◄Unit 8 :Universal gravitational field and potentialUnit 10 :Applications of thermodynamics laws►

Unit 9 : Electric field and electric potential

  • Unit 9 : Electric field and electric potential

    Key Unit Competence

    Analyze the electric field and electric potential. 

    My goals

    By the end of this unit, I will be able to:
    * define electric field and electric potential.
    * explain the relationship between electric potential and electric field 
    intensity.
    * describe functioning of lightening arrestors.

    * identify the dangers of lightening and how to avoid them.

    Introductory activity

    When you pass nearby an electric power station, you may notice some sign 

    post as shown in figure below.

    C

    This is to show that, the high voltage on the grid may affect people who 

    dares to approach it.

    Questions

    1. Discuss about the reasons behind the information given by the sign post.
    2. Is it possible that electricity can affect someone without touching the 
    wires (cables)?
    3. Discuss other areas where you can get such sign post and explain the 

    reason behind.

    Introduction

    Have you ever heard sound due to lightening? If yes, what do you think was 
    the cause?

    If not, ask your friend in your class, at home, or neighbour about lightening.

    Scientifically, lightening and thunder are effects of electric charges created in 

    space (will be discussed later).

    Attraction and repulsion of charges

    Activity 1

    In this section, you will observe the characteristics of the two types of 
    charges, and verify experimentally that opposite charges attract and like 

    charges repel.

    Equipment
    * Two lucite rods
    * One rough plastic rod
    * Stand with stirrup holder
    * Silk cloth

    * Cat’s fur

    C

    Procedure

    1. Charge one lucite rod by rubbing it vigorously with silk. Place the 
    rod into the stirrup holder as shown in Figure.

    2. Rub the second lucite rod with silk, and bring it close to the first 

    rod

    3. What happens? Record the observations in your notes.

    4. Rub the rough plastic rod with cat’s fur, and bring this rod near the 

    lucite rod in the stirrup. Record your observations.

    5. What do you conclude?

    6. Note down observation in your notebook.

    For reference purposes, according to the convention originally chosen by 
    Benjamin Franklin, the lucite rods rubbed with silk become positively charged, 
    and the rough plastic rods rubbed with cat’s fur become negatively charged. 

    Hard rubber rods, which are also commonly used, become negatively charged.

    Coulomb’s law

    Activity 2 

    Materials
    * Coulomb’s Law apparatus
    * Electrophorus (The electrophorus is a simple electrostatic induction 
    device. It’s an inexhaustible source of charge”).
    * Silk cloth.

    * A computer for the graph and quick calculation.

    C

    Procedure

    1. Take a moment to check to position of the hanging ball in your 
    Coulomb apparatus. Look in through the side plastic window. The 
    hanging ball should be at the same height as the sliding ball (i.e. 
    the top of the mirrored scale should pass behind the centre of the 
    hanging pith ball, as in Figure 10.3 below). Lift off the top cover 
    and look down on the ball. The hanging ball should be centred on 
    a line with the sliding balls. If necessary, adjust carefully the fine 

    threads that hold the hanging ball to position it properly.

    2. Charge the metal plate of the electrophorus in the usual way by 
    rubbing the plastic base with silk, placing the metal plate on the 

    base, and touching it with your finger.

    3. Lift off the metal plate by its insulating handle, and touch it 

    carefully to the ball on the left sliding block.

    4. Slide the block into the Coulomb apparatus without touching the 
    sides of the box with the ball. Slide the block in until it is close to 
    the hanging ball. The hanging ball will be attracted by polarization, 
    as in Section III of this lab. After it touches the sliding ball, the 
    hanging ball will pick up half the charge and be repelled away. 
    Repeat the procedure if necessary, pushing the sliding ball up until 

    it touches the hanging ball.

    5. Recharge the sliding ball so it produces the maximum force, and 
    experiment with pushing it towards the hanging ball. The hanging 

    ball should be repelled strongly.

