Chapter 1Basic Concept

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DKS1113 Electric Circuits

Electrical Safety “Danger—High Voltage.”

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Electrical Safety

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International Systems of Units

The following are expressions of the same distance in meters (m):

600, 000, 000 mm 600, 000 m 600 km

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International Systems of Units

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Introduction to Electric Circuits An electric circuit is an interconnection of

electrical elements.

Functions: To transfer energy from one point to another.

Basic concepts: Charge. Current. Voltage. Power. Circuit elements. Energy.

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Charge – q(t) Basic quantity in an electric circuit.

Defined as an electrical property of materials. Exist as negative (electron) and positive (proton)

charges. Measured in Coulombs (C). 1 electron = -1.602*10-19C.

Positive and negative charges move in different direction. Creates electric current. Consider electric current as movement of positive

charge.

Charge may be constant/varying.

Charge – q(t)

Question:

How much charge is represented by 4,600 electrons?

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Charge – q(t)

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Solution:

Each electron has −1.602 × 10−19 C. Hence 4,600 electrons will have −1.602 × 10−19 C/electron × 4,600 electrons = −7.369 × 10−16 C.

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Current – i(t) Defined as the charge flow rate.

Measured in Ampere (A).

Current may be constant/varying.

Current – i(t)

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Voltage – v(t) Defined to be the charge rate of doing

work. Energy required to move a unit charge

through an element. Measured in volts (V). Voltage may be constant/varying. 1 volt = 1 joule/coulomb = 1 newton

meter/coulomb Voltage, ,

Voltage – v(t)

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Power – p(t) Defined to be the time rate of doing work.

Measured in watts (W).

Power can be absorbed or supplied by circuit elements. Positive power element absorbs power. Negative power element supplies power. ‘Sign’ determined by voltage and current.

An ideal circuit: ∑Psupplied + ∑ Pabsorbed = 0.

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Energy Defined as the capacity to do work.

Measured in joules (J).

Example: An energy source forces a constant

current of 2 A for 10 s to flow through a lightbulb. If 2.3 kJ is given off in the form of light and heat energy, calculate the voltage drop across the bulb.

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Solution

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Circuit Elements An element is the basic building block of a

circuit.

Electric circuit is interconnecting of the elements.

Types of elements: Active elements Capable of generating energy

(i.e. batteries, generators). Passive elements Absorbs energy (i.e. resistors,

capacitors and inductors). Voltage and current sources the most important

active elements.

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Source Divided into:

Independent source Does not depend to other elements to supply voltage or current.

Dependent source Reverse of independent.

Constant voltage source: Voltage same for all elements.

Constant current source: Current same throughout the circuits.

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Examples

Calculate the amount of charge represented by two million protons.

Answer: +3.204 × 10−13 C.