# Physics II

This collection explores topics including electrostatics, magnetism, optics, wave-based oscillations, and electrical circuits; explaining underlying principles behind physical phenomena that have changed our world.

• Electric Fields

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

An electric field is generated by a charged object (referred to as the source charge) in the space around it, and represents the ability to exert electric force on another charged object (referred to as the test charge). Represented by a vector at any given point in the space, the electric field is the electrical force per unit test charge placed at that point (the force on an arbitrary charge would be the charge times the electric field). The electric field is fundamental to electricity and effects of charges, and it is also closely related to other important quantities such as electrical voltage. This experiment will use electrified powders in an oil that line up with electric fields produced by charged electrodes to visualize the electric field lines. This experiment will also demonstrate how an electric field can induce charges and how charges respond to the electric field by observing the effect of a charged rod on a nearby soda can.

• Electric Potential

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

Electric potential, also known as "voltage", measures the electric potential energy per unit charge. Electric field is a scalar quantity and is fundamental to many electrical effects. Like potential energy, what is physically meaningful is the difference in the electric potential. For example, the spatial variation in the electric potential is related to the electric field, which gives rise to the electric force on a charge. The difference in the electric potential between two points in a resistor drives the electric current flow. This experiment will use both a volt meter and a fluorescent tube to demonstrate the electric potential (more accurately, the potential difference between two points in space) generated by a charged sphere. The experiment will demonstrate the concept of equipotential surfaces, which are perpendicular to the electric fields.

• Magnetic Fields

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

Magnetic fields can be generated by moving charges, such as an electrical current. The magnetic field generated by a current can be calculated from the Maxwell equation. In addition, magnetic objects such as bar magnets can also generate magnetic fields due to microscopic dynamics of charges inside the material. Magnetic fields will exert magnetic force on other moving charges or magnetic objects, with the force proportional to the magnetic field. Magnetic fields are fundamental to electromagnetism and underlie many practical applications ranging from compasses to magnetic resonance imaging. This experiment will demonstrate magnetic fields produced by a permanent bar magnet as well as an electrical current, using small compass needle magnets that align with magnetic fields. This experiment will also demonstrate the force exerted by the magnetic fields produced by a current on another current-carrying wire.

• Electric Charge in a Magnetic Field

Source: Andrew Duffy, PhD, Department of Physics, Boston University, Boston, MA

This experiment duplicates J.J. Thomson's famous experiment at the end of the 19th century, in which he measured the charge-to-mass ratio of the electron. In combination with Robert A. Millikan's oil-drop experiment a few years later that produced a value for the charge of the electron, the experiments enabled scientists to find, for the first time, both the mass and the charge of the electron, which are key parameters for the electron. Thomson was not able to measure the electron charge or the electron mass separately, but he was able to find their ratio. The same is true for this demonstration; although here there is the advantage of being able to look up the values for the magnitude of the charge on the electron(e) and the mass of the electron (me), which are now both known precisely.

• Investigation Ohm's Law for Ohmic and Nonohmic Conductors

Source: Andrew Duffy, PhD, Department of Physics, Boston University, Boston, MA

This experiment investigates Ohm's law, which relates current, voltage, and resistance.

One goal of the experiment is to become familiar with circuit diagrams and the terminology involved in basic circuits, such as resistor, resistance, current, voltage, and power supply. By the end of the experiment, familiarity is gained with how to wire up a circuit and how to measure both the current passing through a circuit component and the potential difference, or voltage, across it. In a circuit, a battery or power supply provides a voltage measured in volts (V) that makes the charge flow. Other elements in the circuit, such as light bulbs or resistors (which are often just long narrow wires wound into coils) limit the rate at which the charge flows. The rate of flow of the charge is known as current measured in amperes (A), or amps for short, and the degree to which resistors and light bulb filaments limit the flow is known as their resistance measured in ohms (Ω). This experiment involves an exploration of Ohm's law, which relates voltage, current, and resistance. This experiment also explores the difference between a basic circuit component called a resistor, and a light bulb and a light-emitting diode (LED). Light bulbs and LEDs are part of many common devices and are used for various lighting applications, a

• Series and Parallel Resistors

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

This experiment demonstrates how current is distributed in resistors connected in series or parallel, and thus describes how to calculate the total "effective" resistance. Using Ohm's law, it possible to convert between the voltage and current through a resistance, if the resistance is known.

