Lab Experiment on Resistance and Ohm's Law | PHYS 212P, Lab Reports of Physics

Download Lab Experiment on Resistance and Ohm's Law | PHYS 212P and more Lab Reports Physics in PDF only on Docsity! Physics Pre-lab 212P-5 Resistance and Ohm's Law Name:__________________________ Section:_____ Date:__________ (Read this & answer the questions before coming to lab. Note that this prelab requires the use of your experimental kit.) Summary of relevant concepts:  If a potential difference is applied across a conductor, there is a net flow of charge that we call a current. This is no longer a case of static equilibrium; the electric field inside the conductor is NOT zero.  Current is defined as the rate of flow of POSITIVE charge through a cross-sectional area of a conductor: i.e. I = dq/dt  The actual flow of charge in most conductors consists of ELECTRONS. So, by convention, the direction of current is in fact opposite to the actual flow of charge.  Current is a SCALAR and is measured in AMPERES: 1 Ampere = 1 Coulomb/second  Current density J is a VECTOR defined follows:   AdJI  If the current density over the cross-section A of a conductor is uniform, then I = JA.  If a potential difference V applied across a conductor results in a current I, then the resistance of the conductor is defined as: R = V/I; this is NOT by itself a statement of Ohm's Law  The resistivity of a material is defined as = |E|/|J|E|E|/|J|/|E|/|J|J|E|/|J|.  For a conductor of length L and uniform cross-sectional area A, the resistance R and the resistivity  are related by: R L A  Ohm's Law states that -- under certain conditions -- the resistivity of a material is independent of the electric field E or that the resistance of an object is independent of the voltage applied across it.  When a voltage is applied across a conductor that has n electrons per unit volume, individual electrons travel slowly at a drift speed vd related to the current density J = (ne) vd  Resistance in a conductor arises from collisions of the electrons with the vibrating lattice of positive ions. Since the effect of these vibrations increases with temperature, the resistance of a metal increases with temperature. Pre-lab Questions: These prelab questions require you to carry out some simple experiments. It may be convenient to enlist the help of a friend for an extra pair of hands. Q1. How to make really cheap ammeter ("RCA"). Your first goal is to learn how to measure a current using the compass supplied in your kit. Note that you will not understand in detail how this measurement works until later in the semester when we encounter magnetic fields. For now, just treat it as a way to make a really cheap ammeter -- referred to from this point on as an RCA. First, clear up an area on your desk for setting up your work. Make sure that there are no steel or iron objects close by, since you will be using magnetism to measure electric currents. Then, hook up the circuit shown in two types of diagrams below: the one of the left is a non-technical representation, while the one on the right is a "circuit diagram." Note how the batteries are connected to each other -- this is called a "series" arrangement. Note also that -- in the circuit diagram -- the bulb is simply represented by the general symbol for a resistance. From now on, we'll only use the type of diagram shown on the right. For convenience, we've also shown below some useful circuit symbols. Some common circuit symbols: + - + + - - Bulb (resistor) capacitor inductor diode Variable resistor V voltmeter