Work and Energy - xraykamarul electric CURRENT exists whenever electric charge flows through a...

HDR102

SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES

PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH

CHAPTER 4

PHYSICS FOR RADIOGRAPHERS 1

ELECTRICITY

CHAPTER 4: Electricity

LEARNING OUTCOMES

At the end of the lesson, the student should be able to:-

Explain the concept of electric current and EMF.

Describe the direct current and alternating current.

Explain the Band Theory.

Explain the resistance and factors affecting it.

Define the Ohm’s Law.

Describe the resistors.

Explain the series and parallel circuit.

TOPIC OUTLINES

INTRODUCTION

4.1 Electricity 4.8 Series and Parallel Circuits

4.2 Types of Current 4.9 Resistors in Series and Parallel

4.3 Units of Measuring Electricity 4.10 Factors Affecting Resistance

4.4 Band’s Theory 4.11 References

4.5 Resistance (Ohm’s Law)

4.6 Resistors

4.7 Resistivity

INTRODUCTION

Electricity is important to supply the

medical equipment…

BUT…

Does we know how it happens?

Through the chapter, you will find out…

4.1 Electricity

4.1.1 Electric Current

An electric CURRENT exists whenever electric charge flows through a region,

e.g., a simple light bulb circuit.

The magnitude of the current is measured in AMPERES (Amps/A), where

1 ampere = 1 coulomb/second

I = ΔQ/ Δt

4.1 Electricity

Figure 1: A simple basic circuit with using a battery.

4.1 Electricity

4.1.2 Electromotive Force (emf) or Voltage (V)

Is the voltage produced by the energy supplied in an electrical circuit.

The unit is the volt (V).

A difference in charge between two points in a material can be created by an

external energy source such as a battery or generator.

4.1 Electricity

This causes electrons to move so that there is an excess of electrons at one

point and a deficiency of electrons at a second point.

This difference in charge is stored as electrical potential energy known as

emf. It is the emf that causes a current to flow through a circuit.

HIGH LOW

4.2 Types of Current

4.2.1 Direct current:

electrons that flow in the same

direction in a wire. (DC)

from batteries.

Figure 2: A diagram shows a direct current.

4.2 Types of Current

4.2.2 Alternating current:

electrons that flow in different

directions in a wire. (AC)

from generators.

transformers change AC to DC.

Figure 2: A diagram shows an alternating

current.

4.3 Units of Measuring Electricity

4.3.1 Current

Measured in Amperes or amps (A)

4.3.2 Voltage

Measured in volts (V)

Higher voltage, the more work the electrons can do.

4.3 Units of Measuring Electricity

4.3.3 Resistance

The force opposing the flow of electrons.

Measured in ohms

Symbol is Greek letter omega (Ώ)

Thicker wire - less resistance, Longer wire - more resistance

Conductors - low resistance, Insulators - high resistance

4.4 Band’s Theory

Band’s theory is an extension of molecular orbital theory that describes

bonding in solids.

Bands of orbital that are filled or partially filled by valence electrons are

called valence bands.

Higher-energy unoccupied bands in which electrons are free to migrate are

called conduction bands.

The energy gap between the valence and conduction bands is called the band

It does explain the differences in conductor, semiconductor, and insulator.

4.4 Band’s Theory

4.4.1 Conductor

As a potential difference is

applied across the sample, the

electrons in the partly filled band

can accelerate and gain energy

because there are unoccupied

states of higher energy available

and also no band gap.

It is therefore easy for a current

to flow, making it a good

conductor.

Valence Band

Conduction Band

Figure 3: A conductor

4.4 Band’s Theory

4.4.2 Insulator

As a potential difference is

applied across the sample, the

electrons in the valence band

cannot accelerate and increase

their energy since there are no

empty states readily available

and also there is a large band

Such materials are therefore

insulators.

Band Gap

Figure 4: An insulator

4.4 Band’s Theory

4.4.3 Semiconductor

The band structure of pure

(intrinsic) semiconductors is

similar to that of an insulator

except that the valence and

conduction bands are separated

by a smaller band gap Eg.

When a potential difference is

applied across the semiconductor

sample, the electrons in the

conduction band result in a

current flow.

Band Gap

Figure 5: A semiconductor

4.4 Band’s Theory

Valence band

Conduction bandBand gap

Band gap

Figure 6: The diagram shows the conductor, insulator and

semiconductor with regards to band’s theory.

4.5 Resistance (Ohm’s Law)

The magnitude of the electric current that flows through a closed circuit

depends directly on the voltage between the battery terminals and

inversely to the circuit resistance.

