Chapter 14 Current of Electricity

1 Current of Electricity CHAPER 14 Current of Electricity

2 Current of Electricity 1.1 An electron current flows when electrons move through a conductor, such as a metal wire. Electrons flows from the positive terminal of a battery to its negative terminal. 1.2 Conventional current assumes that current flows out of the positive terminal of a source (e.g., battery), through the circuit and into the negative terminal of the source. 1.3 1.4 An is the rate of flow of charges. Symbols and units for current and charges: quantity current charges time symbol unit A name of unit ampere coulomb 1. Electric Current and Charges

3 Current of Electricity 1.5 One coulomb is the amount of charges passing any point in a circuit when a steady current of one ampere flows for one second. QUESTION: When the starter motor of a car is switched on for , of charge passes through the wires in the motor. How large is the electric current? I = Q/t = 16/0.5 = 32 A 2.1 of a power source (e.g., battery) is the work done by the source in driving a unit charge around a complete circuit. In this process, non-electrical energy is converted to electrical energy. 2.2 across an electrical component (e.g., resistor) is the work done in driving a unit charge through the component. In this process, electrical energy is converted to non-electrical energy. quantity work done charges emf pd symbol unit J V name of unit joule coulomb volt == or 2. Electromotive Force and Potential Difference

4 Current of Electricity QUESTION: A battery is labelled as “ ”. (a) Is the “ ” stand for emf or pd? (b) What is the amount of electrical energy produced by the cell when of charge flow through it? (c) How many joules of energy is converted when a current of flows through it for ? (a) emf (b) V= W/Q; W = 1.5 × 0.4 = 6.0 J (c) Q = It = 2 × 0.4 = 0.80 C W = 1.5 × 0.8 = 1.2 J 2.3 When more than one cell is connected in series, the total emf depends on how the cells are arranged. 3.0 V 4.5 V 1.5 V 0 V == anemf, pd, d

5 Current of Electricity 3.1 of an electrical component is the ratio of the potential different across it to the current flowing through it. quantity current potential difference resistance symbol unit V Ω unit (name) ampere volt ohm QUESTION: A potential difference of applied across the heating coil of an electric kettle drives a current of through the coil. Calculate (a) the resistance of the coil, and (b) the current flowing through the coil if the p.d. applied is changed to . (a) R = V/I = 240/8 = 30 Ω (b) I = 110/30 = 3.7 A 3.2 are conductors in electrical circuits with known resistance. They are designed to control the currents in a circuit. 3.2 There are two types of resistors: fixed resistors and variable resistors with symbols: 3. Resistors and Resistance =resistance,

6 Current of Electricity 5.1 When the wire is heated up, the positive ions in the conducting wire vibrate more vigorously because of increase in energy. This causes more collisions between the electrons and the particles and increases resistance. 5.2 The more tightly an atom holds on to its outermost electrons the harder it is for the current flow. 5.3 In longer wires, electrons must travel longer distances and will have more collisions. Resistance of a wire is directly proportional to its length. 5.4 With larger cross-sectional area, there is more space for the electrons to travel through and so fewer collisions. Resistance of a wire is inversely proportional to its cross-sectional area. 5.5 Combining the factors of length and cross-sectional area, the resistance of a wire : QUESTION: A wire has a resistance of . what is its resistance when (a) its length is halved? (b) cross-sectional area is doubled? (c) length is halved, and cross-sectional area is doubled? (a) 7.5/2 = 3.8 Ω (b) 7.5/2 = 3.8 Ω (c) 7.5/(2 × 2) = 1.9 Ω 5. Factors Affecting Resistance  Simulation of factors affecting resistance

7 Current of Electricity 6.1 states that the ratio of the potential difference across a conductor to the current flowing in it is always a constant, provided its physical conditions, such as temperature, remain unchanged. 6.2 are conductors which obey Ohm’s Law. The characteristic graph of an ohmic conductor is a straight line passing through the origin. The resistance of an ohmic conductor is a constant. 6.3 are conductors which do not obey Ohm’s Law, e.g., filament lamp and semiconductor diode. 6. Ohm’s Law reversed direction forward direction Simulation of Ohm’s Law

8 Current of Electricity 6.4 From the characteristics graph of a non- ohmic conductor, resistance at a point ( ) on the graph is −== − , which is the gradient of the reciprocal of the line joining ( ) and ( ). Since this gradient decreases as pd increases, the resistance of the this resistor increases. QUESTION: The graph shows how the currents in a lamp L and in a wire W vary with the potential difference (p.d.) applied. Find the resistance of L and W when the potential difference across them is (a) , and (b) . (a) L: R = 5.0/1.5 = 3.3 Ω W: R = 5.0/0.35 = 14 Ω (b) L: R = 10/1.95 = 5.1 Ω W: R = 10/0.35 = 14 Ω 0 0