced e.m.f. is equal to the rate of change of flux linkage. |

Lenz’s law | The direction of any induced current is in a direction that opposes the flux change that causes it. |

a.c. generator | A generator that, via the use of slip rings, produces an alternating e.m.f. and so an alternating current. |

transformer, step-down | A device that has a greater number of wire turns on the input side than the output side and so reduces the e.m.f. and increases the electric current on the output side. |

transformer, step-up | A device that has a greater number of wire turns on the output side than the input side and so increases the e.m.f. and reduces the electric current on the output side. |

capacitance (C) | The charge stored per unit potential difference, as given by C = Q/V. Measured in farads (F). |

farad | Unit of capacitance (F). 1 F is 1 C of charge stored per volt |

time constant (CR) | The time taken for the charge remaining on a capacitor to fall to 1/e of its original value. It can be found using the expression CR, measured in seconds (s). |

proton number | The number of protons inside a certain nucleus. Also known as the atomic number. |

nucleon number | The number of neutrons added to the number of protons inside a certain nucleus. An alternative name for mass number. |

isotopes | Nuclides with the same number of protons but different numbers of neutrons. |

half-life | The average time taken for the activity of a radioactive source to decrease to one half of its original value. |

binding energy | The energy required to separate an atom into its component parts. |

binding energy per nucleon | The average energy required to remove a nucleon from the nucleus. |

X-rays | A form of electromagnetic wave with wavelengths between 10–12 m and 10–7 m. Used in X-ray photography. |

intensity | The energy incident per square metre of a surface per second, measured in watts per metre squared (Wm–2). |

Doppler effect | The change in wavelength caused by the relative motion between the wave source and an observer. |

piezoelectric effect | The change in volume of certain crystals when a p.d. is applied across them. Alternatively the production of an e.m.f. when certain crystals are placed under stress. |

Olbers’ paradox | A paradox dealing with the finite or infinite nature of the universe. With an infinite number of stars in an infinite universe no matter which way you look you will always see a star. Therefore the night sky will be as bright as the day sky |

Hubble’s law | The speed of recession of a galaxy is directly proportional to its distance from the Earth. |

cosmological principle | On a large scale the universe is uniform (i.e. the universe is the same in all directions and of uniform density, provided a large enough volume is considered). |

critical density of the universe (ρc) | The average density of the universe above which the universe will collapse (closed universe) and below which the universe will expand forever (open universe). ρc = 9.5 × 10–27 kg m–3. |