## IB Physics Glossary

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### A

#### Ampere

is the SI unit of electrical current and is defined through the magnetic force acting between two parallel wires. If the magnetic force between two wires separated by 1m, each of length 1m, carrying equal currents, equals 2 x 10-7 N, then the current in each wire equals 1 Ampere.

#### Anti-node

a region of maximum wave displacement. Amplitude $$2A$$.
 Keyword(s): 9

#### Apparent brightness

the energy from a star per second incident on 1m2 of the Earth's surface.(d)
 Keyword(s): E2

#### Apparent magnitude

a measure of the brightness of a star as it appears from Earth. (d).
 Keyword(s): E3

#### Astronomical Unit

is the average distance between Earth and Sun. (s)
 Keyword(s): E1

#### Attenuation

is the drop in intensity as X-rays pass through a medium.
 Keyword(s): I2

#### Attenuation coefficient

µ, is a constant for a material at a given X-ray energy which allows us to calculate X-ray intensity, I, given any thickness of material x and incident X-ray intensity I0 where $$I = I_0 e^{-\mu x}$$.
 Keyword(s): I2

#### Atwood's Machine

is a pulley system with a difference in mass between the two sides of the pulley:

The acceleration of the pulley, from Newton's 2nd law, is given by

$$a = g \times \frac{m-}{m+}$$
where

m- is the mass difference m2-m1, and m+ is the sum of the masses m1+m2.
 Keyword(s): 2.2

#### Audiogram

is a record of the variation of hearing with frequency for a patient.
 Keyword(s): I1

#### Avogadro's constant

is the number of atoms in 12g of Carbon-12 (6.02x1023). (d)
 Keyword(s): 3.1

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