RI Chap 4 Energy and Fields Lecture Notes

4 ENERGY & FIELDS H2 Physics 9749 Content Page 1.1 Introduction 2 1.2 Systems 2 1.3 Energy Stores & Transfers 3 1.4 Energy Transfers & Work Done 5 1.5 Kinetic Energy 9 1.6 Elastic Potential Energy 10 1.7 Fields 111 1.8 Gravitational Potential Energy 133 1.9 Electric Potential Energy 199 1.10 The Principle of Conservation of Energy 233 1.11 Power & Efficiency 244 Learning Outcomes Candidates should be able to: (a) show an understanding that physical systems can store energy, and that energy can be transferred from one store to another. (b) give examples of different energy stores and energy transfers, and apply the principle of conservation of energy to solve problems. (c) show an understanding that work is a mechanical transfer of energy, and define and use work done by a force as the product of the force and displacement in the direction of the force. (d) derive, from the definition of work done by a force and the equations for uniformly accelerated motion in a straight line, the equation 21 2 KE mv= . (e) recall and use the equation 21 2 KE mv= to solve problems. (f) show an understanding of the concept of a field as a region of space in which bodies may experience a force associated with the field. (g) define gravitational field strength at a point as the gravitational force per unit mass on a mass placed at that point, and define electric field strength at a point as the electric force per unit charge on a positive charge placed at that point.

Page | 2 (h) represent gravitational fields and electric fields by means of field lines (e.g. for uniform and radial field patterns), and show an understanding of the relationship between equipotential surfaces and field lines. [Equipotential surfaces not in H1 syllabus] (i) show an understanding that the force on a mass in a gravitational field (or the force on a charge in an electric field) acts along the field lines, and the work done by the field in moving the mass (or charge) is equal to the negative of the change in potential energy. (j) distinguish between gravitational potential energy, electric potential energy and elastic potential energy. (k) recall that the elastic potential energy stored in a deformed material is given by the area under its force-extension graph and use this to solve problems. (l) define power as the rate of energy transfer. (m) show an understanding that mechanical power is the product of a force and velocity in the direction of the force. (n) show an appreciation for the implications of energy losses in practical devices, and solve problems using the concept of efficiency of an energy transfer as the ratio of useful energy output to total energy input. 1.1 Introduction Now that we have learnt about the basics of forces and its relationship with motion, it is time to turn our attention towards another important concept in Physics – energy. Using energy to analyse the motion of systems can be very powerful, because while forces often deal with analysing how each component of a system behaves at each instant, energy often uses a “big picture” approach to analyse a complex system. We will begin by defining some broad terms relating to systems, before introducing different energy stores and transfers, and how these are all tied together by a very important and fundamental concept in Physics – the principle of conservation of energy. 1.2 Systems In Physics, we often use the word “system” to describe a collection of objects that we would like to focus our analysis on. As an example, when trying to understand how the planets move, we could call our entire Solar System a “system”. However, we could also choose to focus only on the “system” consisting of the Sun and our Earth, leaving everything else (the Moon, the other planets…) outside of our “system”.  Types of systems Systems can be broadly categorised into three different types, based on their ability to interact with their surroundings. 1. Open systems are systems that are able to exchange both energy and matter with their surroundings.

RAFFLES INSTITUTION YEAR 56 PHYSICS DEPARTMENT Page | 3 2. Closed systems are systems that are able to exchange energy, but not matter, with their surroundings. 3. Isolated systems are systems that are unable to exchange energy nor matter with their surroundings. For example, a container of gas with a hole in its walls is an open system, since it is able to exchange both energy and matter (i.e. the gas particles) with the surroundings freely. On the other hand, a sealed container of gas is a closed system. This is because although matter exchange with the surroundings is no longer possible, its walls are not insulated to prevent exchange of thermal energy with the surroundings. Therefore, for example, if the temperature of the gas in the container is higher than the surroundings, thermal energy will be able to flow from the gas to the surroundings. Finally, an isolated system can be represented by a sealed container of gas with perfectly insulated walls, such that both matter and energy exchange with the surroundings are impossible. Note that there should also not be any net external force doing work on an isolated system, because as we will soon learn, work done is actually a process of energy transfer in and out of systems. Hence, for isolated systems to have no energy exchange betw een itself and its surroundings, there must also be no work done on the system. Fig. 1.1 Fig. 4.1 1.3 Energy Stores & Transfers From your previous studies in Physics, you would have learnt about energy as the capacity to do work. It is a quantity measured in Joules (J) as its SI unit. You would also have learnt about the energy of a system being present in different stores, and how systems can transfer energy from one store to another. We are now going to expand on your previous knowledge of energy, energy stores and energy transfers with some new concepts. open closed isolated

Page | 4  Energy stores There are eight different types of energy stores in which systems store energy. They are: Kinetic energy Magnetic potential energy † Gravitational potential energy Electric potential energy Chemical potential energy Nuclear energy Elastic potential energy Internal energy * A simple mnemonic device you can use to help you remember all eight is to visualise a group of men carrying a 1 kg block of ice (in Fig. 4.2): Fig. 4.2 Remembering the words “MEN”, “KG” and “ICE” should help you recall the eight energy stores, because it stands for Magnetic potential, Electric Potential, Nuclear, Kinetic, Gravitational potential, Internal, Chemical potential, Elastic potential. A brief description of the nature of the energy in each store can be found in the table below. Energy stores Description Kinetic Energy due to the motion of an object Gravitational potential Energy due to the position of a mass in an external gravitational field Elastic potential Energy due to the physical deformation of an object (e.g. compressing or stretching a spring) Internal Consists of two components: energy due to the random motion of atoms / molecules in a system (internal kinetic energy) and energy due to interatomic / intermolecular forces (internal potential energy) †: Not covered in H1 / H2 Physics syllabus *Note: You might have previously learnt and referred to this as thermal energy. However, in later topics within H2 Physics, we will learn to distinguish between internal energy and thermal energy. 1 kg

RAFFLES INSTITUTION YEAR 56 PHYSICS DEPARTMENT Page | 5 Chemical potential Energy due to the structural arrangement of atoms / molecules in a substance Electric potential Energy due to the position of a charge in an external electric field Magn