Physics Definitions (Leaving Cert)

Self luminous object

An object that gives out it's own light

Non luminous object

An object that does not give out it's own light.

First law of Reflection

The incident ray, the normal, and the reflected ray are all in the same plane.

Second Law of Reflection

The angle of incidence is equal to the angle of reflection. I.e i = r

First Law of Refraction

The incident ray, the normal and the refracted ray are all on the same plane.

The Second Law of Refraction

The sine of the angle of incidence is proportional to the sine of the angle of refraction. This can be represented mathematically by sin i / sin r = n , where n is a constant.

Principle of Reversibility of Light

Light travels along exactly the same path, if it's direction is reversed; it just goes in the opposite direction.

Light

Is a form of energy that travels away from the source producing it at the speed of about 3x10^8 m s^-1.

Diffuse Reflection

If the surface on which the light shines is rough, the reflected light is scattered in all directions.

Regular Reflection

Is the bouncing of light off an object that is silvered and polished, called a mirror, or a shiny flat surface.

Angle of Incidence

The angle between the incident Ray and the normal.

Angle of Reflection

The angle between the reflected ray and the normal.

Angle of Refraction

The angle between the refracted Ray and the normal.

Refractive Index

The ratio of the sine of the angle of incidence to the sine of the angle of refraction.

Centre of Curvature

The centre of the sphere or circle which a mirror would be part of.

Pole

The centre point of a mirror or lens.

Radius of Curvature

The radius of the sphere or circle which a mirror would be part of.

Principle Axis

The line which passes through the centre of curvature and the pole.

Principle Focus

The point on the principle axis through which all rays traveling parallel to the principle axis are reflected or refracted.

Focal Length

The distance between the principle focus and the pole.

Real Image

An image formed by rays which actually pass through the image. Such an image would form on a screen.

Virtual Image

An image formed by rays which do not actually pass through the image; they only appear to do so.

Parallax

Is the apparent movement of one object relative to the another due to the motion of the observer. the object that is farthest away appears to move with the observer.

Magnification

The ratio of the image height to to the object height. I.e the height of the image divided by the height of the object.

Refraction

The bending of a light ray (or a wave) which occurs when it passes from one medium to another of different density or refractive index.

Critical Angle

When light travels from a denser to a rarer medium, the angle of incidence whose corresponding angle of refraction is 90° is called the critical angle for those two media.

Total Internal Reflection

The reflection of all light, incident at a surface, back into the denser medium due to the critical angle being exceeded.

Optical Fibre

A fibre consisting of a very fine solid strand of high quality glass coated with a film of another glass of lower refractive index. Light is transmitted along it with no loss through the walls, due to total internal reflection occurring at the walls.

Power of Accommodation

The ability of the eye to change the focal length of its lens and so form a sharp image on the retina for objects at different distances. This can be represented mathematically as p = 1 / f , where p is power and f is the focal length.

Short Sightedness

A defect of vision in which the sufferer can successfully accommodate objects a short distance from the eye but not ones far off. (Can be thought as due to the eyeball being too long).

Long Sightedness

A defect of vision in which the sufferer can successfully accommodate objects a long way from the eye but not ones close by. (Can be thought of the eyeball being too short).

Speed

Is the rate of change of distance with respect to time.

Displacement

Is distance in a given direction.

Velocity

Is the rate of change of displacement with respect to time.

Constant Velocity

Is when an object moves in a straight line and does not speed up or slow down.

Acceleration

Is the rate of change of velocity with respect to time.

Scalar Quantity

A quantity that has magnitude only and has no direction in space.

Vector Quantity

A quantity that has both magnitude and direction in space.

Force

Is anything that causes or tends to cause the velocity of an object to change.

Mass

Of a body is a measure of how difficult it is to accelerate that body.

Weight

The weight of an object is the force of the Earth's gravity acting on it. i.e. W =mg

Momentum

Mass x Velocity

Newton's First Law of Motion

States that every body will remain in a state of rest or traveling with constant velocity unless an unbalanced external force acts on it.

Newton's Second Law of Motion

States that when an unbalanced force acts on a body the rate of change of the body's momentum is directly proportional to the force and takes place in the direction of the force.

Newton's Third Law of Motion

States that if body A exerts a force on body B, the body B exerts an equal but opposite force on body A.

The principle/Law of conservation of momentum

In any reaction between bodies in a closed system, the total momentum before the interaction is equal to the total momentum after the interaction.

Density

of an substance is its mass per unit volume. This can be represented mathematically by:

Density=mass/volume (kg m-3)

Pressure

Force per unit area.

Force / Area

Unit of pressure

The pressure is one pascal if a force of 1 N acts over an area of 1 m^2

Archimedes' Principle

When a body is partially of completely immersed in fluid, the upthrust is equal in magnitude to the weight of fluid displaced.

