# Unit 4 Physics Definitions

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- Created by: Lola
- Created on: 26-01-13 12:11

Principle of conservation of linear momentum

The total linear momentum of a system of interacting (e.g. colliding) bodies on which no external forces are acting, remains constant.

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Newton's 2nd law

The rate of change of momentum of a body is directly proportional to the external force acting on the body and takes place in the direction of the force.

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Newton (N)

The newton (N) is that force which produces an acceleration of 1ms when it acts on a mass of 1kg.

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Coulomb's Law

The magnitude of the force F, between two electrically charged bodies, which are small compared with their separation, r, is inversely proportional to r, and proportional to the product of their charges Q and Q'.

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Electric field

An electric field exists in a region if electrical fields are exerted on charged bodies in that region. The direction of an electric field at a point is the direction of the force that would act on a small positive charge placed at that point.

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Electric field strength

Electric field intensity or electric field strength, E, at a point in a field is defined as the force exerted per unit charge on a small charge placed at that point.

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Uniform electric field

Field lines are parallel to each other and evenly spaced. The electric field strength is the same everywhere in the field.

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Capacitor

A device which is used to store charge.

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Capacitance

The measure of the extent to which a capacitor can store charge. Measured in Farads.

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Time constant (τ)

The time taken for the charge or current (or PD across the capacitor) to drop to 37% of their original values.

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Magnetic Field

A magnetic field exists in a region if a magnetic force acts on any magnetic material placed in that region.

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Direction of Magnetic field

The direction of the magnetic field, at a point, is taken to be the direction of the force that would act on an imaginary North Pole placed at that point.

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Magnetic Flux Density

The magnitude (strength) and direction of a magnetic field can be represented by its magnetic flux density (B). Measured in Tesla.

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Magnetic Flux ( Φ )

The magnetic flux through a region is a measure of the number of magnetic field lines passing through the region. Measured in Webers.

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Flux linkage

The product of the number of turns of a coil and the flux through the coil.

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Flux Density ( B )

The flux density of a magnetic field can be defined as the force acting per unit length per unit current on a straight conductor placed at right angles to the magnetic field.

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Tesla

One tesla is the magnetic flux density of a field in which a force of 1 newton acts on a 1 metre length of a conductor which is carrying a current of 1 ampere and is perpendicular to the field.

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Electromagnetic Induction

An EMF is induced in a coil in a magnetic field whenever the flux ( Φ ) through the coil changes.

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Induced EMF

An EMF can be induced in a straight conductor whenever it is caused to cut across magnetic field lines. The magnetude of the induced EMF is proportional to the rate of cutting.

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Faraday's Law

The magnitude of the induced EMF in a coil is directly proportional to the rate of change of flux-linkage or to the rate of cutting of magnetic flux.

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Lenz's law

The direction of the induced EMF is such that the current which it causes to flow (or would flow in a closed circuit) opposes the change in which is producing it.

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Rutherford's nuclear model

Most of the atom is empty space. In its centre is the nucleus, which has most of the mass of the atom and all its positive charge. Electrons orbit the nucleus like planets orbit the sun and are bound to the nucleus by an electrostatic attraction.

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Thermionic emission

The release of electrons from the surface of a metal through heating is known as thermionic emission.

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Deflection of electrons by magnetic fields

An electron moving at right angles to a uniform magnetic field feels a constant force at right angles to its direction of motion. This force causes the electron to move in a circular path at constant speed.

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Particle accelerators

Particle accelerators are very large machines that use electric fields to accelerate charged particles to very high energies and make them collide with other particles which are either at rest or moving in the opposite direction.

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Linear accelerator

Accelerates particles down a straight evacuated tube without the use of very high voltages.

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Beta decay

In a beta minus decay a neutron in the nucleus splits into a proton and an electron. The mass of the neurton is greater than the mass of the proto plus the mass of the electron. The difference in mass is equal to the mass of the energy released.

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Matter and antimatter

A particle and an antiparticle have the same mass, equal amount of charge but of opposite sign and they spin in opposite directions.

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Annihilation

As soon as an anti-particle meets its particle the two destroy each other. Their mass is converted to energy. This is known as annihilation

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Pair production

The opposite of annihilation. High energy photons can vanish creating particle anti-particle pairs in their place.

