## IB Physics Glossary

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### 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

#### Hybrid vehicles

use electric motors with a petrol engine as back-up to provide additional power when necessary. Sophisticated computerized systems switch from the electric motor to the petrol engine and back as required.
 Keyword(s): 8.6