For two resistors connected in series, (meaning that they are wired one after the other), the same current will flow through them. The voltages will add up to a "total voltage", and thus, the total "effective resistance" is the sum of the two resistances. This is sometimes called a "voltage divider" because the total voltage is divided between the two resistors in proportion to their individual resistances. For two resistors connected in parallel, (meaning that they are both wired between two shared terminals), the current is split between the two while they share the same voltage. In this case, the reciprocal of the total effective resistance will equal the sum of the reciprocals of the two resistances. Series and parallel resistors are a key component to most circuits and influence how electricity is used in most applications.

• Capacitance

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

This experiment will use commercial capacitors and a parallel plate capacitor to demonstrate the concept of capacitance. A capacitor stores opposite charges on two conductors, for example two opposite metal plates, leading to a potential difference (voltage drop) between the two conductors. The amount of charge on each conductor is proportional to this voltage drop, with the capacitance as the proportionality factor. If the voltage is changing with time, the current flowing into the capacitor will be proportional to the rate of that change, and again the capacitance is the proportionality factor. The capacitance of the parallel plate capacitor is the product of the dielectric constant with the distance between the plates divided by the area of the plate. This experiment will demonstrate the proportionality with distance by first depositing some charge onto the capacitor and then using a high-impedance voltmeter (electrometer) to monitor the voltage between the plates as the distance is increased. The voltage change will also be monitored with a dielectric material, such as a plastic plate inserted into the space between the metal plates. A capacitance meter will be used to directly measure the capacitance, as well as to measure parallel and series connections of commercially-availa

• Inductance

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

This experiment will use inductive coils to demonstrate the concept of inductor and inductance. Magnetic induction will be demonstrated using a rod magnet inserted into or extracted away from the core of a coil to induce a transient electromotive force (emf) voltage in the coil, measured by a voltmeter. This experiment will also demonstrate the mutual inductance between two coils, where turning on or off a current flowing in a coil can induce an emf voltage in a second coil nearby. Finally, the experiment will demonstrate the self-inductance of a coil, when switching a current off induces an emf to light up a light bulb connected in parallel with the coil.

• RC/RL/LC Circuits

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

Capacitors (C), inductors (L), and resistors (R) are each an important circuit element with distinct behaviors. A resistor dissipates energy and obeys Ohm's law, with its voltage proportional to its current. A capacitor stores electrical energy, with its current proportional to the rate of change of its voltage, while an inductor stores magnetic energy, with its voltage proportional to the rate of change of its current. When these circuit elements are combined, they can cause the current or voltage to vary with time in various, interesting ways. Such combinations are commonly used to process time- or frequency-dependent electrical signals, such as in alternating current (AC) circuits, radios, and electrical filters. This experiment will demonstrate the time-dependent behaviors of the resistor-capacitor (RC), resistor-inductor (RL), and inductor-capacitor (LC) circuits. The experiment will demonstrate the transient behaviors of RC and RL circuits using a light bulb (resistor) connected in series to a capacitor or inductor, upon connecting to (and switching on) a power supply. The experiment will also demonstrate the oscillatory behavior of an LC circuit.

• Semiconductors

Source: Derek Wilson, Asantha Cooray, PhD, Department of Physics & Astronomy, School of Physical Sciences, University of California, Irvine, CA

Semiconductors are materials whose ability to conduct an electrical current depends strongly on their temperature and level of impurity. The most common type of semiconductor material is crystalline silicon. Most pure semiconductors are not outstanding conductors; to improve conductivity, a pure semiconductor is often combined or "doped" with an impurity. These impurities are either donors, like phosphorus and arsenic, that donate electrons to the silicon, or acceptors, like boron and aluminum, that steal electrons from the silicon. When acceptors take electrons from the silicon, they leave regions of positive charge called "holes" that effectively behave as positively charged electrons. A p-type semiconductor is formed when doping makes holes that are the dominant charge carrier in the material. An n-type semiconductor is formed when a semiconductor is doped such that the dominant charge carrier is the electron. As one might expect, a p-n junction is formed at the boundary between the p-type semiconductor and n-type semiconductor. The interaction of electrons and holes at the junction gives rise to the remarkable behavior seen in circuit components such as diodes and transistors. This lab will explore the properties of a single p-n

• Photoelectric Effect

Source: Yong P. Chen, PhD, Department of Physics && Astronomy, College of Science, Purdue University, West Lafayette, IN

Photoelectric effect refers to the emission of electrons from a metalwhen light is shining on it. In order for the electrons to be liberated from the metal, the frequency of the light needs to be sufficiently high such that the photons in the light have sufficient energy. This energy is proportional to the light frequency.The photoelectric effect provided the experimental evidence for the quantum of light that is known as photon. This experiment will demonstrate the photoelectric effect using a charged zinc metal subject to either a regular lamp light, or ultraviolet (UV) light with higher frequency and photon energy.The zinc plate will be connected to an electroscope, an instrument that can read the presence and relative amount of charges. The experiment will demonstrate that the UV light, but not the regular lamp, can discharge the negatively charged zinc by ejecting its excess electrons.Neither light source, however, can discharge positively charged zinc, consistent with the fact that electrons that are emitted in photoelectric effect.

