Physics

Cars, busses, planes, and ships all use fuels as chemical energy stores. They carry their own fuel. Electric trains use energy transferred from fuel in power stations. Electricity transfers energy from the power station to the train. Energy can be stored in different ways and is transferred by heating, waves, an electric current, or when a force moves an object. 

 Some examples:

-       Chemical energy stores include fuels, foods, or the chemicals found in batteries. The energy is transferred during chemical reactions.

-       Kinetic energy stores describe the energy an object has because it is moving.

-       Gravitational potential energy stores are used to describe the energy stored in an object because of its position, such as an object above the ground.

-       Elastic potential energy stores describe the energy stored in a springy object when you stretch or squash it.

-       Thermal energy stores describe the energy a substance has because of it temperature.

Energy can be transferred from one store to another. In a torch the torch’s battery pushes a current through the bulb. This makes the torch bulb emit light, and also get too hot. When an electric 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 kettle. 

Energy transfers

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. All the energy in it kinetic energy store is transferred by heating to the thermal energy store of the object and the floor, and by sound waves moving away from the point of impact. The amount of energy transferred by sound eaves is much smaller than the amount of energy transferred by heating.  

Conservation of energy

 The pendulum would be keep on swinging for ever if it was in a vacuum because there would be no air resistance acting on it, and 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. Because of this, it would be an example of a closed system. A system is an object or a group of objects. Scientists have done lots of tests and 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. 

Bungee jumping

What energy transfers happen to a bungee jumper after jumping off the platform?

-      - When the rope is slack, energy is transferred from the gravitational potential energy store to the kinetic energy store as the jumper accelerates towards the ground due to the force of gravity.

-    -   When the rope tightens, it slows the bungee jumpers fall. This is because the force of the rope reduces the speed of the jumper. The jumper’s kinetic energy store decreases and the rope’s elastic potential energy store increases as the rope stretches. Eventually the jumper comes to a stop – the energy that was originally in the kinetic potential energy store of the rope.  

After reaching the bottom, the rope recoils and pulls the jumper back up. As the jumper rises, the energy in the elastic potential energy store of the rope decreases and the bungee jumper’s energy store increases until the rope becomes slack. After the rope becomes slack, and at the top of the ascent, the bungee jumper’s kinetic energy store decreases to zero. The bungee jumper’s gravitational potential energy store increases throughout the ascent. The bungee jumper doesn’t return to the original height. 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.  

Working out

In a fitness centre or a gym, you have to work hard to keep fit. Lifting weights and pedalling on an exercise bike are just two ways to keep fit. Whichever way you chose to keep fit, you have to apply a force to move something. So the work you do causes a transfer of energy. When an object is moved by a force, work it done on the object by the 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, you need to apply a force to it to overcome the force of gravity on it. If the work you do on the object is 20 J, the energy transferred to it must be 20 J. so its gravitational potential energy store increases by 20 J.

Energy transferred = work done

The work done by a force depends on the size of the force and the distance moved. One joule of work is done when a force of one newton causes an object to move a distance of one metre in the direction of the force. To calculate the work done by a force when it causes displacement of an object, use this equation:

Work done, W = force applied F x distance moved along the line of action of force, S

Changes in gravitational potential energy stores

Every time you lift 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. The force you need to lift up an object velocity is equal and opposite of the gravitational force on the object. So the upward force you need to apply to it is equal to the object’s weight. For example, you need a force of 80N to lift a box of weight 80N.

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

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

1.   1.  How far it is moved vertically (its change of height)

2.    2. Its weight

Using the work done = force applied x distance move in the direction on the force:

Change in object’s gravitational potential energy store = weight x change in height

Energy for a purpose

Where would people be without machines? Washing machines are used to clean your clothes. Machines in factories are used to make the things you buy. You might use exercise machines in a gym to keep yourself fit, and machines are used to get you from place to place. 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 it is wanted in the way this is       wanted.

-       - Wasted energy is the energy that is not usefully transferred.

Friction at work

Next time you use a running machine, think about what happens to the energy transferred by your muscles. As you are exercising, energy is transferred to the thermal energy store of the machine by the force you exert to overcome the friction in the machine. 

