# G485 Definitions

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Electric field strength (E)
Force per unit positive charge (F/q)
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Magnetic flux density (B)
Density of the magnetic field lines per unit area.
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Tesla
Magnetic flux density when a wire, length 1m, carrying a current of 1 ampere at right angles to the field experiences a force of 1m
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Magnetic flux (Φ)
Magnetic flux density x area perpendicular to the flux (Φ=BA)
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Weber
1Wb is the magnetic flux when a magnetic flux density of 1T passes through an area of 1m^2 at right angles
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Magnetic flux x number of turns on the coil it passes through (measured in Weber turns)
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Lenz's Law
The induced e.m.f is always in such a direction as to oppose the change that caused it
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Induced e.m.f is directly proportional to the rate of change of flux linkage
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Capacitance
Charge stored per unit potential difference
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1C of charge stored per volt
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Time constant of a circuit (CR)
Time taken for the charge remaining on a capacitor to fall to 1/e of its original value
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Proton number
Number of protons in a nucleus. Also known as the atomic number
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Nucleon number
Number of neutrons and protons in a nucleus. Also known as mass number
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Isotopes
Nuclides with the same number of protons but different numbers of neutrons
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Half-life
Average time taken for the number of undecayed atoms to halve
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Activity
Number of radioactive decays per unit time. Measured in Becquerels. A=λN
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Decay constant
The probability of radioactive decay λ=A/N
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Binding energy
Energy required to separate all the nucleons in the nucleus
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Binding energy per nucleon
Average energy required to remove a nucleon from the nucleus
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Intensity
Power per unit cross sectional area
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Astronomical Unit (AU)
Average distance from the earth to the sun 1AU= 1.496x10^11m
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Parsecs (pc)
1pc= 3.086x10^16m
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Light Years (ly)
1ly = 9.461x10^15m the distance that EM waves travel through a vacuum in 1 year
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With an infinite number of stars in an infinite universe, no matter what direction you look you will always see a star in your line of sight. Therefore the night sky will be as bright as day sky
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Hubble's Law
The speed of recession of the galaxy is directly proportional to its distance from earth
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Critical Density
The average density of the universe above which the universe would collapse and below it would expand forever
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Cosmological principle
On a large scale the universe is homogeneous and isotropic, so doesn't have a centre
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Doppler effect
The change in wavelength caused by the relative motion between the wave source and the observer
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piezoelectric effect
The change in volume of certain crystals when a p.d is applied across them.
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## Other cards in this set

### Card 2

#### Front

Density of the magnetic field lines per unit area.

#### Back

Magnetic flux density (B)

### Card 3

#### Front

Magnetic flux density when a wire, length 1m, carrying a current of 1 ampere at right angles to the field experiences a force of 1m

#### Back ### Card 4

#### Front

Magnetic flux density x area perpendicular to the flux (Φ=BA)

#### Back ### Card 5

#### Front

1Wb is the magnetic flux when a magnetic flux density of 1T passes through an area of 1m^2 at right angles

#### Back 