## IB Physics Glossary

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### E

#### Eclipsing binary stars

are identified by a periodic dip in their combined brightness-time curve.
 Keyword(s): E2

#### Effective half-life

the time taken for the activity of a medical radioactive isotope to reduce by half, taking into account both physical and biological removal of activity from the body such that

$\frac{1}{T_E} = \frac{1}{T_P} + \frac{1}{T_B}$

or

$T_E = \frac{T_P \times T_B} {T_P + T_B}$

where TE is the effective half-life, TP is the physical half-life and TB is the biological half-life.

Note the decay constants add:

$\lambda_E = \lambda_P + \lambda_B$.
 Keyword(s): I3

#### Efficiency

is the ratio of useful power of a system to the input power or the ratio of useful energy transformed to the total energy input. (d)

Approximate overall energy efficiencies for different types of power station are Coal: 35%, Gas: 45% & Oil: 38%.
 Keyword(s): 2.3, 8.1

#### Elastic collisions

occur when the total kinetic energy of a system remains constant.
 Keyword(s): 2.3

#### Electric current

is defined in terms of force per unit length between two parallel current-carrying conductors. (d)

The electric current flowing in a circuit is the rate of flow of charge, $I = \frac{\Delta q}{\Delta t}$.
 Keyword(s): 5.1

#### Electric field strength

is the force per unit charge exerted on a positive test charge placed in the field.

$E = \frac{F}{q}=\frac{k\frac{Qq}{r^2}}{q}=k \frac{Q}{r^2}$.

Its unit is NC-1. It is a vector quantity and is always directed from the positive charge (+) to the negative charge (-) and points tangentially ($\perp$) to the surface of the charge. Note:
1. An attractive force
2. A repulsive force (no field lines at centre)
3. Positive point charge to ground
4. Parallel plates (no edge effects) with constant electric field For charges of unequal magnitude, the field lines are shifted towards the lower magnitude charge. The field line pattern is pear-shaped with the "apex" closer to the lower magnitude charge.

The field around a charged conductor

At equilibrium, the charge and electric field follow these guidelines:
• the excess charge lies only at the surface of the conductor
• the electric field is zero within the solid part of the conductor
• the electric field at the surface of the conductor is perpendicular to the surface
• charge accumulates, and the field is strongest, on pointy parts of the conductor
[source, http://physics.bu.edu/~duffy/py106/Electricfield.html]

For a sphere of radius R the electric field varies with distance r from centre of sphere as shown below. #### Electric Potential Difference

is the work done per unit charge in moving a small positive charge between two points. (d)
 Keyword(s): 5.1

#### Electrical circuit symbols Keyword(s): 5.2

#### Electrical power

can be expressed in three ways

$P = V I$

$P = I^2 R$ - used for power dissiapation due to heating in the transmission of electricity.

$P = \frac{V^2}{R}$.
 Keyword(s): 5.1

#### Electronvolt

is the work done in moving an electron through a potential difference of 1 volt. (d)

A charge, q, gains kinetic energy $q V$ when it is accelerated through a potential difference $V$:

$q V = \frac{1}{2}m v^2$.

 Keyword(s): 5.1

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