    6. You are going to measure the displacement of the hanging ball. You 
    do not need to measure the position of its centre, but will record 
    the position of its inside edge. Remove the sliding ball and record 
    the equilibrium position of its inside edge that faces the sliding 
    ball, which you will subtract from all the other measurements to 

    determine the displacement d.

    7. Put the sliding ball in, and make trial measurements of the inside 
    edge of the sliding ball and the inside edge of the hanging ball. The 
    difference between these two measurements, plus the diameter of 
    one of the balls, is the distance r between their centres. Practice 
    taking measurements and compare your readings with those of your 
    lab partner until you are sure you can do them accurately. Try to 

    estimate measurements to 0.2 mm.

    C

    8. Take measurements, and record the diameter of the balls (by 

    sighting on the scale).

    9. Remove the sliding ball, and recheck the equilibrium position of 

    the inside edge of the hanging ball.

    10. You can record and graph data in Excel or by hand (although if 
    you work by hand, you will lose the opportunity for 2 mills of 
    additional credit below). Recharge the balls as in steps 1 – 4, 
    and record a series of measurements of the inside edges of the 
    balls. Move the sliding ball in steps of 0.5 cm for each new 

    measurement.

    11. Compute columns of displacements d (position of the hanging ball 
    minus the equilibrium position) and the separations r (difference 
    between the two recorded measurements plus the diameter of one 

    ball).

    C

    C

    Knowledge of the forces that exist between charged particles is necessary for 
    an understanding of the structure of the atom and of matter. The magnitude of 
    the forces between point charges was first investigated quantitatively in 1785 

    by Coulomb, a French scientist. The law he discovered is stated as follows:

    “The force between two point charges is directly proportional to the 

    product of charges divided by the square of their distance apart”.

    C

    C

    C

    C

    C

    C

    C

    C

    Electric field

    Notions and definitions

    Questions to think about 

    a) You have learned about Coulomb’s law and you have seen 
    that when an electric charge is brought near to another, there 
    is an attractive or a repulsive force. Does that force acts when 

    charges are in contact or it acts even at a certain distance?

    b) If so, what can be the reason?

    c) Does that force increase or decrease when the distance 

    between charges increases? 

    After responding to those questions, you’ll see that around an electric charge 
    is a region so that when another charge is placed in it, it undergoes an electric 

    force. That region is called electric field created by the first charge. 

    An electric field can be defined as a region where an electric force is obtained. 

    It’s a region where an electric charge experiences a force.

    C

    C

    C

    C

    Field lines (lines of force)

    Activity 4: Lab zone 

    Existence of field lines 

    This shows the shape of electric fields, in much the same way that magnetic 

    fields are demonstrated with iron filings.

    Materials
    * Power supply, EHT, 0-5kV.          * Electric fields apparatus.

    * Semolina.                                          * Castor oil.

    Procedure

    a) Fill the electrode unit with a layer of castor oil to a depth 
    of about 0.5cm. Sprinkle a thin layer of semolina over the 
    surface. (A thin piece of glass tubing drawn out to give a 
    fine pointed stirrer is helpful so that the semolina is evenly 
    distributed.) It is better to start with too little semolina than 
    to start with too much. You can always increase the quantity 

    later.

    b) Place the electrodes in the castor oil. Connect the positive 
    and negative terminals of the EHT power supply to the 
    electrodes. Adjust the supply to give 3,000 to 4,000 volts. 
    When the voltage is switched on, the field lines will be 

    clearly visible.

    c) Try electrodes of different shapes. For example, one can be 
    a ‘point’ electrode whilst the other is a plate, or two point 
    electrodes can be used. A wire circular electrode with a 
    point electrode at the centre will show a radial field. The 
    field with two plates quite close together should also be 

    shown.

    C

    A line of force or field lines is defined as a line such that the tangent to it at a 

    point is in the direction of force on a small positive charge placed the point. 

    Arrows on the lines of force show the direction of the force on a positive 

    charge; the force on a negative charge. 

