## IB Physics Glossary

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### O

#### Ohm's law

states that current is proportional to voltage, at constant temperature, $$I \prop V$$. (d)
The graph should be a straight line passing through the origin.
For non-ohmic conductors current is not proportional to voltage, $$I \cancel{ \prop} V$$. The graph is non-linear. Examples of non-Ohmic conductors include

• filament light bulbs
as $$I \uparrow, T \uparrow, R \uparro \Rightarrow$$ gradient is not constant, or, as $$I \downarrow, T \downarrow, R \downarrow \Rightarrow$$ gradient is not constant.
Typical graphs can be
y-axis: I, x-axis: V, $$R \uparrow \Rightarrow \frac{V}{I} \uparrow \Rightarrow$$ gradient decreases;

y-axis: V, x-axis: I, $$R \uparrow \Rightarrow \frac{V}{I} \uparrow \Rightarrow$$ gradient increases.
• thermistors
NTC (negative temperaure coefficient): $$T \uparrow \downarrow R \downarrow \uparrow$$;
PTC (positive temperature coefficent): $$T \uparrow \downarrow R \uparrow \downarrow$$.

• Light dependent resistors (LDRs)
brightness $$\uparrow \downarrow$$, $$R \downarrow \uparrow$$;

• Strain gauge
a variation in strain of a material (deformation per unit length due to the effect of an applied load/mass) produces a variation in electrical resistance of material

 Keyword(s): 5.2

#### One Mole

of substance has as many molecules as there are atoms in 12g of Carbon-12. (d)
 Keyword(s): 3.1

#### Oppenheimer-Volkoff limit

is approximately 2-3 solar masses for mass of core for Super Red Giant;
for core masses < 2-3 solar masses (above 1.4 solar masses) $$\rightarrow$$ Neutron star
for core masses > 2-3 solar masses $$\rightarrow$$ Black hole.
 Keyword(s): E5

#### Optically active

substances rotate the plane of polarization of the incident light. The amount of rotation may depend on the concentration of the substance/solution and the amount/length/thickness. Keyword(s): 4

#### Orbital Motion

occurs when the gravitation provides the centripetal force for circular orbital motion: $$F = G \frac{Mm}{r^2}=\frac{mv^2}{r}$$.
 Keyword(s): 9.4