The relationship that connects current, voltage and resistance is known as

OHM'S LAW and is written as follows:

I = V/R or V = IR

The current is measured in amperes, the voltage in volts and the resistance in

ohms (Ώ).

4.6 Resistors

Resistors are used to control the amount of current flowing in a circuit.

Resistors have resistances from less than 1 ohm to millions of ohms.

The two main types of resistors:-

Figure 7: Wire-wound (coil of fine

wire)resistors

Figure 8: Composition (carbon) resistors

4.6 Resistors

Symbol on a schematic diagram

For the color code, the first two colors represent the first two digits in the

value of the resistor, the third represents the power of ten that it must be

multiplied by, and the fourth is the tolerance.

4.6 Resistors

Figure 9: A resistor and list of color coding.

4.7 Resistivity

RESISTIVITY is actually when electric charge flows through a circuit it

encounters electrical RESISTANCE.

The resistance of a metal conductor is a property which depends on its

dimensions, material and temperature.

At a specific temperature, the resistance (R) of a metal wire of length (L) and

cross-sectional area (A) is given by:

R = ρ L/A

ρ is a constant of proportionality called the RESISTIVITY.

The unit of resistance is the ohm (Ώ) and the unit of resistivity is ohm-meter

4.8 Series and Parallel Circuits

4.9 Resistors in Series and Parallel

In Series the total resistance is:-

R = R1 + R2 + R3

R1 R2 R3

In Parallel the total resistance is:-

1 = 1 + 1 + 1

R R1 R2 R3

4.10 Factors Affecting Resistance

Resistance depends on:-

1. Temperature

2. Material of conductor

3. Length

4. Cross-sectional area

4.10.1 Temperature

a) The resistance of a metallic

conductor increases as the

temperature increases e.g.

copper.

b) The resistance of a

semiconductor/insulator

decreases as the temperature

increases e.g. thermistor.

4.10.2 Length

a) Resistance of a uniform conductor is

directly proportional to its length.

i.e. R L

4.10.3 Cross-sectional area

a) Resistance of a uniform conductor is

inversely proportional to its cross-

sectional area.

i.e. R 1

4.10.4 Material

a) The material also affects the resistance of a conductor by a fixed

amount for different materials. This is known as resistivity ().

R = L = constant of proportionality

A Unit: ohm meter m

= Rd 2 (For a wire with circular cross-sectional area)

Answer the question.

ACTIVITY

When there is an electric current passing through a wire, the particles moving are _____________.

protons

electrons

SUMMARY

An electric CURRENT exists whenever electric charge flows through a region,

e.g., a simple light bulb circuit.

Electromotive force (emf) or Voltage (V) Is the voltage produced by the

energy supplied in an electrical circuit.

There are TWO types of current: Direct Current (DC) and Alternating Current

Band’s theory is an extension of molecular orbital theory that describes

bonding in solids.

SUMMARY

Ohm’s Law states The magnitude of the electric current that flows through a

closed circuit depends directly on the voltage between the battery

terminals and inversely to the circuit resistance.

RESISTIVITY is actually when electric charge flows through a circuit it

encounters electrical RESISTANCE.

Resistance depends on temperature, material used, length, and cross

sectional area.

NEXT SESSION PREVIEW

CHAPTER 5: ELECTROMAGNETISM

In chapter 5, students will be taught the basic concept and theory

of electromagnetism.

4.12 References

No. REFERENCES

1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for

radiographers. Blackwell.

2 Bushong, S. C. (2008). Radiologic science for technologists. Canada:

Elsevier.

APPENDIX

FIGURE SOURCE

Figure 1 http://www.actors.co.ke/en/news/Energy1.jpg

Figure 2 http://intechweb.files.wordpress.com/2012/03/shutterstock_77399518.jpg

Figure 3 http://www.solarenergybook.org/wp-content/uploads/2009/12/solar-energy-

example.gif

Figure 4 http://www.petervaldivia.com/technology/energy/image/potencial-and-

kinetic.bmp

Figure 5 http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%206/S

B7-2b.JPG

Figure 6 http://www.petervaldivia.com/technology/energy/image/potencial-and-

kinetic.bmp

Figure 7 http://www.physics4kids.com/files/art/motion_energy1_240x180.jpg

Figure 8 http://www.sciencebuilder.com/michigan/science/images/p/potentialenergy.j

Figure 9 http://4.bp.blogspot.com/_V7DuEO3c2E8/S-

b2PZfOXZI/AAAAAAAAADk/KKXoueyon2I/s1600/One-balanced-rock.jpg

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