Tiêu đề Chapter 11 Electricity.pdf ✅

SCIENCE (Physics) Chapter 11: Electricity
(1) 11 ELECTRICITY Electricity Electric Current ● Electric current is expressed as the amount of charge flowing through a particular area in unit time. ● Quantitatively, electric current is defined as the rate of flow of electric charge. Current,I = Charge flowing (Q) Time taken (t) ● The S.I. unit of current is ampere (A), where 1 ampere = 1 coulomb/second. ● 1 mA = 10−3 A, 1 μ A = 10−6 A ● The conventional direction of electric current is the one in which positive charges move orderly. ● An instrument called ammeter measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured. Electric circuit and circuit diagram The electric circuits are closed loop or path which forms a network of electrical components, where electrons are able to flow. This path is made using electrical wires and is powered by a source, like a battery. The start of the point from where the electrons start flowing is called the source whereas the point where electrons leave the electrical circuit is called the return. Representation of an electric circuit through symbols is called a circuit diagram. A schematic diagram of an electric circuit comprising – cell, electric bulb, ammeter and plug key Example: A current of 1A is drawn by a filament of an electric bulb for 20 minutes. Find the amount of electric charge that flows through the circuit. Ans: The given data is, I = 1A and t = 20 minutes t = 20 × 60 t = 1200 seconds Therefore, I = Q t Electric charge is q = It
(2) 11 ELECTRICITY q = 1 × 1200 q = 1200 C Coulomb’s Law According to Coulomb’s law, the force of attraction or repulsion between two charged bodies is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. It acts along the line joining the two charges considered to be point charges. F ∝ q1q2 d2 Electric Potential Electric potential is the work done per unit charge in bringing the charge from infinity to that point against electrostatic force. In a conductor, electrons flow only when there is a difference in electric pressure at its ends. This is also called potential difference. Electric Potential Different ● Electric potential difference (pd) between two points in an electric circuit, carrying some current, is the amount of work done to move a unit charge from one point to another. Potential difference (pd) = Work done (W) Quanity of charge moved (Q) ● The S.I. unit of pd is volt (V), where 1 volt = 1 joule 1coulomb Example: How much work is done in moving a charge of 2 C across two points having a potential difference 12 V? Ans: The amount of charge Q, that flows between two points at potential difference V (= 12 V) is 2 C. Thus, the amount of work W, done in moving the charge [from Eq. (12.2)] is W = VQ = 12 V × 2 C = 24 J. Electric Circuit ● A continuous conducting path between the terminals of a source of electricity is called an electric circuit. ● A drawing showing the way various electric devices are connected in a circuit is called a circuit diagram. ● Some commonly used circuit elements are given below: Sr. No. Element Symbol 1 An electric cell
(3) 11 ELECTRICITY 2 A battery 3 Plug key or switch (open) 4 Plug key or switch (closed) 5 A wire joint 7 Bulb 6 Wires crossing without joining 8 Resistor 9 Variable resistor or Rheostat 10 Ammeter 11 Voltmeter Ohm’s law ● According to Ohm’s law, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, provided its physical conditions remain the same. V ∝ I V l = Constant V l = R V = IR where R is a constant of proportionality called resistance of the conductor. ● Resistance is the property of a conductor to resist the flow of charges through it. ● The S.I. unit of resistance is ohm ( Ω ). From R ∝ V l 1 ohm = 1 volt ampere