## IB Physics Glossary

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### E

#### 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 $V$:

$q V = \frac{1}{2}m v^2$.

 Keyword(s): 5.1

#### Elevator weight

is used to show how the reaction force, between a person standing in an elevator and the floor of the elevator, varies with up/down acceleration of the elevator:

Note, the actual weight of the person does not change but an increase or decrease in the reaction force produces a change on the weighing scale in elevator.
 Keyword(s): 2.2

#### Emf

or electromotive force, is the total energy supplied by a cell/battery per unit charge as it flows through the cell/battery. (d)
 Keyword(s): 5.2

#### Emissitivity

is the ratio of power emitted (per unit area) by a body to the power emitted (per unit area) by a black body of the same dimensions at the same temperature
or

the ratio of power emitted by a body to the power emitted if it were a black body. (d)
 Keyword(s): 8.5

#### Energy density

is the energy liberated per unit mass of fuel consumed. Its unit is Jkg-1.(d)
 Keyword(s): 8.2

#### Entropy

is a system property that expresses the degree of disorder in the system.

 Keyword(s): 10.2

#### Equilibrium position

is the position where a particle would remain at rest if not disturbed. (d)
 Keyword(s): 4

#### Escape velocity

is the velocity that an object needs at the surface of the planet with sufficient kinetic energy to escape the gravitational attraction of the planet.
$v_{esc} = \sqrt{\frac{2GM}{R}} = \sqrt{2Rg}$
where G is the universal gravitational constant, M is the mass of the planet, R is the radius of the planet and g is the acceleration due to gravity.

For a Black Hole, the escape velocity equals c, the velocity of light in empty space. The above equation becomes
$c= \sqrt{\frac{2GM}{R}}$
and
$R= R_s = \frac{2GM}{c^2}$
where Rs is the Schwarzchild radius.
 Keyword(s): 9.4

#### Explosive collisions

occur when the total kinetic energy of a system increases.
 Keyword(s): 2.3

### F

#### Far point

is the most distant point from the eye for which the eye can focus on (assumed to be $\infty$).
 Keyword(s): A1

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