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PHYSICS ENERGY

Mr Rishi Gopie

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ENERGY, WORK AND POWER

Consider some forms of energy: electrical, chemical, internal, Heat or thermal, light, sound, nuclear, magnetic, kinetic, and potential.

The principle law of conservation of energy states that the total amount of energy in the universe is constant and that energy cannot be created or destroyed but can be changed from one form to another.

Examples of systems in which energy transformations occur are:

a) The production of hydroelectricity

Diag. 36

b) The combustion of fossil fuels (such as coal, peat, natural gas and petroleum products such as gasoline, dieseline)

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Diag. 37

Alternative sources of energy (to the combustion of fossil fuels) include:

i) Hydroelectricity ii) Geothermal energy iii) Wind energy iv) Wave energy v) Tidal energy vi) Solar energy vii) Nuclear energy viii) Chemical energy in the form of biomass and biogas

Consider some procedure/techniques to conserve energy

i) Switch off unnecessary appliances (such as lights, fans etc.) ii) Use low power or fluorescent lighting where possible

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iii) Do not put hot objects into a refrigerator or freezer iv) Use covers on cooking utensils (such as pots and pans) v) Adjust flame below utensil to suit the size of the bottom of the utensil vi) Do not heat more material (such as water) in a utensil (such as a kettle) than is necessary vii) For electrical appliances such as kettles, with heating elements (or filaments) the element

should be completely covered with the material being heated (water say) viii) Use full loads of washing in a washing machine ix) Do not wash up in running water x) Do not drive vehicles (using fossil fuels as power sources) to fast (and especially avoid large

acceleration) xi) Efficiently insulate heating systems (such as stoves and ovens) and cooling systems (such as

refrigerators and freezers and air conditioned rooms) xii) Keep vehicles in proper working order.

WORK

Work (w) is defined as the product of force (F) and displacement (s) in the direction of the force, i.e. W = F x s.

Energy can be regarded as the ability to perform work and work as the transfer of energy. The work-‐energy theorem considers work and energy to be interchangeable and equivalent to one another.

The unit of both energy and work is the Joule (symbol J). The joule is defined as the work done or energy used when a force of one newton is displaced through one metre in the direction of the force.

Kinetic energy, Ek, is the energy possessed by a body because of its motion and is given by Ek = 1/2mv2, where Ek is kinetic energy, m is the mass of the body and v is the speed of the body at a given instant.

Potential energy, Ep, is the energy possessed by a body because of its position or its state (i.e. condition) there are two types of potential energy

i) The energy stored or possessed by a body because of its state or condition is known as conditional potential energy and exists as elastic potential energy (or strain energy) in certain circumstances (such as when the body is stretched or compressed)

ii) The energy possessed by a body because of its position in a gravitational field is known as gravitational potential energy. A change in gravitational potential energy ΔEp, is associated with a change in vertical height (i.e. vertical displacement) of the body in the gravitational field is given by ΔEp = mgh, where m is the mass of the body, g is the gravitational field strength (or acceleration due to gravity) and h is the change in vertical height (or vertical displacement).

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POWER (P)

Power is defined as the rate of performing work (w) or the rate of conservation/transfer of energy (E), i.e. P = W/t or P = E/t, where an amount of work, W, or an amount of energy, E, is involved over a time interval t. The unit of power is the watt (symbol W) and it is defined as a rate of working, or a rate of transferring/converting energy, of a joule per second.

Friction is that force which opposes the motion of one surface over or through another-‐ for instance, friction opposes the motion of a solid body through a fluid or over another solid body.

When work is done energy is used in overcoming friction, some form of energy is transferred into thermal energy, i.e. heat is produced, and this represents ‘a loss of energy’. Friction is therefore a source of loss of efficiency in many systems (such as machines).

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Tutorial

June 2009 paper 2 # 4

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June 1994 paper 2 #4

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January 1999 paper 3 # 2

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January 1999 paper 2 #1

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