Physics+2025+Lab+6A

Phys 2025 Lab 6A - Static Electricity and Charge Sensors Your Name: Time of Day Class Starts: Day of Week: Date: Your partner’s names: In the theoretical section of this lab, we explore the relationship between charge distributions and voltage, (particularly ground voltage), and explain the operation of a charge sensor. The experiments consist mainly of using the charge sensor to determine the charges on various objects. We also cover charging by induction. Charge Distributions and Voltage Imagine a cluster of positive charges grouped closely together, as shown below. Your intuition should tell you that the charges will spread out as much as possible, since like charges repel each other. Q Q Q We can also think about this in terms of voltages. The voltage V a distance r from the center of a charge distribution with charge Q is given by V = Q 4 ⇡" 0 r So the farther away you get from the center of the charge distribution, the lower the voltage. The positive charges start o ↵ at a high voltage, since they are clustered so closely together. Positive charges have a natural tendency to flow from high voltage to low voltage, so the charges flow outward to where the voltage is lower. A ground is a large conductor that allows charges inside it to spread out as much as possible. In an ideal ground, the distance between charges is virtually infinite, so the voltage is 0. V ground = 0 The circuit symbol for a ground element is shown below. 1

Charge Sensors A circuit diagram showing the inside of a charge sensor is shown below. V Charge Sensor A charge sensor consists of two main components: a capacitor and a voltmeter in parallel. By measuring the voltage across the capacitor ( V ), and knowing the capacitance of the capacitor ( C ), the charge sensor is able to determine the amount of charge stored in the capacitor ( Q ) by using the capacitance equation Q = CV The capacitor has a capacitance of about 10 nF, which is quite small. You’ll remember from a previous lab that a voltmeter connected in parallel with a low-capacitance capacitor can drain the capacitor relatively quickly, because of the small RC time constant. In order to compensate, the charge sensor uses a high-impedance voltmeter , which has a very large internal resistance. This results in a longer RC time constant, which prevents the charge from draining o ↵ too quickly. Initial Setup For this experiment, you will need • 1 Desktop Computer • 1 Vernier ® LabQuest Stream or LabPro Interface • Vernier ® Logger Pro or Graphical Analysis Software • 1 Vernier ® Charge Sensor • 1 Vernier ® Electrostatics Kit (containing) – 1 Faraday Cage – 1 Faraday Pail – 1 Plastic Base Place the green plastic base onto your lab bench, and insert the Faraday pail (a metal cup) and the Faraday cage (a cylindrical metal cage) onto the base. Connect the LabQuest Stream to your computer, and connect the charge sensor to the LabQuest Stream. Connect the ground probe of the charge sensor directly to the Faraday cage. This will connect the Faraday cage directly to Earth ground. The Faraday Cage acts as a shield that absorbs stray charge particles and diverts them directly to ground. (Note, it is important to use a desktop computer because it will be connected directly to Earth ground through the ground 2

In this case, the charge on the capacitor is referred to as an induced charge , because instead of being transferred directly from the source (through contact), it was induced by the electric field emanating from the source. Experiment 1 - Charging through Contact When objects made of two di ↵ erent materials are rubbed together, sometimes one material will have a stronger a ffi nity for electrons than the other, and will actually steal electrons from the other material. This results in the two objects developing opposite charges. In this lab, you will be using a charge sensor to measure the charge on various objects that have been charged through contact, and determine which material was the electron acceptor, and which material was the electron donor. For this experiment, you will need • Your initial setup • 1 Vernier ® Electrostatics Kit (containing) – 1 White Plastic Rod – 1 Black Plastic Rod – 1 Brown Cloth – 1 Green Plastic Sheet – 1 White Plastic Disk – 1 Black Plastic Disk Rub one end of the white plastic rod with the green plastic sheet for several seconds. Dip the charged end of the rod into the cup, without touching the sides. What charge do you measure on the rod? ( Question 1A ) Which material was the electron donor? ( Question 1B ) Which material was the electron acceptor? ( Question 1C ) Rub one end of the black plastic rod with the brown cloth for several seconds. Dip the charged end of the rod into the cup, without touching the sides. What charge do you measure on the rod? ( Question 1D ) Which material was the electron donor? ( Question 1E ) Which material was the electron acceptor? ( Question 1F ) Gently rub together the white and black plastic disks for several seconds. Dip the white disk into the cup, without touching the sides, and measure the charge. Then dip the black disk in and measure the charge. What charge do you measure on the white disk? ( Question 1G ) What charge do you measure on the black disk? ( Question 1H ) Which material was the electron donor? ( Question 1I ) Which material was the electron acceptor? ( Question 1J ) 4

Experiment 2 - Charging By Induction For this experiment, you will need • Your initial setup • 1 Vernier ® Electrostatics Kit (containing) – 1 Black Plastic Rod – 1 Brown Cloth • 1 Alligator Clips Read all of the instructions (all the way to Question 2D) before starting the experiment. Rub one end of the black plastic rod with the brown cloth for several seconds. Dip the charged end of the rod into the cup, without touching the sides, and hold it there. What charge do you measure on the rod? ( Question 2A ) With the rod still hovering in the cup, connect one end of the alligator clips to the faraday cage, and touch the other end to the cup for several seconds. What charge do you measure now? ( Question 2B ) With the rod still hovering in the cup, disconnect the alligator clip from the faraday cage and remove them. What charge do you measure now? ( Question 2C ) Remove the rod from the cup. What charge do you measure now? ( Question 2D ) What you have just done is called charging by induction . In the first step, you induce a charge - Q on the pail and a charge of Q on the capacitor by placing the charge source in the pail. Q V Pail ( - Q ) Charge Sensor Q In the next step, the capacitor is shorted with a wire, and all of the charge drains away. However, a charge - Q remains on the pail, attracted to the charge source. 5

Answer Sheet Experiment 1 A) What is the charge on the white plastic rod? B) Which material was the electron donor? C) Which material was the electron acceptor? D) What is the charge on the black plastic rod? E) Which material was the electron donor? F) Which material was the electron acceptor? G) What is the charge on the white plastic disk? H) What is the charge on the black plastic disk? I) Which material was the electron donor? J) Which material was the electron acceptor? Experiment 2 A) What charge do you measure on the black rod? B) What charge do you measure with the cup shorted to ground? C) What charge do you measure with the cup disconnected from ground? D) What charge do you measure with the rod removed from the cup? E) What would happen if we repeated this experiment using the white rod and the green plastic sheet? 7 +5MC Green plastic sheet white Plastic rod - TMC Brown cloth Back plastic rod + 3MC - 3MC Black plastic disk white Rastic disk - IINC onc - 12MC - -MC # would result in a positive charge induced on the cup .