L3 Physical Science Principal of Electrical Circuits

PRINCIPLES OF ELECTRIC CIRCUITS Lecturer: Semase M.P Physical Science L3 Downloaded from hlayiso.com Sefikeng Campus

Specific Outcome State, analyse and apply principles in electric circuits Learning Outcomes • Define and apply Ohm’s law • Determine the proportionality between current and potential difference • Distinguish between resistance, equivalent resistance and internal resistance • Identify symbols used for electric circuits Downloaded from hlayiso.com

Introduction • Ohm’s law deals with the relationship between potential difference, current and resistance. • In its mathematical form, it enables us to perform calculations to obtain values for any quantities. Downloaded from hlayiso.com

Ohm’s Law • Ohm’s law states that: The potential difference across the ends of a resistor is directly proportional to the current in the resistor provided the temperature remains constant. Downloaded from hlayiso.com

Importance of ohm’s law • Ohm’s law is applicable to a very limited range of situations • As the statement of the law says, the temperature must remain constant. Thus is a rare situation because all resistors heat up when current flows through them • The law is only valid for metallic conductors at very low currents • Conductors that obey ohm’s law are called ohmic conductors • Other types of conductors such as light bulbs, diodes and electrolytes do not obey ohm’s law and are called non-ohmic conductors Downloaded from hlayiso.com

Equivalent resistance • Ohm’s law defines resistance as the potential difference required to make a current flow in the resistor. • A resistor has a resistance of one ohm if a potential difference of one volt is needed to make a current of one ampere flow in the resistor. • Equivalent resistance refers to the combined value of a number of resistors, connected either in series or in parallel. • The equivalent resistance is the value of a single resistor that has the same effect as a number of resistors connected together. Downloaded from hlayiso.com

Resistors Resistors in series Resistors in parallel • Resistors are connected in • Resistors are connected in series when there is only parallel when they are one path connected in such a way • For the current to flow and that there is more than one so the same current flows path for the current to flow through all the resistors • The equivalent resistance is • The equivalent resistance is less than that of the greater than any individual smallest individual resistor resistance Downloaded from hlayiso.com

Internal resistance • Internal resistance refers to the resistance inside a cell or a battery when a current flows through it. • Some of the energy that is transferred to the charges as they move through the cell is transferred to heat through the cell. • A measure of the energy supplied to the charges is given by the reading on a voltmeter connected to the cell when no current flows • When a current flows through the cell, a circuit is closed, a voltmeter connected across the cell reads the potential difference across the circuit • The difference between the emf and the potential difference is often called the lost volts. Downloaded from hlayiso.com

Summary • Ohm’s law which, which states that potential difference across the ends of a resistor is directly proportional to the current in the resistor; provided the temperature remains constant. • A conductor that obeys ohm’s law is called an ohmic conductor. • Equivalent resistance is the combined resistance of a number of individual resistors. • Resistance is the potential difference needed to make a certain current flow in a conductor • Ohm’s law can be illustrated experimentally Downloaded from hlayiso.com

Summative assessment • Define electric current (2) • Define potential difference(2) • What is meant by resistance(3) • Define the ohm and give symbol (3) • What is meant by equivalent resistance(2) • What is emf(1) • What is meant by internal resistance of a battery(2) Downloaded from hlayiso.com