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Specific heat capacity

The specific heat capacity, (c) of a substance is the heat required to produce unit temperature rise in unit mass of the substance.

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Internal Energy (U)

The internal energy of any object is defined as the sum of all the microscopic kinetic energy and potential energies of the molecules within the object.

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The Pressure law

For a fixed mass of gas at constant volume, the pressure is directly proportional to the temperature measured in kelvins.

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Boyle's law

For a fixed mass of gas at constant temperature, the product of pressure and volume is constant.

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Charles' law

For a fixed mass of gas at constant pressure, the volume is directly proportional to the temperature measured in kelvin.

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Ideal gas

A gas that obeys the gas laws exactly.

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Mole

1 mole (mol) of a substance is defines as the amount of it that contains the same number of elementary units(atoms, molecules) as there are atoms in 12g of carbon12.

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Avogadro constant

The Avogadro constant is numerically equal to the number of atoms in a mole.

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Boltzman's constant

The gas constant per molecule.

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Simple Harmonic Motion

A body is moving with simple harmonic motion if the body is moving in such a way that its acceleration is directed towards a fixed point in its path and is directly proportional to its distance from that point.

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Damped oscillations

The amplitude of the oscillations of a simple pendulum gradually decreases to zero due to the resistive force that arises from the air. The motion is therefore not a perfect s.h.m. and is said to be damped by air resistance.

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Forced oscillations

A system is undergoing forced oscillations if it is being forced to oscillate by some other system at the frequency of that system.

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Resonance

When a system is being forced to oscillate, but at its own natural frequency, it does so with a large amplitude, and it is said to be in resonance with the forcing system.

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Radioactivity / Radioactive decay

Some isotopes have atoms whose nuclei are unstable. These nuclei disintegrate and emit radiations in an attempt to aquire a more stable state.

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α - particle

An α - particle consists of 2 protons and 2 neutrons, i.e. it is identical to a helium nucleus.

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β - particle

β - particles are emitted by nuclei which have too many neutrons to be stable. Such a nucleus attains a more stable state when one of its neutrons changes into a proton and an electron.

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β - decay

Nucleon number does not change & proton number increases by 1.

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γ - rays

Electromagnetic radiation of very small wavelength. They produce little ionisation and are very penetrating.

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Artificial radioactivity

Radioactive nuclides which do not occur in nature can be produced by bombarding naturally occuring nuclides with atomic particles - with neutrons in a nuclear reactor.

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Background radiation

The radiation that is around us all the time. This is mainly due to Nature and partly due to human activity. E.g. uranium deposits in the ground, radon gas and cosmic rays.

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Cosmic rays

Fast moving nuclei from outer space which bombard the atmosphere, sending showers of particles to the Earth's surface.

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Geiger - Muller tube

The G-M tube is a very sensitive type of ionisation chamber which can detect single ionisation events.

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Activity

Number of disintegrations per unit time.

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Decay constant (λ)

The fraction of undecayed atoms, N, that decay in a second.

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Half life

The average time taken for half the nuclei present to decay.

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Gravitational Field

The region surrounding a mass in which another mass experiences a gravitational force of attraction is called a gravitational field.

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Direction of Gravitational Field

The direction of a gravitational field at a point is the direction of the force that would act on a mass placed at that point.

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Newton's law of universal gravitation

Every particle in the Universe attracts every other with a force which is proportional to the product of their masses and inversely proportional to the square of their separation.

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Universal Gravitational costant (G)

6.67 × 10

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Gravitational Field Strength

The gravitational field strength at a point in a gravitational field is defined as the force per unit mass acting on a mass placed at that point.

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## Other cards in this set

### Card 2

#### Front

The rate of change of momentum of a body is directly proportional to the external force acting on the body and takes place in the direction of the force.

#### Back

Newton's 2nd law

### Card 3

#### Front

The newton (N) is that force which produces an acceleration of 1ms when it acts on a mass of 1kg.

#### Back

### Card 4

#### Front

The magnitude of the force F, between two electrically charged bodies, which are small compared with their separation, r, is inversely proportional to r, and proportional to the product of their charges Q and Q'.

#### Back

### Card 5

#### Front

An electric field exists in a region if electrical fields are exerted on charged bodies in that region. The direction of an electric field at a point is the direction of the force that would act on a small positive charge placed at that point.

#### Back

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