September 18, 2011

4.11 Gravitational Potential Energy

Recall and use the relationship:

$\small Gravitational\, potential\, energy\, =\, mass\times gravitational\, field\, strength\times height$
$\small GPE=m\times g\times h$

Increasing either your mass, height or gravitational field strength (going to another planet is the easiest way to do this) will give you a higher GPE, and vice versa, lower your weight/height and you lower your GPE.

Example:

If a 750kg car is at the top of a 50m slope with a vertical height of 10m, how many Joules of GPE does the car have? (g on earth is 10N/kg)

$\small GPE=m\, g\, h$
$\small GPE=750kg\times 10N/kg\times 10m$
$\small GPE=75,000J\, or\, 75kJ$

Note: we use the vertical height (10m) appose to the slope height (50m) because weight acts in straight line from the ground.

Important: we often say that an object has no GPE when it is on the ground, but if we were to have a swinging pendulum it obviously wouldn’t touch the ground so we say that it has 0J of GPE when it is at its lowest point. Basically during experiments we say that the object has 0J of GPE when it is at the lowest point possible, this is because theoretically we could measure all objects GPE from the centre of the earth, but it is not convenient to do so.

September 13, 2011

4.10 Energy transferred Vs. Work done

Understand that work done is equal to energy transferred
• Work done (measured in Joules) is equal to the Force (in Newton’s) multiplied by the distance moved (in metres).
• 1 Joule is the work done when a force of 1 Newton moves through a distance of 1 metre (in the direction of the force) i.e. when 1 Newton is transferred 1 metre
• \Work done = energy transferred

4.9 Work

Recall and use the relationship between work, force and distance moved in the direction of the force:

$Work\: done\: =\: force\: \times \: distance\: moved$

Examples
1. A man pushes a 40N wheelbarrow 20m, how much work is he doing?
$Wd\: =\: F\: \times \: d$
$Wd\: =\: 40N\: \times \: 20m$
$Wd\: =\: 800j$

2. A skier skis 50m down a slope with 10N of friction acting against his motion, he descends 10m vertically down the slope. How much work has he done?

What measurement do we use as the distance? Well, we must remember to use the distance running in the same direction as the force acting against the object, so we use the length the skier travelled (50m) appose to his vertical height, so:

$Wd\: =\: 10N\: \times \: 50m$
$Wd\: =\: 500J$

September 3, 2011

4.8 Insulation

Describe how insulation is used to reduce energy transfers from buildings and the human body.

Insulation is used in housing to trap heat inside the house and keep the house warm (or in warmer countries like Thailand, the insulation is used to stop the heat from getting into the house, not to stop the cold getting out, there is a difference!)

The walls of houses are filled with ‘Insulation’ to stop heat escaping; this insulation is made of a material that is a very poor conductor (often fibreglass) in some cases this fibreglass is then coated in a silver material that reflects the infra-red radiation.

This means that both the relevant types of heat are being reduced; Conduction is reduced by a poor conductor and Radiation is stopped by something that reflects the heat. And convection isn’t really a problem in houses as convection can’t pass through the solid doors and walls of a house.

The same principles work for the human body… or anything for that matter, If you want to stay cool wear something white as it will reflect a large amount of the radiation. On the other hand if you want to stay warm, it is also a good idea to wear white, this is because if you were to wear a dark colour it would absorb the heat from your body and then radiate it out into the atmosphere which is not going to keep you cool.

4.7 Convection

Describe the role of convection in everyday phenomena.

There are several examples of convection that we come to contact with every day.
An example of convection is the motion of air in your room when you have a radiator on, the radiator warms the air close to the radiator so it rises up and away. Cold air from below takes its place and is in turn heated. As this air moves away from the radiator it cools, and therefore falls and return back to the radiator in a circular motion. The diagram below shows this principle.