Spreading out

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 therefore increase, as do their respective temperatures.

Useful energy is eventually transferred to the surroundings too. For example, the useful energy supplied to turn wheels of a car is eventually transferred from the kinetic energy stores of the wheels to the thermal energy stores of the tyres by heating – increasing the thermal energy stores of the tyres. This energy is then transferred to the thermal energy store of the road and surrounding air.

Energy becomes less useful the more it spreads out. For example, the hot water from a central heating boiler in a building is pumped through pipes and radiators. The thermal energy store of the hot water decreases as it transfers energy by heating to the thermal energy stores of the radiators – heating the rooms in the building. But the energy supplied to heat these rooms will eventually by transferred to the surrounding air.

Everyday electrical appliances

The energy you use in your home is mostly supplied by electricity, gas, and oil. Although all three of these energy supplies can be used for cooking and heating, your electricity supple is vital because you use electrical appliances for so many purposes everyday. The charge that flows through these appliances transfers energy to them, which they then transfer usefully. But some of the energy you supply to them is wasted.

Choosing an electrical appliance

You use electrical appliances for many purposes. Each appliance is designed for a specific purpose, and it should waste as little energy as possible. Suppose you were a rock musician at a concert. You would need electrical appliances that transfer the variations in sound waves to electricity and then back to sound waves. But you wouldn’t want the appliances to transfer lots of energy to the thermal energy store of the surroundings or themselves. 

Energy and power

When you use a lift to go up, a powerful electric motor pulls you and the lift upwards. The electric current through the lift motor transfers energy to the gravitational potential energy store of the lift when the lift goes up at a steady speed. You also get energy from the electric current transferred to the thermal energy store of the motor and the surroundings due to friction between the moving parts of the motor. In addition, energy is transferred to the thermal energy store of the surroundings by sound waves created bu the lift machinery.  

-          -     The energy you supply to the motor per second is the power supplied to it.

-     -  The more powerful the lift motor is, the faster it moves a particular load.

The more powerful an appliance is, the faster the rate at which it transfers energy.

The power of an appliance is measured in watts (W) or kilowatts (kW).

1 watt is equal to the rate of transferring 1 joule of energy in 1 second (i.e., 1 W = 1 J/s)

1 kilowatt is equal to 1000 watts (i.e., 1000 joules per second or 1 KJ/s).

Appliance

Power Rating

A torch

1 W

An electric light bulb

100 W

An electric cooker

10,000 W = 10 kW (where 1 kW = 1000 watts)

A railway engine

1,000,000 W = 1 megawatt (MW) = 1 million watts.

A Saturn V space rocket

100 MW

A very large power station

10,000 MW

World demand for power

10,000,000 MW

A star like the sun

100,000,000,000,000,000,000 MW

Wasted power

In any energy transfer, the energy wasted = the input energy supplied – the useful output energy. Because power is energy transferred per second:

Power wasted = total power in – useful power out.   

Energy transfer by conduction

When you have a BBQ, you need to know which materials are good conductors and which ones are good insulators. If you can’t remember you are likely to burn your fingers!

Testing rods of different materials as conductors

The rods need to be the same width and length for a fair test. Each rod is coated with a thin layer of wax near one end. The uncoated ends are then heated together. The wax melts fastest on the rod that best conducts energy.

-     -  Metals conduct energy better than non – metals.

-      - Copper is a better conductor than steel.

-       - Glass conducts better than wood.  

Insulation matters

Materials that are good insulators are necessary to keep you warm in winter, whether you are at home or outdoors. Good insulators need to be materials that have low thermal conductivity, so energy transfer through them is as low as possible.

The energy transfer per second through a layer of insulating material depends on:

-    -   The temperature difference across the material.

-    -   The thickness of the material.

-      - The thermal conductivity of the material.

To reduce the energy transfer as much as possible in any given situation:

-      - The thermal conductivity of the insulating material should be as low as possible.

-      - The thickness of the insulating layer should be as thick as is practically possible.

