Tuesday, 21 January 2020, 6:06 AM
Site: The Science Portal: 21C Science with Style
Course: The Science Portal: 21C Science with Style (21CScience)
Glossary: IB Physics Glossary
A

#### Absolute magnitude

M, is a measure of how bright an object appears or the apparent magnitude, when it is observed from a distance of 10 pc. (d).

#### Absorbed dose

D, is the amount of energy absorbed by a unit mass of material or body tissue where D = absorbed energy/mass. Its units are Jkg-1 or the Gray (Gy).

#### Acceleration

is the rate of change of velocity. (d.)
$a=\frac{\Delta v}{\Delta t} = \frac{v-u}{t}$.

#### Accommodation

is the process by which the lens in the eye can focus on both near and far objects. When we focus on distant objects the lens is longer and thinner due to the effect of the cilary muscles and the taut fibers. When we focus on nearby objects the lens becomes shorter and thicker due to contraction of the ciliary muscles.

#### Accuracy

tells us how close the measured value of a quantity is to its true value. An accurate measurement is "close" to a true value. An inaccurate measurement is "far" from a true value.

#### Acoustic impedance

of a substance is the product of medium density, $\rho$, and speed of sound waves, c, in the medium $Z= \rho \times c.$ Its unit is kgm-2s-1. Impedance matching occurs when the impedance of two media are equal. The incident waves are transmitted (100%) through the media. When there is a large difference in impedance between media most of the incident sound waves are reflected.

﻿occurs without any energy transfer to or from the gas.

A rapid expansion or compression is approximately adiabatic since $\Delta Q \approx 0$.

The 1st law of thermodynamics
$Q = \Delta U + W$
becomes
$0 = \Delta U + W$
or
$\Delta U = - W$.

An adiabatic expansion, $W > 0 \Rightarrow U < 0$, is a cooling process.

An adiabatic compression, $W < 0 \Rightarrow U > 0$, is a heating process.

#### Albedo

is the ratio of total solar energy reflected back from Earth into space to the total incident solar energy. (d)

#### Alpha decay

$^A_ZX \rightarrow~{}^{A-4}_{Z-2}Y + ^{4}_{2}\alpha$

or

$^A_ZX \rightarrow~{}^{A-4}_{Z-2}Y + ^{4}_{2}He$.

Where X is the parent nucleus, Y is the daughter nucleus, A is the number of nucleons and Z is the number of protons.
An example decay is
$^{241}_{95}Am \rightarrow~{}^{237}_{93}Np + ^{4}_{2}\alpha$.

Properties of alpha particles include
• discrete energy spectrum
• +2 charge
• relatively heavy
• strongly ionizing
• short range in air (cm)/low penetration
• can be blocked by a sheet of paper
• deflected by magnetic-fields and electric-fields (in direction opposite to beta particles)

#### Ammeter

measures current flowing through a component. It is placed in series with the component. An ideal ammeter has zero resistance so that it has no effect on the current flowing.