    C

    C

    Electric field due to a distribution of electric charges

    Activity 5

    Electric field due to a distribution of charges

    Materials
    * A sheet of paper
    * A pen

    * A ruler

    Procedure
    1. Represent a distribution of charges where you have charges of 
    different signs.
    2. Represent a point A where you want to find the total electric field.
    3. At the point, A represents directions of electric fields vectors 
    produced by each charge.
    4. Do the sum of electric fields. Remember that an electric field is 
    a vector. When they make a certain angle between them, use the 
    method of parallelogram. When they have the same direction or 
    opposite directions, use the appropriate method. 
    5. Establish a mathematical relation of the total electric field due to the 

    distribution of charges.

    C

    C

    C

    C

    Potential difference

    Work of electric force

    Activity 6

    Find the expression of the work done by an electric force

    a) When can we say that we have a uniform electric field?

    b) Draw a diagram showing two plates of opposite signs (the left 

    plate is positive and the right one is negative) between which the 

    electric field is uniform.

    c) Show the direction of field lines in the electric field.

    d) Between the two plates, put a positive charge at a point A which 

    has to travel toward a point B in the field.

    e) Represent the direction of the vector force on the line joining A and B.

    f) Write down the expression of the force undergone by the charge.

    g) What is the expression of the work done if the charge has to move 

    from A to B (in the final formula)?

    Particles that are free to move, if positively charged, normally tend towards 
    regions of lower voltage (net negative charge), while if negatively charged 

    they tend to shift towards regions of higher voltage (net positive charge).

    However, any movement of a positive charge into a region of higher voltage 
    requires external work to be done against the field of the electric force, work 
    equal to that electric field would do in moving that positive charge the same 
    distance in the opposite direction. Similarly, it requires positive external work 
    to transfer a negatively charged particle from a region of higher voltage to a 

    region of lower voltage.

    The electric force is a conservative force: work done by a static electric field is 
    independent of the path taken by the charge. There is no change in the voltage 
    (electric potential) around any closed path; when returning to the starting 

    point in a closed path, the net of the external work done is zero. 

    Potential in a field

    Activity 7

    Understanding the potential in a field
    1. What kind of energy has a body when it’s held above the earth? If 
    the body has to move under the force of gravity, does it move from 

    a point of great height to one of less or it’s the inverse?

    2. Do you agree or not that points in the earth’s gravitational field 

    have potential values depending on their heights?

    3. According to you, can this theory be similar to the one established 

    for electric field? Explain.

    4. For charges, instead of saying gravitational potential for 
    gravitational field, can we say electric potential for the case of 

    electric field? Explain.

    5. Can points around the charge be said to have electric potential?

    6. How can we define the electric potential at a point? 
    Potential generally refers to a currently unrealized ability. The term is used in 
    a wide variety of fields, from physics to the social sciences to indicate things 
    that are in a state where they are able to change in ways ranging from the 

    simple release of energy by objects to the realization of abilities in people.

    Although the concept of electric potential is useful in understanding electrical 
    phenomena, only differences in potential energy are measurable. If an electric 
    field is defined as the force per unit charge, then by analogy an electric potential 
    can be thought of as the potential energy per unit charge. Therefore, the work 
    done in moving a unit charge from one point to another (e.g., within an electric 

    circuit) is equal to the difference in potential energies at each point.

    Potential difference, work, energy of charges

    Activity 8

    C

    C

    Electric potential is a location-dependent quantity that expresses the amount 
    of potential energy per unit of charge at a specified location. When a Coulomb 
    of charge (or any given amount of charge) possesses a relatively large quantity 
    of potential energy at a given location, then that location is said to be a location 
    of high electric potential. And similarly, if a Coulomb of charge (or any given 
    amount of charge) possesses a relatively small quantity of potential energy 
    at a given location, then that location is said to be a location of low electric 
    potential. As we begin to apply our concepts of potential energy and electric 
    potential to circuits, we will begin to refer to the difference in electric potential 

    between two points.

    Relation between E and V

    Activity 9

    Relation between E and V

    1. What is the relation to find the work done by an electric force to 

    move a charge from A to B, knowing that the distance between A 

    and B is d?

