Exploring physics

Electric current

Electric current

Rate of flow of charge

Coulomb

1 Coulomb of electric charge passes through a point in 1 second when there is a current of 1 ampere

Kirchoff's First Law

Conservation of charge. At any point in an electrical circuit, the sum of the currents into that point is equal to the sum of the currents out of that point

Kirchoff's Second Law

Conservation of energy. In a closed loop of an electrical circuit, the sum of the emfs is equal to the sum of the pds

Mean drift velocity

The average velocity of electrons as they move through a wire

Number density

The number of free electrons per cubic metre of a material

Potential difference

The energy transferred from electrical energy to other forms, per unit charge

Volt

1V is the pd across a component when 1J is transferred per 1C passing through the component

Electromotive force

The amount of energy transferred from chemical to electrical energy, per unit charge

Resisstance

A property of a component that regulates the current through it, calculated by dividing the potential difference across it by the current in it

Ohm

The resistance of a component that has a potential difference of 1 volt per ampere

Ohm's law

For an ohmic conductor at a constant temperature, the current is directly proportional to the potential difference across it

Threshold voltage

The minimum potential difference at which a diode begins to conduct

LDR

An electrical component with a resistance that decreases as light intensity incident on it increases, because the number density of charge carriers increases.

Resistivity

A property of a material defined as the product of the resistance of a component made of the material and its cross-sectional area, divided by its length

Thermistor

An electrical component with resistance that decreases as temperature increases

Internal resistance

The resistance of a source due to its construction, which causes a loss in energy as charge passes through the source

Terminal pd

The potential difference measured across the terminals of an electrical power source - when there is no current this is equal to the emf but if there is a current in the source this is equal to the emf minus the lost volts

Lost volts

The potential difference across the internal resistor of a source of emf

Potentiometer

An electrical component with 3 terminals and a sliding contact that can be adjusted to vary the pd between 2 of the terminals

Progressive wave

A wave that transfers energy away from a source as a result of oscillations

Longitudinal wave

A wave for which oscillations are parallel to the direction of energy transfer

Transverse wave

A wave for which oscillations are perpendicular to the direction of energy transfer

Displacement

The distance from the equilibrium in a given direction

Amplitude

The maximum displacement from the equilibrium position

Wavelength

The minimum distance between a point on a wave and the identical point on the adjacent wave

Period

The time taken for one complete pattern of oscillation

Phase difference

The difference in displacements of particles along a wave or on different waves, measured in degrees or radians, with one wavelength representing 360 degrees or 2pi radians

Frequency

The number of complete wavelengths passing a point per unit time

Speed of a wave

The distance travelled by the wave per unit time

Reflection

The change in direction of a wave at the boundary between 2 media, so that the wave remains in the original medium

Refraction

The change in direction of a wave when it changes speed as a passes between media

Polarisation

The phenomenon in which oscillations and direction of travel of a transverse wave are limited to a single plane

Diffraction

The phenomenon in which waves passing through a gap or around an obstacle spread out. It becomes significant when the wavelength is comparable to the gap width

Intensity

The radiant power passing through a surface per unit area

Properties of electromagnetic waves

Travel at the speed of light in a vacuum, are transverse

Electromagnetic spectrum

The full range of frequencies of electromagnetic waves

Plane polarised wave

A transverse wave in which the oscillations are limited to only one plane

Refractive index

Related to a specific material, n=c/v, where c is the speed of light in a vacuum and v is the speed of light through the material

Critical angle

The angle of incidence at the boundary between two media that will produce an angle of refraction of 90degrees

Total internal reflection

The reflection of all light hitting the boundary between two media back into the original medium when it is travelling through the medium with the higher refractive index, and the angle of incidence is greater than the critical angle

Principle of superposition of waves

When two waves meet at a point the resultant displacement is equal to the sum of the individual displacements of the two waves

Interference

Superposition of two progressive waves from coherent sources

Coherence

Two wave sources with constant phase difference

Path difference

The difference in the distance travelled by two waves from the source to a specific point, measured as a fraction of the wavelength

Constructive interference

Superposition of two waves in phase so that the resultant wave has amplitude greater than the original waves

Destructive interference

Superposition of two waves in antiphase so that the waves cancel each other out and the resultant amplitude is less than the original waves

