IB Physics Glossary
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A 

Ampereis 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.

Antinodea region of maximum wave displacement. Amplitude $$2A$$.
 
Apparent brightness the energy from a star per second incident on 1m^{2} of the Earth's surface.(d)  
Apparent magnitude a measure of the brightness of a star as it appears from Earth. (d).  
Astronomical Unit is the average distance between Earth and Sun. (s)  
Attenuation is the drop in intensity as Xrays pass through a medium.  
Attenuation coefficient ยต, is a constant for a material at a given Xray energy which allows us to calculate Xray intensity, I, given any thickness of material x and incident Xray intensity I_{0} where $$I = I_0 e^{\mu x}$$.  
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 $$a = g \times \frac{m}{m+}$$ wherem_{} is the mass difference m_{2}m_{1}, and m_{+} is the sum of the masses m_{1}+m_{2}.  
Audiogram is a record of the variation of hearing with frequency for a patient.  
Avogadro's constant is the number of atoms in 12g of Carbon12 (6.02x10^{23}). (d)  