{"id":10264,"date":"2026-07-03T17:39:42","date_gmt":"2026-07-03T17:39:42","guid":{"rendered":"https:\/\/kapdec.com\/help\/?p=10264"},"modified":"2026-07-03T17:39:42","modified_gmt":"2026-07-03T17:39:42","slug":"current-resistance-and-power","status":"publish","type":"post","link":"https:\/\/kapdec.com\/help\/current-resistance-and-power\/","title":{"rendered":"Current, Resistance, And Power"},"content":{"rendered":"
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KAPDEC® | Elite STEM Learning Platform | https:\/\/kapdec.com<\/a><\/p>\n

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Unit: <\/strong>Electric Circuits<\/strong><\/h1>\n

Chapter: Current, Resistance and Power <\/strong><\/h2>\n

Reference: AP Physics Electricity and Magnetism, Electric Circuits, Current, Resistance and Power, <\/em>Electric Charge, Conductors and Insulators, Charging by Induction, Three basic properties of electric charge, Coulomb\u2019s law, Facts about Coulomb\u2019s law, <\/em>Electric current<\/em>, <\/em>Electric Power<\/em><\/p>\n

After studying this chapter, you should be able to,<\/strong><\/p>\n

    \n
  • state the basic properties of electric charges;<\/li>\n
  • explain the concepts of quantisation and conservation of charge;<\/li>\n
  • explain Coulomb\u2019s law of force between electric charges;<\/li>\n
  • state the electric current<\/li>\n
  • know the concept of electric power.<\/li>\n<\/ul>\n

    Electric Charge: <\/strong><\/p>\n

    \u2022 Electrostatic charge is a fundamental property of matter due to which it produces and experiences electrical and magnetic effects.<\/p>\n

    \u2022 Properties of atoms, molecules and bulk matter are determined by electric and magnetic forces.<\/p>\n

    \u2022 It can be inferred from simple experiments based on frictional electricity that there are two types of charges in nature: negative and positive; and like charges repel and unlike charges attract.<\/p>\n

    \u2022 By convention, the charge on the electron is considered as negative and the charge on the proton is considered as positive and the charge present is equal. The S.I. unit of electric charge is the coulomb. Its C.G.S unit is stat coulomb.<\/p>\n

    \u00a0\u2022 The nature and amount of electric charge present in a charged body is detected by a Gold-leaf electroscope.<\/p>\n

    \u2022 Total charge on a body is expressed as q = \u00b1 ne.<\/p>\n

    Conductors and Insulators <\/strong><\/p>\n

    \u2022 Objects that allow charges to flow through them are called Conductors (metals) and objects that do not allow charges to flow through are called Insulators (rubber, wood, and plastic).<\/p>\n

    \u2022 Objects that behave as an intermediate between conductors and insulators are called semiconductors, for example- silicon.<\/p>\n

    \u2022 The process of sharing charges with the earth, when we bring a charged body in contact with the earth is called grounding or earthing.<\/p>\n

    Charging by Induction <\/strong><\/p>\n

    \u2022 Charging by induction means charging without contact.<\/p>\n

    \u2022 If a plastic comb is rubbed with wool, it becomes negatively charged.<\/p>\n

    Three basic properties of electric charge<\/strong><\/p>\n

    \u2022 Quantization: When the total charge of a body is an integral multiple of a basic quantum of charge, this is known as quantization of electric charge. i.e., q = ne where n = \u00b11, \u00b12, \u00b13, ……………..<\/p>\n

    \u2022 Additivity: It means that the total charge of a system is the algebraic sum (adding taking into account negative and positive signs) of all the charges in the system.<\/p>\n

    \u2022 Conservation of charge: Conservation of electric charges means that there will be no change in the total charge of the isolated system with time. There is the transfer of the electric charge from one body to another, but no charge will be created or destroyed.<\/p>\n

    Coulomb\u2019s law:<\/strong><\/p>\n

    The force between two-point charges q1<\/sub> and q2<\/sub> is directly proportional to the product of the two charges (q1<\/sub> q2<\/sub>) and inversely proportional to the square of the distance between them (r2<\/sub>) and it acts along the straight line joining the two charges.<\/p>\n

    F = force on q2<\/sub> due to<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/p>\n

    where<\/p>\n

    \u00a0\u00a0<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    The experimental value of the constant \u03b50<\/sub> is 8.854 \u00d7 10\u201312<\/sup>C2<\/sup> N\u20131<\/sup>m\u20132<\/sup> Therefore, the approximate value of k is 9 \u00d7 109 <\/sup>Nm2<\/sup> C\u20132<\/sup><\/p>\n

    Facts about Coulomb\u2019s law: <\/strong><\/p>\n

    \u2022 Coulomb’s law is not valid for charges in motion; it should only be used for point charges in a vacuum at rest.<\/p>\n

    \u2022 The electrostatic force obeys Newton\u2019s third law of motion and acts along the line joining the two charges.<\/p>\n

    \u2022 Presence of other charges in the neighbourhood does not affect Coulomb\u2019s force.<\/p>\n

    \u2022 The ratio of electric force and the gravitational force between a proton and an electron is represented by<\/p>\n

    \u00a0<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    \u00a0<\/p>\n

    Electric current:<\/strong><\/p>\n

    When the net amount of charge flowing through the area in the time interval is defined as the electric current across the area.<\/p>\n

    The current is the rate of flow of charge through a surface area placed perpendicular to the direction of flow. If charge \u0394q flows in time \u0394t, the average current is defined as:<\/p>\n

