Transcript Chapter 5
Chapter 5 Electricity and Magnetism What is electricity? • Electricity describes all of the phenomena caused by positive and negative charges – Examples … • Protons carry a positive charge (+) whereas electrons carry a negative charge (-) • If a body (atom/molecule or object) has MORE PROTONS than electrons, it will be positively charged. • If a body has MORE ELECTRONS than protons, it will be negatively charged. Illustrating charges • Draw two balloons, as shown below: Positively charged balloon Negatively charged balloon Forces of Attraction / Repulsion • Electrically charged objects, which have gained or lost ELECTRONS, tend to exert a force on other objects nearby. • Like charges (+ + or - - ) will tend to REPEL each other. • Opposite charges (+ - ) will tend to ATTRACT each other. • Charges, like matter or energy, are conserved. • http://www.youtube.com/watch?v=Fyko81W AvvQ Conductors and Insulators Charging an Object • Charging an object means creating an imbalance between the number of protons and electrons. • When an object has the exact same number of protons and electrons it is called NEUTRAL. • Not all objects behave the same way when they receive a charge. Conducting Continuum • Depending on what happens to an object when it receives a charge, we can say that it is a conductor or an insulator • Conductors allow charges to flow freely and spread evenly throughout the object • Insulators are objects that impede the flow of electrical charges and don’t allow them to move easily through the material. • Superconductors: a substance capable of becoming superconducting (lose all electrical resistance) at sufficiently low temperatures. • Example: mercury, niobium-tin, yttrium-barium-copper oxide. • Semi-conductors: a substance that has a conductivity between that of an insulator and that of a conductor (metal). • Examples: silicon, germanium Conducting Continuum • Every object fits somewhere along this continuum • Where do you think the human body would fit in the above diagram? Behaviour of charges • When a charge is acquired by a conductor, the charges will tend to distribute themselves across the surface. When a charge is acquired by an insulator, the charges will tend to remain at the initial location of charging. Static electricity • Static means “not moving” • Static electricity describes all of the phenomena related to charges that are at rest. • Charges are at rest when they are INSULATED • Static charge is often detected by a device known as an electroscope. • There are three methods for charging objects Methods of charging an object Method Friction Conduction Induction Before During After Triboelectric Series Dynamic Electricity • Dynamic means “moving” • Dynamic electricity describes all of the phenomena related to electrical charges in motion. • Charges move when they are free to do so, namely when they are in contact with conductors Electric Current • Electric current: refers to the orderly flow of electrons through a conductive medium • Flow is from negative to positive • Conventional current: is the direction in which a positive particle would flow in an electrical circuit [EVEN THOUGH THIS DOESN’T REALLY HAPPEN] • Flow is from positive to negative! Electric Circuits Symbols Explanation • There are certain symbols that will be important as we study circuits. Here are some of the most important. • Also learn the symbols for: – Switch – Light bulb Electric Circuit • There are three main categories of electric circuits (but we’re only going to study two): No choices… I wish I had choices! Ah choices…it’s nice to have choices! Definitions in Electric Circuits • There are three definitions that are very important in the study of circuits. • These are: – Current Intensity – Potential Difference (Voltage) – Resistance Current Intensity • Symbol: I Unit: Amperes (Amps, A) • is the number of charges that flow past a given point in a circuit per second. • Formula: I = q/t • Measured with a device called an AMMETER, which must be connected in series! Potential Difference (Voltage) • Symbol: V Unit: Volts (V) • the amount of energy transferred between two points in an electrical circuit. • Formula: V = E/q • Measured with a device called a VOLTMETER, which must be connected in parallel! Resistance • Symbol: R Unit: Ohms (Ω) • is the ability of a material within an electrical circuit to hinder the flow of electric current. • Formula: R = V/I • Measured with a device called an OHMMETER, which must be disconnected in from the circuit to take a measurement! • Resistance is dependent on many factors (see notes) Ohm’s Law • Is the mathematical relationship between current intensity, potential difference and resistance. • It states that for a given resistance, the potential difference is directly proportional to the current intensity. • V=IxR or I=V/R or R = V / I Power • Symbol: P Unit: Watt (W) • is the rate at which work is performed by an electrical device in a circuit per second. • Formulae: P = Work/time (W/t) P = V•I (which can be measured directly from the circuit by using an ammeter and voltmeter) Electrical Energy • the amount of energy used by an electrical device is often the same as the work done by the device. In order to calculate the amount of money it costs to operate an electrical device, it is important to know the energy rating. • You can convert between Energy and Power by using E = P • t • Pay attention to units!** ** Sometimes Energy is expressed in kilowatt hours (kWh) and sometimes it’s expressed in Joules (J). ** Electricity costs $$$ Cost = cost of electricity • amount of electricity used $ = $/kWh • kWh Magnetism • Magnetism describes all of the phenomena caused by magnets. • Magnets are objects that can attract other objects containing primarily iron, cobalt or nickel. Theory of Domains Magnets have poles • Just like in charges opposites attract and likes repel • The north pole of a magnet is the end that naturally seeks the Earth’s magnetic pole near the geographic North Pole. •The other side of the magnet is its south pole. Compass • A compass is a device that has a freely turning magnetized needle. This needle will always try to point north, and so is helpful for navigational purposes. Note: when a compass is brought near a magnet, it will align itself with the magnetic field, and is no longer a reliable navigation tool. Magnetic Fields • Magnetic fields are the areas in space in which the force of a magnet can act on another magnet. • These fields are represented by lines with arrows, and the closer these lines are to one another, the stronger the field. • The direction of the arrows indicate the direction of the magnetic field, which is always from NORTH SOUTH. • Magnets come in different shapes and sizes. Magnetic Fields in Straight Wires • Current-carrying wires, like magnets, will exert a magnetic field. • The direction of the magnetic field is determined by the RIGHT HAND RULE which states: “when the thumb of the right hand points from + to -, the fingers of the right hand point in the direction of the magnetic field”. Examples of Magnetic Fields