Definitions - Unit 3

Flashcards on Physics Definitions

Period (T)

Time taken for one complete circuit.

Frequency (f)

The number of circuits or cycles per second.

Radian

A unit of measurement of angles equal to about 57.3°, equivalent to the angle subtended at the centre of a circle by an arc equal in length to the radius. UNIT: rad

Angular velocity (ω)

For an object describing a circle at uniform speed, the angular velocity ω is equal to the angle θ swept out by the radius in time ∆t divided by t (ω =∆θ / ∆t) UNIT: rad s-1

Simple harmonic motion (shm)

Shm occurs when an object moves such that its acceleration is always directed toward a fixed point and is proportional to its distance from the fixed point. (a = – ω 2x)

Period (T) for an oscillating body

The time taken for one complete cycle.

Amplitude (A) of an oscillating object

The maximum value of the object’s displacement (from its equilibrium position).

Phase

The phase of an oscillation is the angle (ωt + ε) in the equation x = A cos (ω t + ε). [ε is called the phase constant.] UNIT: rad

Frequency (f)

The number of oscillations per second. UNIT: Hz

Free oscillations [Natural oscillations]

Free oscillations occur when an oscillatory system (such as a mass on a spring, or a pendulum) is displaced and released. [The frequency of the free oscillations is called the system’s natural frequency.]

Damping

Damping is the dying away, due to resistive forces, of the amplitude of free oscillations.

Critical damping

Critical damping is the case when the resistive forces on the system are just large enough to prevent oscillations occurring at all when the system is displaced and released.

Forced oscillations

These occur when a sinusoidally varying ‘driving’ force is applied to an oscillatory system, causing it to oscillate with the frequency of the applied force.

Resonance

If, in forced vibrations, the frequency of the applied force is equal to the natural frequency of the system (e.g. mass on spring), the amplitude of the resulting oscillations is large. This is resonance.

Ideal gas

An ideal gas strictly obeys the equation of state pV = nRT, in which n is the number of moles, T is the kelvin temperature and R is the molar gas constant. R = 8.31 J mol-1 K-1. With the exception of very high densities a real gas approximates well to an ideal gas.

The mole

The mole is the S.I. unit of an ‘amount of substance’ . It is the amount containing as many particles (e.g. molecules) as there are atoms in 12 g of carbon-12.

Avogadro constant

This is the number of particles per mole. (NA = 6.02 × 1023 mol-1)

Internal energy (U) of a system

This is the sum of the kinetic and potential energies of the particles of a system.

Heat (Q)

This is energy flow from a region at higher temperature to a region at lower temperature, due to the temperature difference. In thermodynamics we deal with heat going into or out of a system. It makes no sense to speak of heat in a system.

Work (W)

If the system is a gas, in a cylinder fitted with a piston, the gas does work of amount p∆V when it exerts a pressure p and pushes the piston out a small way, so the gas volume increases by ∆V. Work, like heat, is energy in transit from (or to) the system.

First law of thermodynamics

The increase, ∆U, in internal energy of a system is ∆U = Q – W in which Q is the heat entering the system and W is the work done by the system.

Specific heat capacity (c)

The heat required, per kilogram, per degree celsius or kelvin, to raise the temperature of a substance. UNIT: J kg-1 K-1 or J kg-1 °C-1

Alpha (α) radiation

Fast moving particles, helium nuclei, ejected from certain radioactive nuclei.

Beta (β) radiation

Electrons with speeds just less than the speed of light, ejected from certain radioactive nuclei.

Gamma (γ) radiation

Photons of high energy (high frequency, short wavelength) ejected from radioactive nuclei.

A X notation
Z

X is the chemical symbol of the element, A the mass number (number of protons plus number of neutrons) and Z the atomic number (number of protons).

Half-life (T1/2) of a nuclide

The time taken for the number of radioactive nuclei, N (or the activity A) to reduce to one half of the initial value. Unit: s or any unit of time

Activity (A)

The rate of decay (number of disintegrations per second) of a sample of radioactive nuclei. Unit: becquerel (Bq) = s-1

Decay constant (λ)

The constant which appears in the exponential decay law N = N0 e−λt and determines the rate of decay (the greater λ, the more rapid the rate of decay). λ is related to half-life by λ = ln2 / T1/2Unit: s-1 or any (unit of time)-1

Unified atomic mass unit (u)

The unified atomic mass unit is defined as exactly one twelfth of the mass of one atom of carbon-12. Thus one atom of carbon-12 has a mass of exactly 12 u. (1 u = 1.66 × 10-27 kg)

Binding energy of a nucleus

The energy that has to be supplied in order to dissociate a nucleus into its constituent nucleons. [It is therefore not energy which a nucleus possesses.] Unit: J [or MeV]

Conservation of mass-energy

Energy cannot be lost or gained, only transferred from one form to another. We can measure the energy in a body by multiplying its mass by c2.