CHAPTER 1 : INTRODUCTION TO

ELECTRICAL CIRCUITS

i. Electromagnetic Force (EMF)oEmf provided by a source of energy such as

a battery or a generatoroSymbol - EoUnit - Volt (V)

ii. Electrical ChargeoHas two types, positive charge and negative

charneoSymbol - QoUnit - Coulomb (C)oElectric field of positive and negative charge

1.1 ELECTRICAL QUANTITIES

iii. Currento Is a flow of electrical charge through a

conductive mediumoSymbol - IoUnit - Ampere (A)

iv. Electrical Potentialo A change in electrical potential between

two points in an electrical circuit is called potential difference

oSymbol - VoUnit - Volt (V)

v. Resistanceo the opposition to the passage of an

electrical current through that elementoSymbol - RoUnit - Ohm (Ω)

vi. Conductoro is a material which contains movable

electric charge o such as copper or aluminiumo the movable charged particles are electrons

vii. Insulatoro is a material whose internal electric charges

do not flow freelyo such as glass, paper

o Resistivity (ρ) - is a measure of how strongly a material opposes the flow of electric current

R ρo Length (ℓ) – length of the wire

R ℓo Cross Sectional Area (A) – cross sectional

area of the wireR

o Temperature (T)R T

1.1.1 Factors that effect the resistance of conductor material

o factors can be represented in the form of the equation below:

Where as:R = resistance (Ω) = resistivity (Ωm) = length (m)A = cross sectional ()

o E.g Calculate the resistance of aluminium wire of length 1.5km. Given wire diameter is 10mm and the resistivity is 0.025 μΩm

i. Complete Circuito connection is closed and allows

current to flow properly

i. Open Circuito loads in the circuit is open

ii. Short Circuito The load is short circuit using

conductor

1.2 Types of electrical circuit

i. Voltmeter - instrument used for measuring electrical potential difference between two points in an electric circuit

ii. Ammeter - is a measuring instrument used to measure the electric current in a circuit

iii. Ohmmeter - is an electrical instrument that measures electrical resistance

1.2.1 Use of meters

i. states that the current (I) is proportional to the potential difference (V) and inversely proportional to the resistance (R).

Where: I = current (A)V = voltage (V)R = resistance (R)

1.3 Ohm’s Law

ii. E.g 1

Determine the current resulting from the application of a 9V battery across the network with a resistance of 2.2Ω

iii. E.g 2

Calculate the voltage that must be applied across the soldering iron to establish a current of 1.5A through the iron if its internal resistance is 80Ω.

i. Is an indication of how much work can be accomplished in a specified amount of time.

Where P = power (Watt)W = Work (Joule)t = time (second)

ii. Derive from ohm’s law formula:

Where I = current (A)V = voltage (V)

1.4 Electrical Power

1.4.1 Use of Wattmeter

Examplei. E.g 1 An electric heater works on 120V and draws 3A of current. How much power does it use?ii. E.g 2What is the power when there are 500mA of a current through at 4.7kΩ resistor.iii. E.g 3 If a 75V source is supplying 2A to a load, what is the resistance value of the load.

i. Is the ability to do work and power is the rate at which energy is used

Where E = Energy (kWj / Joule)P = Power (W)t = time (s)

1.5 Electrical Energy

i. f

1.5.1Use of kilowatt-hour meter

1.5.2 Relationship between electrical energy and heat energy

i. Joule – Watt secondii. Kilowatt hour – Jouleiii. Joule - Calorie

1.5.3 Convertion

i. A particular electronic device uses 100 mW power. If it runs for 24 hours. How many joules of energy does it consume?

ii. For how many seconds must there be 5 A of current through a 47 Ω resistor in order to consume 25 J

iii. A 12V source is connected across a 10 Ω resistor

a. How much energy is used in two minutes?b. If the resistor is disconnected after one

minute, is the power during the first minute greater than, less than or equal to the power during a two minute interval

Example

i. + ii. = iii. +

1.6 Series and Parallel Circuit

1.6.1 Series Circuit

CSVD = CURRENT SAME VOLTAGE DIFFERENT

i. Three 470 Ω resistors are connected in series with a 48V source.

ii. Four equal value resistor are in series with a 5V battery, and 2.23mA are measured. What is the value of each resistor.

Example

i. + ii. =

1.6.2 Parallel Circuit

VSCD = VOLTAGE SAME CURRENT DIFFERENT

Example

1.6.3 Voltage Divider Rule

1.6.4 Current Divider Rule

1.6.5 Series – Parallel Circuit

1.7 Kirchoff’s Law1.7.1 Kirchoff’s Current Law

1.7.2 Kirchoff’s Voltage Law