• Reflection and Refraction

Source: Derek Wilson, Asantha Cooray, PhD, Department of Physics & Astronomy, School of Physical Sciences, University of California, Irvine, CA

Light travels at different speeds depending on the material through which it is propagating. When light travels from one material to another, it will either slow down or speed up. In order to conserve energy and momentum, the light must change the direction in which it propagates. This bending of light is known as refraction. Some fraction of the light is also reflected at the interface between the two materials. In special cases, a light beam can be refracted so sharply at an interface that it is actually completely reflected back into the medium from which it was coming. Lenses make use of the principle of refraction. Lenses come in two varieties with different curvatures: convex lenses and concave lenses. Convex lenses are often used to focus light but can also be used to create magnified images of objects. When a convex lens causes the light rays coming from an object to diverge, the human eye judges the light to be coming from some point behind the actual object from which the light is originating. The image of the object will in this case be magnified. This type of image is called a virtual image. Concave lenses can also cause light rays to diverge and create virtual images, though the image will be demagnified. This lab will demonstrate the funda

• Interference and Diffraction

Source: Yong P. Chen, PhD, Department of Physics & Astronomy, College of Science, Purdue University, West Lafayette, IN

Interference and diffraction are characteristic phenomena of waves, ranging from water waves to electromagnetic waves such as light. Interference refers to the phenomenon of when two waves of the same kind overlap to give an alternating spatial variation of large and small wave amplitude. Diffraction refers to the phenomenon of when a wave passes through an aperture or goes around an object, different parts of the wave can interfere and also give rise to a spatial alternation of large and small amplitude. This experiment will demonstrate the wave nature of the light by observing diffraction and interference of a laser light passing through a single slit and double slits, respectively. The slits are simply cut using razor blades in an aluminum foil and the characteristic diffraction and interference patterns manifest as patterns of alternating light and dark fringes on a screen placed after the foil, when the light is shone through the slit(s) on the foil. Historically, the observation of diffraction and interference of light played important roles in establishing that light is an electromagnetic wave.

• Standing Waves

Source: Arianna Brown, Asantha Cooray, PhD, Department of Physics & Astronomy, School of Physical Sciences, University of California, Irvine, CA

Standing waves, or stationary waves, are waves that appear not to propagate and are produced by the interference of two waves traveling in opposite directions with the same frequency and amplitude. These waves appear to vibrate up and down with no linear movement and are most easily identified in vibrating finite media like a plucked guitar string, water in a lake, or air in a room. For example, if a string is fixed at both ends and two identical waves are sent traveling along the length, the first wave will hit the end barrier and reflect back in the opposite direction, and the two waves will superpose to produce a standing wave. This motion is periodic with frequencies defined by the length of the medium and is a visual example of simple harmonic motion. Simple harmonic motion is motion that oscillates or is periodic, where the restoring force is proportional to the displacement, meaning the farther something is pushed, the harder it pushes back. The goal of this experiment is to understand the roles of wave superposition and reflection in creating standing waves, and exploit those concepts to calculate the first few resonant frequencies, or harmonics, of standing waves on a slinky. Each frequency that an object produces has its own standing

• Sound Waves and Doppler Shift

Source: Arianna Brown, Asantha Cooray, PhD, Department of Physics & Astronomy, School of Physical Sciences, University of California, Irvine, CA

Waves are disturbances that propagate through a material medium or empty space. Light waves can travel through a vacuum and some forms of matter, and are transverse in nature, which means that the oscillations are perpendicular to the direction of propagation. However, sound waves are pressure waves that travel through an elastic medium like air, and are longitudinal in nature, which means the oscillations are parallel to the direction of propagation. When sound is introduced to a medium by a vibrating object, like the vocal chords of a person or strings in a piano, the particles in the air experience forward and backward motion as the vibrating object moves forward and backward. This results in regions in the air where the air particles are compressed together, called compressions, and other regions where they are spread apart, called rarefactions. The energy created by a sound wave oscillates between the potential energy created by the compressions and the kinetic energy of the small movements and speeds of the particles of the medium. Compressions and rarefactions can be used to define the relationship between sound wave velocity and frequency. The goal of this experiment is to measure the speed of sound in air and explore the apparent change in frequency