# Unit 4 physics definitions

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Principle of conservation of momentum
In any interaction between bodies, linear momentum before equals linear momentum after, provided no external force acts on the bodies
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Elastic collision
a collision in which kinetic energy is conserved (eg gas molecules)
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Inelastic collision
a collision in which KE is not conserved (eg a car crash)
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Principle of conservation of energy
Energy cannot be created or destroyed but can be changed from one form to another. (Remember mass and energy are interchangeable by E=mc2 )
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centripetal force
force required to maintain circular motion
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centripetal acceleration
acceleration of an object moving in a circular path (always perpendicular to velocity so no change of speed)
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Electric field
region in which a charge will experience an electrostatic force
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Electric field strength
Force acting per unit charge
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Coulomb's Law
The force between two point charges is directly proportional to the product of the charges and indirectly proportional to the square of the separation.
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Uniform field
Field strength is constant in magnitude and direction at all points (shown by parallel field lines pointing in same direction)
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Field around a point (charge/mass) shown by field lines radiating (outwards from +ve charge, inwards towards a negative charge)
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Capacitance
The charge stored per unit volt by a capacitor
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time constant
the time taken for the charge (or p.d.) of a capacitor in series with a resistor to decay to 37% of its initial value. (37% = 1/e)
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magnetic flux density
magnetic flux density
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magnetic flux
the product of the flux density through a loop and the area of the loop, F = BA
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the product of the flux through a loop of a coil and the number of turns in that coil
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Induced emf in a loop of a conductor is directly proportional to the rate of change of fluxed linked by the loop.
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Lenz's Law
the induced current in a conductor will set up a magnetic field which will oppose the changes which are creating the induced current
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nucleon number (mass number)
number of neutrons + number of protons in a nucleus
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thermionic emission
process in which electrons are released from a heated filament
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annihilation
process in which particles and antiparticles combine to form a photon of energy (mass converted to energy)
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particle made of quarks (baryon or meson). They are held together by the strong nuclear force
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Baryon
particle made of three quarks (or three antiquarks) (eg proton or neutron)
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Meson
particle made of one quark and one antiquark (eg pion)
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Lepton
a set of fundamental particles which do not interact by strong nuclear force (eg electron, electron neutrino)
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## Other cards in this set

### Card 2

#### Front

Elastic collision

#### Back

a collision in which kinetic energy is conserved (eg gas molecules)

### Card 3

#### Front

Inelastic collision

### Card 4

#### Front

Principle of conservation of energy

### Card 5

#### Front

centripetal force