Physics definitions A levels

all the definitions for physics a levels

Electric field

A region of space in which a force acts on a stationary charge

Electric field strength

Electric force per unit positive charge exerted on a test charge placed at that point.

Coulomb’s Law

Electrostatic force between 2 point charges is directly proportional to the product of the charges and inversely proportional to the square of the separation. The direction of the force acts along the line joining the two point charges.

Electric potential

Work done per unit positive charge in bringing a small test charge from infinity to that point.

Equipotential lines

Lines joining points in a field that have the same potential, they meet perpendicular to the electric field lines

Angular displacement

Angle through which an object turns

Angular velocity

Rate of change of angular displacement of a radius that joins the body to the centre of a circle

Kinetic theory of gases

1. No forces of attraction or repulsion between atoms unless they are in collision with each other or the walls of the vessel

2. Gas molecules are in constant, random motion and obey Newton’s laws of motion

3. Time of collisions is negligible compared with time between collisions

Ideal gas

Hypothetical gas that obeys the equation of state (pV=nRT0 for an ideal gas perfectly for all pressure p, volume V, amount of substance, n and temperature T

Thermal equilibrium

When two objects in thermal contact are in thermal equilibrium, there is no net heat transfer between them, they are at same temp

Principle of superposition

When two or more waves of the same type meet at a point at the same time, the displacement of the resultant wave is the vector sum of displacements of individual waves at that point at that time.

Stationary waves

When two progressive waves of the same type that are travelling in opposite directions with the same speed, frequency and amplitude meet, they undergo superposition to form stationary waves

Diffraction

Spreading of waves through a gap or an obstacle

Rayleigh criterion

For the two patterns to be just distinguishable, the central maximum of one must lie on the first minimum of the other

Interference

Two sources need to be coherent, waves produced by the sources have a constant phase difference

Two source

1. Waves must overlap

2. Waves must be coherent

3. Waves must have equal amplitudes

Transverse

Displacement of the particles in the wave are at right angles to the direction of transfer of energy

Longitudinal

Displacement of the particles in the wave are along the direction of transfer fof energy of the wave

What affects the resistance of conductor

1) No. of charge carrying conductors, n → decreases R
2) Thermal vibrations of the lattice structure → Increases R

Electric current

Flow of charged particles

Potential difference

Work done per unit charge when electrical energy is transferred to non electrical energy when the charge passes from one point to another

E.m.f.

Work done per unit charge when non electrical energy is transferred to electrical energy when the charge moves round one complete circuit

Resistance

Ratio of potential difference across the conductor to the current passing through it

Resistivity

Relationship between the dimensions of a specimen of a material and its resistance that is constant at constant temperature, calculated by p = RA/L where R is resistance, A is cross sectional area, L is the length

Polarised wave

Vibrations / oscillations of the wave are restricted to only one direction, in the plane normal to the direction of energy transfer

Transverse waves

Vibrations are perpendicular to the direction of transfer of energy of the wave.

Longitudinal waves

Vibrations are parallel to the direction of transfer of energy of the wave

Progressive waves

Energy is carried from one point to another by means of vibrations or oscillations within the waves, without transporting matter

Intensity

Rate of energy transmitted per unit area perpendicular to the wave velocity

Photon

Discrete packet of energy of electromagnetic radiation

Work function energy

Minimum energy of photon to cause emission of electron from surface of a metal

Threshold frequency

Lowest frequency of emi that gives rise to the ejection of electrons from the metal surface

Threshold wavelength

Highest wavelength of emi radiation that gives rise to the ejection of electrons from the metal surface

Why the energy levels are negative (quantum)

1. Energy level at n = infinity is 0eV

2. Electron bonded to the nucleus by electric attractive force since the neutron and electron are oppositely charged

3. Atom gains energy to cause electron to break free, hence energy levels are negative

Nuclear fusion

Combining of two nuclei of low nucleon number to produce a larger nucleus with the release of energy.

