## IB Physics Glossary

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### G

#### 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

### H

#### Half-life

$$t_{\frac{1}{2}}$$, is the time required for the initial activity of a radioactive sample to be reduced by a factor of 2 or the time requird for the initial number of radioactive nuclei to be reduced by a factor of 2. (d)

 Keyword(s): 7.2

#### Half-value thickness

is the length of material through which X-rays must travel to reduce their intensity by 2 where $$x_{\frac{1} {2}} = \frac{ln 2} {\mu}$$, where $$\mu$$ is the attenuation coefficient.
 Keyword(s): I2

#### Heat exchanger

allows the nuclear reactions to occur in a place that is sealed off from the rest of the environment. The reactions increase the temperature in the core. This thermal energy is transferred to heat water, and the steam that is produced turns the turbines.
 Keyword(s): 8.3, 8.4

#### Hubble's law

states the recessional speed, $$v$$, of a galaxy is proportional to its distance, $$r$$, from the Earth or galaxies move away from each other with a speed proportional to their separation: $$v \propto r$$ or $$v=H \times r$$, where $$H$$ is Hubble's constant.
 Keyword(s): E6

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