    \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    \u00a0<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    \u00a0 The motion of charges inside a conductor of surface area A<\/p>\n

    If the rate of flow of charge varies with time, the current also varies with time. The instantaneous current is expressed as:<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    The electric current through a conductor is the rate of transfer of charge across a surface placed normally to the direction of flow.<\/strong><\/p>\n

    The SI unit of current is ampere. Its symbol is A:<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    Electric Power:\u00a0<\/strong><\/p>\n

    The rate at which electric energy is consumed or dissipated in an electric circuit.<\/p>\n

    P = VI<\/p>\n

    \u21d2 P = I2<\/sup>R = V2<\/sup>\/R<\/p>\n

    The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus,<\/p>\n

    1 W = 1 volt <\/strong>\u00d7 <\/strong>1 ampere = 1 V A <\/strong><\/p>\n

    The unit \u2018watt\u2019 is very small. Therefore, in actual practice we use a much larger unit called \u2018kilowatt\u2019. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (W h). One watt hour is the energy consumed when 1 watt of power is used for 1 hour. The commercial unit of electric energy is kilowatt hour (kW h), commonly known as \u2018unit\u2019.<\/p>\n

    1 kW h = 1000 watt \u00d7 3600 second<\/strong><\/p>\n

    = 3.6 \u00d7 106 <\/sup>watt second<\/strong><\/p>\n

    = 3.6 \u00d7 106<\/sup> joule (J)<\/strong><\/p>\n

    Example:<\/strong> A total of 6.0 \u00d7 1016<\/sup> electrons pass through any cross-section of a conducting wire per second. Determine the value of the current in the wire.<\/p>\n

    Solution:<\/strong> Total charge passing through the cross-section in one second is \u0394Q = ne = 6.0 \u00d7 1016<\/sup> \u00d7 1.6 \u00d7 10\u201319<\/sup> C = 9.6 \u00d7 10\u20133<\/sup>C<\/p>\n

    \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/p>\n

    \"\"<\/p>\n

    Source: Kapdec.com<\/p>\n<\/div>\n

    \u00a0<\/p>\n

    \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 = 9.6 \u00d7 10\u20133<\/sup> A = 9.6 mA<\/p>\n

    Example:<\/strong><\/p>\n

    An electric oven rated 600 W operates 4 hour\/day. What is the cost of the energy to operate it for 15 days at Rs 6.00 per kW h?<\/p>\n

    Solution:<\/strong><\/p>\n

    The total energy consumed by the refrigerator in 15 days would be<\/p>\n

    600 W \u00d7 4.0 hour\/day \u00d7 15 days = 36000 W h = 36 kW h<\/p>\n

    Thus, the cost of energy to operate the refrigerator for 15 days is<\/p>\n

    36 kW h \u00d7 Rs 6.00 per kW h = Rs 216.00<\/p>\n

    Key point:<\/strong><\/p>\n

      \n
    • Currents are not always steady<\/li>\n
    • SI unit of current is ampere.<\/li>\n
    • The smaller units of current are milliampere, 1 mA = 10\u20133<\/sup>A, and microampere,<\/li>\n<\/ul>\n

      1\u03bcA = 10\u20136<\/sup> A.<\/p>\n

        \n
      • The current can arise due to the flow of negative charges (electrons), as in metals<\/li>\n
      • The electron charge is usually represented by\u00a0the symbol e or q<\/strong><\/em>. This is the basic physical constant used to represent 1.1602 \u00d7 10-19<\/sup>\u00a0coulomb.<\/li>\n<\/ul>\n
          \n
        • The electric charge has three basic properties: quantisation, quantisation, additivity and conservation.<\/li>\n
        • The additivity of electric charges means that the total charge of a system is the algebraic sum (i.e., the sum taking into account proper signs) of all individual charges in the system.<\/li>\n
        • Conservation of electric charges means that the total charge of an isolated system remains unchanged with time.<\/li>\n
        • Electric and magnetic forces determine the properties of atoms, molecules and bulk matter.<\/li>\n
        • The quantisation\u00a0The quantisationof electric charge means that the total charge (q) of a body is always an integral multiple of a basic quantum of charge (e) i.e., q = n e, where n = 0, \u00b11, \u00b12, \u00b13, …. Proton and electron have charges +e, \u2013e, respectively.<\/li>\n
        • Coulomb\u2019s law is a short-range force (about 10-10<\/sup>).<\/li>\n
        • Coulomb is a large unit of charge. For the small amount, we can use micro coulomb and milli coulomb.<\/li>\n
        • Here \u03f50\u00a0is the permittivity of free space, when we place the charge in any medium then we have to use \u03f5<\/em>\n
          \"\"<\/p>\n

          Source: Kapdec.com<\/p>\n<\/div>\n

          \u00a0as the permittivity of the medium.<\/li>\n

        • The value of \u03f5\u00a0is depend on the medium.<\/li>\n
        • Conservation of the total charge of an isolated system is a property independent of the scalar nature of the charge<\/li>\n
        • Force on one charge due to another charge is unaffected by the presence of other charges, and there are no additional three-body, four-body, etc., forces which arise only when there are more than two charges.<\/li>\n<\/ul>\n
            \n
          • The rate at which electric energy is consumed or dissipated in an electric circuit.<\/li>\n
          • P = VI<\/li>\n<\/ul>\n

            \u00a0<\/p>\n

            <\/p>\n