Nuclear fission

Splitting of nuclei of high nucleon number to produce a smaller nucleus with the release of energy and neutrons.

Background radiation

Detected by radiation counter when no radioactive sources are nearby

Activity

Number of nuclear disintegrations per unit time of the nuclei

Decay constant

The constant probability of decay per unit time of a nucleus

Half life

The average time taken for the initial number of nuclei of thatt particular radioactive nuclide to reduce to half of its initial concentration

Force

Rate of change of momentum

Centre of gravity

Point at which the weight may seem to act

Hooke’s Law

Force is directly proportional to extension, provided that the elastic limit has not been exceeded

Upthrust

Vertically upward force exerted by surrounding fluid when a body is submerged fully or partially in a fluid

Equal in magnitude and opposite in direction to the weight of fluid displaced by the body

Object in equilibrium

Net force must be 0 in any direction

Net torque must be zero about any axis of rotation

Moment

Product of the magnitude of force and perpendicular distance of the force from the pivot

Torque

Product of one of the forces and the perpendicular distance between the forces

Magnetic flux density ****

Magnetic flux density of a magnetic field is defined as the magnetic force per unit current per unit length acting on a straight current carrying conductor placed perpendicular to a uniform magnetic field

Magnetic flux

Product of an area and the component of the magnetic flux density perpendicular to that area

Magnetic flux linkage

Product of the magnetic flux passing through the coil and the number of turns on the coil

Faraday’s Law

Magnitude of induced e.m.f is directly proportional to the rate of change of magnetic flux linkage

Lenz Law

Induced current is in a direction so as to produce effects which oppose the change in magnetic flux

Principle conservation of energy

Energy can neither be created nor destroyed in any process, it can be transformed from one form to another and transferred from one body to another but the total amount remains constant

Principle of moments

When a system is in equilibrium, the sum of clockwise moments will be equal to the sum of anticlockwise moments about the same axis

Conservative forces

Work the force does on an object moving between 2 points is independent of the path the object takes between the two points

Power

Work done per unit time

Why gravitational potential is negative

Gravitational force is always attractive, external force required to bring a small test mass from infinity to a point in the gravitational field of the mass always acts in the opposite direction to the displacement of the small test mass. The work done per unit mass by the external force is thus negative and gravitational potential is always negative.

Newton’s Law of gravitation

Gravitational force of attraction between two point masses is directly proportional to the product of their masses and inversely proportional to the square of the separation between their centres

Gravitational field strength

Gravitational force per unit mass exerted on a small test mass placed at that point

Gravitational potential

Work done per unit mass needed to bring a small test pass from infinity to that point

Simple harmonic motion

Oscillary motion of the particle whose acceleration is directly proportional to its displacement from a fixed point and this acceleration is always in opposite direction to displacement

Damped oscillations

Continuous dissipation of energy to the surroundings such that total energy in system decreases, hence amplitude of the motion progressively decreases with time

Resonance

Resonance occurs when the resulting amplitude of the system becomes a maximum when the external driving force equals to the natural frequency of the system

Maximum transfer of energy from driving system to the driven system

Progressive vs stationary wave

1. Amplitude → particles on a progressive wave all have the same amplitude, while the particles on a stationary wave have different amplitudes

2. Wavelength → progressive wave: shortest distance between 2 particles that are in phase, stationary wave: twice the distance between 2 adjacent nodes

3. Phase → progressive all the particles are in different phases, transverse: all the particles in the same loop vibrate in the same phase, with the particles in the opposite loop having a phase difference of pi rad

Temperature vs heat

1. Temperature is able to measured using a thermometer while heat is unable to be measured usuing an instrument

Absolute scale

Does not depend on any thermometric property of any particular substance and has absolute zero and triple point of water as fixed points

Spontaneous decay

The decay of particles is not affected by any external or environmental factors

Random decay

There is a fixed probability of decay per unit time, the time of a decay of a nucleus cannot be predicted