Chapter One

Página 1

P1 - 1.1 - Changes In Energy Stores

Energy can be stored in different was and is transferred by heating, waves, an electric current, or when a force moves an object. Like:

Chemical Energy Stores - these include fuels, foods or chemicals found in batteries. These use chemical reactions to transfer energy

Kinetic Energy Stores - this describes the energy an object has because it is moving

Gravitational Potential Energy Stores - this describes the energy stored in n object because of its position.

Elastic Potential Energy Stores - this describes the energy stored in a springy object when you stretch or squash it

Thermal Energy Stores - this describes the energy a substance has because of its temperature

Energy can be transferred from one store to another. When a kettle is used to boil water, the current in the kettle's heating element transfers energy to the thermal energy store of the water and the kettle.

Chemical energy store in the battery ---> electric current in the wires ---> energy transferred to the surroundings --> light waves OR increase in the thermal energy store of the surroundings.

When an object starts to fall freely, it speeds up as it falls. The force of gravity acting on the object causes energy to be transferred from its gravitational potential energy store to its kinetic energy store.

Página 2

P1 - 1.2 _Conservation Of Energy

When a roller coaster climbs an incline, its gravitational potential energy store increases. This energy is them transferred to other energy stores as the roller coaster races downwards. As the roller coaster descends, its GPE store decreases. Most of this energy is transferred to its kinetic energy store, which therefore increases. However, some energy is transferred to the thermal energy store of the surroundings by air resistance and friction, and some energy is transferred by sound waves.

A pendulum would keep swinging forever if it was in a vacuum because there would be no air resistance acting on it, so no energy would be transferred from any of its energy stores. There would be no net change to the energy stored in the system, so it would be a closed system. A system is an object or a group of objects. Scientists research have concluded that the total energy of a closed system is always the same before and after energy transfers to other energy stores within the closed system.

energy cannot be created or destroyed - this is the principle of conservation of energy.

Energy can be stored in different ways, for example:

When a rubber band is stretched, its elastic potential energy store increases.

When an object is lifted, its gravitational potential energy store is increased

When bungee jumping, the jumper doesn't return to the original height after bouncing once. This is because some energy was transferred to the thermal energy store of the surroundings by heating as the rope stretched and then shortened again.

Página 3

P1 - 1.3 - Energy and Work

When an object is moved by force, work is done on the object by force. So the force transfers energy to the object. The amount of energy transferred to the object is equal to the work done on it. For example: to raise an object, you need to apply force to it to overcome the force of gravity on it. If the work you do on the object is 20J, the energy transferred to it must be 20J. So its gravitational potential energy store increases by 20J.

energy transferred = work done

work done (W) = force applied (F) x distance moved along the line of action of the force (s)

joules (j) newtons (N) meters (m)

Worded Example

A builder pushed a wheelbarrow a distance of 5m across flat ground with a force of 50N. How much work was done by the builder?

Solution

work done = force applied x distance moved

= 50N x 5m

=250 J

Friction at Work

Work done to overcome friction is mainly transferred to thermal energy stores by heating

If you rub your hands together vigorously, they become warm. Your muscles do work to overcome the friction between your hands. The work you do is transferred as energy that warms your hands

Brake pads on a vehicle become hot if the brakes are applied for too long. Friction between the brake pads and the wheel discs opposes the motion of the wheel discs opposes the motion of the wheel. The force of friction does work on the brake pads and wheel discs. As a result, energy is transferred from the kinetic energy store of the vehicle to the thermal energy store of the brake pads and the wheel discs. This makes them become hot and transfer energy by heating tot he thermal energy store of the surrounding air.

Página 4

P1 - 1.4 - Gravitational Potential Energy Stores

Every time you life an object up, you do some work Some of your muscles transfer energy from the chemical energy store in the muscle to the gravitational energy store of the object. In calculations we refer to the energy in this store as gravitational potential energy Ep.

When an object is moved upwards, the energy in its gravitational potential energy store increases. This increase is equal to the work done on it by the lifting force to overcome the gravitational force on the object

When an object moves down, the energy in its gravitational potential energy store decreases. This decrease is equal the work done by the gravitational force acting on it.

The work done when an object moves up or down depends on:

how far it is moved vertically (its change of height)

its weight

change in object's gravitational potential energy store = weight x change of height

joules (J) newtons (N) meters (m)

Astronauts on the Moon can lift objects much more easily than they can on the Earth. This is because the gravitational field strength on the Moon's surface is only about a sixth of the gravitational field strength on the Earth's surface

When an object is lifted or lowered, the change to its gravitational potential energy store is equal to its weight x its change of height. Therefore:

change of GPE store = mass (m) x gravitational field strength (g) x change of height

joules (J) kg N/kg m

Página 5

P1 - 1.5 - Kinetic Energy and Elastic Energy Stores

The amount of energy in the kinetic energy store of an object can be calculated using the kinetic energy equation below

kinetic energy = 1/2 x mass x speed^2

joules kg m/s

When you stretch a rubber band or a spring, the work you do is stored in it as elastic potential energy

for a spring to be stretched to an extension e we can calculate the energy in its elastic potential energy store using the equation below

elastic potential energy = 1/2 x spring constant x extension^2

J N/m m^2

Página 6

P1 - 1.6 - Energy Dissipation

A machine transfers energy for a purpose. Friction between the moving parts of a machine causes the parts to warm up. So, not all of the energy supplied to a machine is usefully transferred. Some of the energy is wasted

Useful energy is energy transferred to where is its wanted in the way that is wanted

Wasted energy is the energy that is not usefully transferred

Whenever energy is transferred for a purpose in any system, some of the energy is transferred usefully. The rest is dissipated (spread out) and may be stored in less useful ways. this energy is described as waste energy.

Friction in machines always causes energy to be wasted. When a bike rider brakes, friction acts between the brake blocks and the wheels. Energy is wasted as it is transferred from the kinetic energy stores of the bike and cyclist, to the thermal energy stores of the brake locks and the wheels, which are heated by friction.

As you are exercising, energy is transferred to the thermal energy store of your muscles and to the thermal energy store of the exercise machine by the force you exert to overcome the friction in the machine.

Wasted energy is dissipated to the surroundings for example, the gears of a car get hot because of friction when the car is running. here, energy is transferred from the kinetic energy store of the gear box to the thermal energy stores of the gear box and the surrounding air. the thermal energy stores of the gear box and the surrounding air therefore increase, as do their respective temperatures.