The law of Flotation

States that the weight of a floating body is equal to the weight of the fluid it displaces.

Boyle's Law

States that at a constant temperature the volume of a fixed mass of gas is inversely proportional to its pressure.

Newton's Law of Universal Gravitation

States that any two point masses in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

The moment of a Force

the moment of a force, about an axis, is the product of the force times the perpendicular distance between the axis and the line of action of the force.

Conditions for equalibrium

-When a body is in equilibrium, the sum of all forces acting on it is zero.

-The sum of the clockwise moments about any axis is equal to the sum of the anticlockwise moments.

Couple / Torque

Two parallel forces with the same magnitude acting in opposite direction.

Work

Work is done when a force moves a body. The amount of work done is equal to the product of the force times the distance moved in the direction of a force.

The joule

One joule is the work done when a force of 1 Newton acts for a distance of 1 metre in the direction of the force.

Energy

Is the ability to do work. The amount of energy something has is the amount of work it can do.

Principle of Conservation of energy

States that energy cannot be created or destroyed but can only be converted from one form to another.

Kinetic Energy

Of a body is the energy that body has due to motion.

Potential Energy

Of a body is the energy it has due to its position or condition in a force field.

Renewable source of energy

A source of energy that does not get used up.

Power

Is the rate at which work is done.

Unit of power

The power is one watt if work is being done or energy is being converted at a rate of one joule per second.

Angular velocity

Is the rate of change of angle with respect to time

Unit of angular velocity

radian per second

Centripetal Force

Any force that causes a body to move in a circular path. The force constantly acts at right angles to the direction of the body's velocity.

Centripetal acceleration

Its the acceleration a body has towards the center of the circle if that body is moving in a circle.

Period of an orbit

The time taken for a satellite to go once around the central body is called the periodic time or simply the period (T) of the orbit.

Hooke's Law

States that when an object is bent, stretched or compressed by a displacement s, the restoring force F is directly proportional to the displacement - provide the elastic limit is not exceeded.

A body is moving in simple harmonic motion if

1. its acceleration is directly proportional to its distance from a fixed point on its path.

2. and its acceleration is always directed towards that point.

Periodic time (T)

of a particle executing simple harmonic motion is the time for one complete oscillation. T is measured in seconds.

Frequency (f)

Is the number of cycles per second. It is measure in cycles per second - Hertz (Hz).

Temperature

is the measure of the hotness or coldness of a body.

Thermometric Property

is any physical property that changes measurably and uniformly with temperature.

Heat capacity

Of an object is the heat energy needed to change the temperature of that object by 1kelvin

Specific Heat capacity

Of an object is the heat energy need to change the temperature of 1kg of that substance by 1kelvin

Latent heat

Of an object is the heat energy needed to change its state without a change in temperature.

Specific latent heat

Of a substance is the amount of heat energy needed to change the state of 1 kg of that substance without a change in temperature

Specific latent heat of fusion

Is the amount of heat energy needed to change 1kg of a substance from a solid to liquid without a change in temperature

Specific latent heat of vaporisation

Of a substance is the amount of heat energy needed to change 1kg of that substance from a liquid to a gas without a change in temperature

Conduction

Is the movement of heat energy through a substance by the passing of vibrating molecules. There is no overall motion of the substance.

Radiation

Is the transfer of heat energy from one place to another in the form of electromagnetic waves.

Convection

Is the transfer of heat through a fluid by means of circulating currents of fluid cause by heat

U-value

Of a structure is the amount of heat energy conducted per second through 1 metre squared of that structure when a temperature difference of 1 degrees Celsius is maintained between its ends.

Solar constant

The average amount of the sun's energy falling per second perpendicularly on 1 metre squared of the the Earth's atmosphere is the solar constant.

Traveling Mechanical wave

Is a disturbance carrying energy through a medium without any overall motion of that medium

Traveling wave

Either mechanical or electromagnetic, is a disturbance that travels out from the source producing it , transferring the energy from the source to other places through which it passes

Transverse wave

Is a wave where the direction of the vibration is perpendicular to the direction in which the wave travels

Longitudinal wave

Is a wave where the direction of vibration is parallel to the direction in which the wave travels

Amplitude

The maximum distance of any particle from its undisturbed position

Oscillation or cycle

The distance produced by one complete vibration of the source

Wavelength

The distance from any point on one cycle to the corresponding point on the next cycle

Frequency

The number of cycles per second. The unit of frequency is the hertz

Velocity (c)

Is the distance traveled by one cycle in one second.

Reflection of waves

Is the bouncing of waves off of an obstacle in their path

Refraction of waves

Is the changing of direction of a wave when it enters a region where it's speed changes

Diffraction

The sideways spreading of waves into the region beyond a gap or around an obstacle

Interference

Is when waves from two sources meet, a new wave is produced. The displacement produced at any point by this wave is the algebraic sum of the displacements that each wave would produce on its own.