Chapter 1

Basic Concepts

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Introduction 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.

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Current – i(t)

Defined as the charge flow rate.

Measured in Ampere (A).

Current may be constant/varying.

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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.

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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).

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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 Determine the current

flowing through an element if the charge flow is given by q(t)=(9t2+2t-2)C.

Find the charge q(t) flowing through a device if the current (i(t))=(2t+5) mA and q(0)=0.

The charge entering a certain element is shown below. Determine current at:

t= 1 ms t= 6 ms t= 10ms

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Examples Given the graph,

plot the corresponding current.

Calculate total charge that pass through an element at: t = 1s t = 3s t = 5s

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Examples Find the power absorbed by each of the

elements.

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Measuring Instruments Ammeter Voltmeter Wattmeter