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- Know that energy and work are measured in joules (J), and power in watts (W).
When a force is acting on an object, work is being done on that object. Note that work is only done if the object moves/ is compressed or decompressed in the same direction as the force applied on it.
The unit for both energy and work is joules.
This is because the two are interchangeable – ‘work done’ is the same as ‘energy expended’.
Let me give you some examples:
You push a box along the floor. The box was initially at rest, but since you are now doing work on the box, the box moves, and thus gains kinetic energy. Here, work gives kinetic energy.
Now imagine a game of pool – you hit the cue ball towards one of the other balls on the table. As the cue ball moves, it has kinetic energy. The cue collides with another ball, and thus applies a force on it – which means the cue ball does work on this other ball – and the other ball starts to move. The cue ball was only able to do work on the other ball because of its kinetic energy.
The unit of power is watts (W). I don’t think this one requires an explanation 😊
- Demonstrate understanding that an object may have energy due to its motion (kinetic energy, K.E.) or its position (potential energy, P.E.), and that energy may be transferred and stored.
If an object is in motion, it has kinetic energy – the faster the object moves, the greater its K.E.
Another type of energy is potential energy. In this case, energy is stored due to the object’s position in a gravitational field.
This makes more sense when you remember the law of Conservation of Energy. The law states that the total energy of an isolated system remains constant – it is said to be conserved over time. This means that energy cannot be created or destroyed – only converted from one form to another.
So if you hold a ball that weighs 1kg at a height of 1m above the ground, then it has a P.E. approximately equal to 9.81J. When you let go, the ball falls and gains speed because of the work done on it by gravity. It gains kinetic energy and loses potential energy at the same time – the potential energy of the ball is converted to kinetic energy.
Because the ball had a P.E. of 9.81J before it falls, its maximum K.E. (the K.E. it attains just before it hits the ground) is also 9.81J.
Therefore, the position of the ball allowed it to store a P.E. of 9.81J, and as it fell, this energy was released as K.E.
Energy can also be transferred from one object to another.
Remember the pool example – the cue ball initially had kinetic energy, but then it collided with another ball.
What do you think happens next? Does the cue ball continue to travel at the same speed? No! Some of the kinetic energy of the cue ball is transferred to the other ball, so that ball starts to move and the cue ball loses speed.
- Recall and use the expressions K.E. = ½mv2 and P.E. = mgh
‘m’ is the mass of the object, measured in kg.
‘v’ is the velocity of the object, measured in m/s.
‘g’ is the acceleration of free fall – i.e. the acceleration of an object as it falls freely in Earth’s gravity. This is the same for all objects as gravity does the same amount of work on every single object, no matter its mass. In real life, any differences in how fast different objects fall is caused by air resistance. The approximate value of g is 9.81m/s2.
‘h’ is the height of the object above the base level. In most cases, the base level is taken to be the ground. However, sometimes, the base level may be taken as a shelf so the P.E. of any objects on the shelf will be 0J, the P.E. of any objects below the shelf will be negative and the P.E. of any objects above the shelf will be positive.
You will need to use these two formulae to calculate K.E. and P.E., or in some cases, to calculate m, v, g or h if you have the other values.
You should practise past papers to become good at this.
- Give and identify examples of energy in different forms, including kinetic, gravitational, chemical, strain, nuclear, thermal (heat), electrical, light and sound.
Kinetic energy: objects in motion have kinetic energy. The faster the motion, the greater the kinetic energy. Examples include vehicles, moving your hand, kicking a ball, the earth orbiting the sun, vibrating atoms, etc.
Gravitational potential energy: the energy stored in an object due to its position in a gravitational field. Examples include: the energy of a book on a shelf, the energy of a person sitting on a swing at the exact moment that the swing reaches its highest point, the energy of a stationary cyclist at the top of a hill, etc. (Note that moving object can also have gravitational potential energy)
Chemical energy: the potential energy stored in the bonds of chemical compounds. Different bonds have different amounts of chemical energy. When a bond breaks, it releases the chemical energy as heat energy, and when bonds form, heat energy is taken away from the surroundings and stored as chemical energy. That’s why there are changes in temperature during a chemical reaction. If more heat energy is released than used, the reaction becomes exothermic (it releases heat to its surroundings), and if more heat energy is used in the formation of bonds than the heat released in the breaking of bonds, then the reaction is endothermic (it takes away heat from the surroundings).
Strain energy: the energy stored in an elastic body due to deformation. For example, when you compress a spring, the energy you use to compress it (in other words, the work done on the spring) is stored in the spring as strain energy. Therefore, strain energy is also a form of potential energy. When you let go, the spring can resume its normal shape by converting its strain energy to kinetic energy. Other examples include the stretching of a rubber band, the use of an elastic in slingshots, etc.
Nuclear energy: the energy released during nuclear fission or fusion. Nuclear fission is the splitting of a large nucleus to form two or more smaller nuclei. It is normally achieved by firing an accelerated neutron at a large nucleus. Nuclear fission is how energy is produced in nuclear power plants. Nuclear fusion is when two very light nuclei combine to make a larger nucleus. It is much harder to achieve than fission because it requires a lot of energy to overcome the repulsion between the two highly positive nuclei. I’m going to explain why both fission and fusion release energy, because I’ve been asked this question a lot of times, but you do not need to know this information for your exam. This is because Iron-56 is the most stable nucleus (its nucleons are most tightly bound) and as a general rule, the closer a nucleus gets to resembling an Iron-56 nucleus, the more stable it becomes. So when very large nuclei split, they become more stable and thus release energy (they require less energy to hold themselves together) and when very small nuclei fuse, they become more stable and release energy.
Thermal (heat) energy: this is the energy that comes from heat. The hotter something is, the greater the thermal energy. Heat results in the vibration of atoms and molecules (kinetic energy), so we measure temperature using the average kinetic energy of the particles in the object. This is why temperature is not equivalent to thermal energy. Thermal energy can be transferred from one object to another. For example, when we put ice in a drink, thermal energy is transferred to the ice from the drink – this is why the ice melts and the drink becomes colder.
Electrical energy: the energy required to move charged particles through a conductor. In other words, the energy required to create a current. Batteries and cells provide electric energy when connected in a complete circuit.
Light energy: Light is energy. It’s the only kind of energy we can see, although there are certain types of light that aren’t visible to the naked human eye (like Infrared radiation, Ultraviolet radiation, X rays, etc.) Everything on the electromagnetic spectrum is considered light. Light energy can be created from heat, kinetic energy, even chemical energy etc. It is created due to disturbances in the electric and magnetic fields.
Sound energy: Sound is the movement of energy through the vibration of matter in longitudinal waves. Longitudinal waves are waves in which the vibrations occur in the same direction as the movement of energy. And this energy is sound energy. Let me explain using an example – hitting a drum causes its surface to vibrate (it rapidly moves up and down). This causes waves of vibration to travel through the air. These waves carry energy and this energy is sound energy. Other examples: singing, talking, strumming a guitar, playing the flute, etc.
- Give and identify examples of the conversion of energy from one form to another, and of its transfer from one place to another.
I’ve already explored this point in length over the course of the previous four points, so I need not explain it again here.
- Apply the principle of energy conservation to simple examples.
This point has also been thoroughly explored in point 2.
The best way to improve your knowledge on this point is to practise past papers.
If you don’t already know, we’ve provided links to several websites that you can get past papers for the IGCSE syllabus, on the Resources page.
Notes submitted by Sarah.
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