Electricity Lesson Objectives: • Describe the Bohr model of the atom • Describe electron flow • Describe conventional current flow • Describe conductors, insulators & semiconductors • Define e.m.f. & p.d. • Define direct current and resistance. 1 To understand electricity we need to look inside the atoms that make up all forms of matter. Since we cannot actually do this, we need to use a simple model of the atom. This model is known as the Bohr model, see diagram below, this shows a single atom consisting of a central nucleus with orbiting electrons. Within the nucleus: • Protons are positively charged. • Neutrons have no charge (Neutral) - Orbiting the nucleus: • Electrons are negatively charged Electron Nucleus The electrical charge on Protons & Electrons are equal and opposite. + + Electron - The Bohr model of the atom Protons and Neutrons A stable atom will have the same number of Protons and Electrons and is therefore electrically neutral (no charge) When an atom loses an electron it becomes a positive ion When an atom gains an electron it becomes a 2 negative ion • The number of electrons occupying a given orbit within an atom depends on the position of the element within the periodic table. • The electrons in all atoms sit in a particular position (orbit or shell) dependant on their energy level. • Each of these shells within the atom is filled by electrons from the nucleus outwards • The first shell (inner) of these shells can have up to two electrons; the second shell can have up to eight and the third eighteen. Outer electron orbit Loosely bound electron • When atoms have single or small numbers of electrons in their outer orbit (or shell), these are loosely bound to the atom and can be easily dislodged. • Once dislodged from the atom they are called Free Electrons. 3 Conductors • A material that has many free electrons available to act as charge carriers and thus allows current to flow freely is known as a conductor. • These free electrons become the charge carriers within the material. • Examples of good conductors are aluminium, copper, gold, iron. Free electrons moving randomly around the material + The application of an external force causes free electrons to move in a uniform direction, then an electric current is said to flow In a material containing free electrons their direction of motion is random, as shown above, but if an external force (electro motive force) is applied, that causes the electrons to move in a uniform manner and an electric current is said to flow. 4 Insulators • Materials that do not conduct electrical charge are called insulators. • The electrons in insulators are tightly bound to the nuclei of the atoms. (i.e. no free electrons) • Examples of insulators include plastics, glass, rubber and ceramic materials. Semiconductors • These materials combine some of the electrical properties of conductors and insulators. • In these materials, there may be a number of free electrons sufficient to allow a small current to flow. • It is possible to add foreign atoms (called impurity atoms) to the semiconductor material that modify the properties of the semiconductor • Varying combinations of these additional atoms are used to produce various electrical devices such as diodes and transistors • Common types of semiconductor materials are silicon, germanium, selenium and gallium. 5 Temperature effects. • All materials offer some resistance to current flow. • In conductors the free electrons collide with the relatively large and solid nuclei of the atoms. • As the temperature, increases the nuclei vibrate more and further obstruct the path of the free electrons causing more collisions. • The result is that the resistance of conductors increases with temperature rise. • In insulators there are no free electrons, but as temperature increases, a few outer electron manage to break free from their fixed position and act as charge carriers • The result is that the resistance of insulators decreases with temperature rise. • In semiconductors, as temperature increases large numbers of electrons break free to act as charge carriers. • The result is that the resistance of semiconductors decreases rapidly with temperature rise. • Special alloys, such as eureka and manganin combine the effects of insulators and conductors. The resistance of these materials remains constant with temperature rise. 6 Insulators Resistance R Semiconductors Conductors Special alloys (e.g. Eureka, Manganin) Temperature T Variation of resistance with temperature 7 Electromotive Force (e.m.f.) and Potential Difference (p.d.) (Voltage) • The ability of an energy source to (e.g. a battery) to produce current within a conductor may be expressed in terms of electromotive force (e.m.f). • Whenever an e.m.f. is applied to a circuit a potential difference (p.d.) exists. • Both the e.m.f. and the p.d. are measured in volts (V). • When the power source is connected, a p.d. will be developed across each component in the circuit. p.d. Positive terminal + e.m.f - p.d. Negative terminal p.d. 8 Conventional current flow - + • Conventional current flow is from positive potential to negative potential. • You will note that this is in the opposite direction to electron flow, which is towards the positive potential. • The reason for this is that the direction of conventional current flow was decided before it was known that negative electrons carried the electrical charge. 9 Direct current • Direct current results from the application of a direct e.m.f. (derived from batteries or a DC power supply). • An essential characteristic of these supplies is that the applied emf does not change its polarity. (even though its value might be subject to fluctuation). • Direct current flow is in a single constant direction. Resistance • The amount of current that will flow in a conductor when a given e.m.f. is applied is inversely proportional to the resistance. • Resistance therefore, can be thought of as an opposition to current flow. • The higher the resistance the lower the current that will flow. 10 The simple electric circuit shown below comprises a battery, which provides the e.m.f. The battery is connected to a resistor in a closed circuit in which current will flow. The e.m.f. (V) supplied by the battery will be equal to the potential difference (V) developed across the resistor. (R). Current I + Electromotive force V + Resistance, R - Potential difference, V Current I 11
