## IB Physics Glossary

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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

### G

#### g versus E

 Gravitational Electric Force $$F = G \frac{m_1 m_2}{r^2}$$ $$F = k \frac{q_1 q_2}{r^2}$$ Constant G k Field strength $$g = \frac{F}{m} = G \frac{M}{r^2}$$$$F = m g$$ $$E = \frac{F}{q} = G \frac{Q}{r^2}$$$$F = q E$$ Other details Force is always attractive Force can be attractive (for like charges) or repulsive (for opposite charges) Acts on particles with mass with electric charge
 Keyword(s): 6.2

#### Galactic cluster

is a collection of galaxies grouped together due to the gravitational attraction between them. (d)
 Keyword(s): E1

#### Galactic supercluster

is a group of galactic clusters. (d)
 Keyword(s): E1

#### Galaxy

is a large group of stars gravitationally bound together (approx. 1010 stars, diameter 105 ly). (d)
 Keyword(s): E1

#### Gamma decay

$$^A_ZX^* \rightarrow~{}^{A}_{Z}X + ^{0}_{0}\gamma$$

where X* is an excited nucleus, X a de-excited nucleus, A is the number of nucleons and Z is the number of protons. A sample gamma decay is
$$^{40}_{18}Ar^* \rightarrow~{}^{40}_{18}Ar + ^{0}_{0}\gamma$$

Properties of gamma particles include
• discrete energy spectrum
• zero charge
• mass-less (but have momentum!)
• weakly ionizing
• long range in air (light-years)/high penetration
• can be blocked by thick lead sheets ( approx. 10 cm)
• not deflected by magnetic-fields and electric-fields

 Keyword(s): 7.2

#### Gas laws

can be found, for an ideal gas, using the ideal gas equation $$P V=n R T$$. Note T is the absolute temperature and its unit is Kelvin (ºC + 273). Typical units: pressure P, Pa of kPa; volume V, m3 .

1. For a fixed temperature of gas (and amount of gas, ie. n fixed)
$$P V = constant$$ or $$P \propto \frac{1}{V}$$.
Pressure is inversely proportional to Volume.

2. For a fixed volume of gas (and amount of gas, ie. n fixed)
$$P = constant \times T$$ or $$P \propto T$$.
Pressure is proportional to Temperature.

3. For a fixed pressure of gas (and amount of gas, ie. n fixed)
$$V = constant \times T$$ or $$V \propto T$$.
Volume is proportional to Temperature.

More generally one can write

$$\frac{P_2 V_2}{P_1 V_1} = \frac{n_2 T_2}{n_1 T_1}$$.
 Keyword(s): 3

#### Gravitation versus Electric Potential  Keyword(s): 9.2,9.3

#### Gravitational field strength

is the force per unit mass on a small (test) mass at the point. (d)

$$g = \frac{F}{m}=\frac{G\frac{Mm}{r^2}}{m}=G \frac{M}{r^2}$$.

Its unit is Nkg-1. It is a vector quantity and is always directed towards the centre of the mass and points tangentially ($$\perp$$) to its surface. To compare the gravitational field strength for different masses/planets

$$\frac{g_x}{g_y}=\frac{M_x}{M_y} \left(\frac{r_y}{r_x}\right)^2$$.

 Keyword(s): 6.1

#### Greenhouse effect

The atmosphere is transparent to many frequencies of electromagnetic radiation. Much of the power received from the Sun is in the visible and ultraviolet regions. This causes the surface of the Earth to warm up and radiate in the infrared (heat). Some of this infrared radiation is absorbed by gases in the atmosphere, causing the atmosphere to warm up, and re-radiated in all directions. The net effect is that the atmosphere and the surface of the Earth are warmed.
 Keyword(s): 8.5

#### Greenhouse gases

are gases in the atmosphere that absorb infrared radiation. The principle greenhouse gases are carbon dioxide ($$CO_2$$), methane ($$NH_4$$), water vapour ($$H_2O$$), and nitrous oxide ($$NO_2$$). Ozone and chlorofluorocarbons (CFCs) also contribute to the greenhouse effect.
 Keyword(s): 8.5