Specific heat capacity

A car in strong sunlight can become very hot. A concrete block of equal mass would not become as hot. Metal heats up more easily than concrete. Investigations show that when a substance is heated, its temperature rise depends on:

-       -  The amount of energy supplied to it

-       -  The mass of the substance

-       -  What the substance is.

Heat loss and prevention from a house

We should be able to describe how heat energy is lost from buildings and explain how these losses can be reduced. Heat is lost in a variety of ways:

    Through the roof

    Through windows

    Through gaps around the door

    Through the walls

       Through the Door. 

Heat energy is transferred through the home by conduction through the walls, floor, roof and window. It is also transferred by convection. Very cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the roof tiles. Heat energy also leaves the house by radiation. There are a variety of ways to reduce heat loss. Some simple ideas might be, including fitted carpet, curtains and drought excluders. Heat loss through windows can be reduced by having double glazing windows. Heat loss through walls can be gained by using cavity wall insulation. Heat loss through walls can be reduced by laying lost insulation.

Heating and insulating buildings

Reducing the rate of energy transfer at home

Houses are heated by electric or gas heaters, oil or gas central heating systems, or by solid fuel in stoves or in fireplaces. Whichever form or heating you have in your home, the heating bills can be expensive. When your home heating system is transferring energy into your home to keep you warm, energy is also transferring energy into your home to keep you warm, energy is also transferring to the surroundings outside your home.  

Solar panels

Heating a home using electricity or gas can be expensive. Solar panels absorb infrared radiation from the Sun and are used to generate electricity directly (solar cell panels) or to heat water directly (solar heating panels). In the northern hemisphere, a solar panel is usually fitted on a roof that faces south so that it absorbs as much infrared radiation from the sun as possible.  

Most of the energy we use comes from burning fossil fuels, mostly gas or oil or coal. The energy in home, offices, and factories is mostly supplied by gas or by electricity e=generated in coal or gas – fired power stations. Oil is needed to keep road vehicles, ships, and aeroplanes moving. Burning one kilogram of fossil fuel releases about 30 million joules of energy. We use about 5000 joules of energy each second, which is about 150 thousand million joules each year. But because of the inefficiencies in how much energy is distributed and used, a staggering 10,000 kg of fuel is used each year to supple the energy needed just for you!  

Wind power

A wind turbine is an electricity generator at the top of a narrow tower. The force of the wind drives the turbine’s blades around. This turns a generator. The power generated increases as the wind speed increases. Wind turbines are unreliable because when there is little or no wind they do not generate electricity.  

Wave power

A wave generator uses the waves to make a floating generator move up and down. This motion turns the generator so it generates electricity. A cable between the generator and the shoreline delivers electricity to the grid system.  Wave generators need to withstand storms, and they don’t produce a constant supply of electricity. Also, lots of cables (and buildings) are needed along the coast to connect the wave generators to the electricity grid. This can spoil areas of coastline. Tidal flow patterns might also change, affecting the habits of marine life and birds.

Supply and demand

The demand for electricity varies during each day. It is also higher in winter than in summer. Electricity generators need to match these changes in demand. Power stations can’t just start up instantly. The start – up time depends on the type of power station.  

Cost comparisons

The overall cost of a new energy facility involves capital costs to build it, running costs for fuel and maintenance, and more capital costs to take it out of use at the end of its working life.  

The costs of new energy facilities are usually passes on the consumes through increased fuel bills. Energy – saving schemes such as low – energy light bulbs in your home would reduce the need for more power stations. Schemes such as improved home insulation reduce the demand for non – renewable energy resources (e.g. gas). Home owners pay upfront and eventually get their money back through reduced fuel bills.  

The variable demand for electricity is met by:

-       -  Using nuclear and coal – fired power stations to provide a constant amount of electricity (the base load demand)

-       -  Using gas – fired power stations and pumped storage schemes to meet daily variations in demand and extra demand in winter

-     -    Using renewable energy resources when demand is high and when the conditions for renewable energy generation are suitable (e.g. use of wind turbines in winter and when wind speeds are high enough.)

-     -    Using renewable energy resources when demand is low to store energy in pumped storage schemes.