    2. What is the relation of the work using the potential difference?

    3. Equalize the two relations and deduce the value of E. The relation 

    found is the one between E and V.

    4. From the expression found, deduce the new unit of the electric field E.

    5. Write down the relation between E and V found, express in 
    equation of V, write the electric field produced by a charge at a 
    point deduce the electric potential created by a charge at a point 

    situated at a distance d from it. 

    The effect of any charge distribution can be described either in terms of 
    electric field or in terms of electric potential. Electric potential is often easier 
    to use since it is a scalar whereas electric field is a vector. There is an intimate 
    connection between the potential and the electric field. Let us consider the 
    case of a uniform electric field, such as that between the parallel plates (fig.) 

    whose difference of potential is Vba.

    c

    C

    Examples

    In figure below, the potential difference between the metal plates is 40v. 
    a) Which plate is at the higher potential?
    b) How much work must be done to carry a +3.0C charge from B to 
    A? from A to B?
    c) How do we know that the electric the electric field is in the 
    direction indicated? 

    d) If the plate separation is 5.0mm. what is the magnitude of E?

    C

    C

    Motion of electric charges in an electric field

    Activity 10

    C

    C

    C

    There are so many applications of cathode ray tube which is a practical example 
    of the motion of electrons in an electric field in daily life. For example TV 

    sets, oscilloscope, etc. use cathode ray tubes. 

    C

    C

    Lightening and lightening arrestor 

    Activity 11 

    Lightening and lightening arrestor

    a) Surely, you have heard a thunder before the rainfall. What do 
    you observe in the sky during it?
    b) According to you, this is due to what?
    c) Is the fact observed dangerous?
    d) If yes, do you know some consequences which you have 
    observed or heard?
    e) If yes, is there a way to be protected from it?
    f) Do some research on internet to know more about it and 

    submit the result of your research to the teacher.

    Some explanation

    What you observe is called Lightening which is a sudden electrostatic 
    discharge (the sudden flow of electricity between two electrically charged 
    objects caused by contact, an electrical short, or dielectric breakdown) during 
    an electrical storm between electrically charged regions of a cloud (called intra-
    cloud lightening or IC), between that cloud and another cloud (CC lightening), 
    or between a cloud and the ground (CG lightening). The charged regions in the 
    atmosphere temporarily equalise themselves through this discharge referred 
    to as a strike if it hits an object on the ground. Although lightening is always 
    accompanied by the sound of thunder, distant lightening may be seen but be 
    too far away for the thunder to be heard. Lightening strikes can be damaging 

    to buildings and equipment, as well as dangerous to people.

    C

    C

    Buildings often use a lightening protection or lightening rod system consisting 
    of a lightening rod (also called a lightening conductor) and metal cables to 
    divert and conduct the electrical charges safely into the ground. Another form 
    of lightening protection system creates a short circuit to prevent damage to 
    equipment. The electrically conducting metal skin of commercial aircraft is 

    isolated from the interior of to protect passengers and equipment.

    Often, the lightening protection is mounted on top of an elevated structure, 
    such as a building, a ship, or even a tree, electrically bonded using a wire 
    or electrical conductor to interface with ground or “earth” through an electrode, 
    engineered to protect the structure in the event of lightening strike. If lightening 
    hits the structure, it will preferentially strike the rod and be conducted to the 
    ground through the wire, instead of passing through the structure, where it 
    could start a fire or cause electrocution. Lightening rods are also called finials, 

    air terminals or strike termination devices.

    C

    In a lightening protection system, a lightening rod is a single component of 
    the system. The lightening rod requires a connection to earth to perform its 
    protective function. Lightening rods come in many different forms, including 
    hollow, solid, pointed, rounded, flat strips or even bristle brush-like. The main 
    attribute common to all lightening rods is that they are all made of conductive 
    materials, such as copper and aluminum. Copper and its alloys are the most 

    common materials used in lightening protection.

    C

    C

    ◄Unit 8 :Universal gravitational field and potentialUnit 10 :Applications of thermodynamics laws►