Stationary/standing wave

A wave that remains in a constant position with no net transfer of energy, characterised by its nodes and antinodes

Fundamental frequency

The minimum frequency at which an object can vibrate

Harmonic

A whole number multiple of the fundamental frequency

Node

A point where there is no displacement so the amplitude is always zero

Antinode

A point on a stationary wave where there is maximum displacement

Photon

A quantum of energy of electromagnetic radiation

Electron volt

The energy transferred to or from an electron when it passes through a potential difference of 1 volt

Photoelectric effect

The emission of electrons (photoelectrons) from a metal surface when electromagnetic radiation above a threshold frequency is incident on it

Work function

The minimum energy needed to remove a single electron from the surface of a particular metal

Threshold frequency

The minimum frequency of EM radiation that will cause the emission of an electron from the surface of a particular metal

Capacitance

The charge stored per unit potential difference across a capacitor

Farad

The unit of capacitance. 1F is equal to 1C per V

Capacitor

An electrical component in which charge is separated. 2 metallic plates separated by a dielectric

Uses of a capacitor

Flash in a camera, back-up energy source

Time constant

The product of capacitance and resistance for a capacitor-resistor circuit. It is equal to the time taken for the pd, charge or current to decrease to 1/e of its original value

Coulomb's law

Any two point charges exert an electrostatic force on each other that is directly proportional to the product of their charges and inversely proportional to the square of their separation

Electric field strength

The force experienced by unit positive charge at that point

Electrical potential at a point

The work done in moving unit positive charge from infinity to that point (zero at infinity)

Magnetic flux density

The strength of a magnetic field. B=F/IL. Measured in Tesla.

Velocity selector

A device that uses both electric and magnetic fields to select charged particles of a particular velocity

Magnetic flux

The product of a cross-sectional area and the component of magnetic flux density that is perpendicular to it. Measured in Weber

Magnetic flux linkage

The product of the number of turns in a coil and the magnetic flux

Faraday's law of electromagnetic induction

The magnitude of the induced emf is equal to the rate of change of flux linkage

Lenz's law

The direction of the induced emf or current is always such as to oppose the change producing it

Strong nuclear force

A force carried by gluons which acts on hadrons and holds nuclei together. Attractive to about 3fm and repulsive below 0.5fm.

R0

Radius of one nucleon, about 1.2fm

Neutrino

A lepton that carries no charge but may have very tiny mass, less than a millionth of the mass of an electron

Radioactivity

The process by which unstable nuclei decay, emitting ionising radiation

Activity

The rate at which nuclei decay in a radioactive source, measured in Bq or decays per second

Decay constant

The probability that an individual nucleus will decay, per unit time

Half-life

The average time taken for the activity of a source to fall to half of its original value

Annihilation

The complete destruction of a particle and its antiparticle in an interaction that releases energy in the form of identical photons

Pair production

The replacement of a single photon with a particle and antiparticle of the same total energy

Mass defect

The difference in the mass of a nucleus and the mass of its completely separated constituent nucleons

Binding energy

The minimum energy required to separate a nucleus into its constituent nucleons

Binding energy per nucleon

The binding energy divide by the number of protons and neutrons in the nucleus

Induced fission

Nuclear fission occurring when a nucleus becomes unstable upon absorbing another particle (such as a neutron)

Nuclear fusion

A process in which two smaller nuclei join together to form one larger nucleus

Attenuation

The decrease in intensity of electromagnetic radiation as it passes through matter and/or space

Attenuation coefficient

A measure of the absorption of X-ray photons by a substance

Contrast media

Have high proton numbers, so higher attenuation coefficients than soft tissue. Iodine in liquids, barium in digestive system

Medical tracer

A radioisotope that has been combined with elements that will target the desired tissues. Technetium-99m for gamma camera, Fluorine-18 for PET scan

Gamma camera

Collimator, scintillator, photomultiplier tubes

Ultrasound

A sound wave with frequency beyond the range of human hearing, so more than 20kHz

Ultrasound transducer

A device that emits and receives ultrasound

Piezoelectric effect

Some crystals produce an emf when they are distorted

Acoustic impedance

The product of the density of a substance and the speed of ultrasound in that substance

Acoustic matching

The use of two substances with similar acoustic impedance to minimise the reflection of ultrasound at the boundary between them