## IB Physics Glossary

Browse the glossary using this index

Special | A | B | C | D |

**E**| F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ALL

## E |
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## Eclipsing binary starsare identified by a periodic dip in their combined brightness-time curve. | |

## Effective half-life the time taken for the activity of a medical radioactive isotope to reduce by half, taking into account both physical and biological removal of activity from the body such that $$\frac{1}{T_E} = \frac{1}{T_P} + \frac{1}{T_B}$$ or $$T_E = \frac{T_P \times T_B} {T_P + T_B}$$ where T is the effective half-life, _{E}T is the physical half-life and _{P}T is the biological half-life._{B}Note the decay constants add: $$\lambda_E = \lambda_P + \lambda_B$$. | |

## Efficiency is the ratio of useful power of a system to the input power or the ratio of useful energy transformed to the total energy input. (d) Approximate overall energy efficiencies for different types of power station are Coal: 35%, Gas: 45% & Oil: 38%. | |

## Elastic collisions occur when the total kinetic energy of a system remains constant. | |

## Electric current is defined in terms of force per unit length between two parallel current-carrying conductors. (d) The electric current flowing in a circuit is the rate of flow of charge, $$I = \frac{\Delta q}{\Delta t}$$. | |

## Electric field strengthis the force per unit charge exerted on a positive test charge placed in the field.
$$E = \frac{F}{q}=\frac{k\frac{Qq}{r^2}}{q}=k \frac{Q}{r^2}$$. Its unit is NC ^{-1}. It is a vector quantity and is always directed from the positive charge (+) to the negative charge (-) and points tangentially ($$\perp$$) to the surface of the charge.
For charges of unequal magnitude, the field lines are shifted towards the lower magnitude charge. The field line pattern is pear-shaped with the "apex" closer to the lower magnitude charge. The field around a charged conductor At equilibrium, the charge and electric field follow these guidelines:- the excess charge lies only at the surface of the conductor
- the electric field is zero within the solid part of the conductor
- the electric field at the surface of the conductor is perpendicular to the surface
- charge accumulates, and the field is strongest, on pointy parts of the conductor
For a sphere of radius R the electric field varies with distance r from centre of sphere as shown below. |

## Electric Potential Difference is the work done per unit charge in moving a small positive charge between two points. (d) | |

## Electrical circuit symbols | |

## Electrical powercan be expressed in three ways $$P = V I$$ $$P = I^2 R$$ - used for power dissiapation due to heating in the transmission of electricity. $$P = \frac{V^2}{R}$$. | |

## Electronvolt is the work done in moving an electron through a potential difference of 1 volt. (d) A charge, q, gains kinetic energy $$q V$$ when it is accelerated through a potential difference $$V$$: $$q V = \frac{1}{2}m v^2$$. | |