the Physic of Electricity

8/10/2019 the Physic of Electricity

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1. The Physics of Electr ici ty

Jurusan Fisika, FMIPAUniversitas Riau


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An Atom

Everything is made of

atoms which contain

Positive particles calledProtons and Negative

particles called Electrons.

An atom will usually have

the same number ofpositives and negatives.

This makes the atom

Neutral.


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What is Electr icity?

Electrons have a negative

charge (Q) measured in

coulombs (C) move round acircuit from negative to

positive giving rise to an

electric current.

So electricity is movement

of charge round a circuit,

and we call this electric

current.


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Electr ic Cur rent

An electric current is a flow

of electric charge.

In electric circuits thischarge is often carried by

moving electrons in a wire.

It can also be carried byions in an electrolyte, or by

both ions and electrons such

as in a plasma.

A simple electric circuit,

where current is represented

by the letter i.


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The F low of Electr ic Cur rent

The electrons, the

charge carriers in an

electrical circuit,

flow in the opposite

direction of the

conventional electriccurrent.


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SI Unit of Electr ic Current

The SI unit for measuring an

electric current is the

ampere, which is the flow ofelectric charges through a

surface at the rate of one

coulomb per second.

Electric current can be

measured using an ammeter.

The relationship betweenthe voltage (V), resistance

(R), and current (I) is V=IR;

this is known as Ohm's

Law.


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Effects of Electr ic Currents

The reason electricity is so useful is that it is

easily converted into other forms of energy.

Electric currents cause many effects, notablyheating, but also induce magnetic fields, which

are widely used for motors, inductors and

generators.


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Ohm's law

Ohms law states that the current I through a given

conductor is directly proportional to the potential

difference V between its end points.

Ohms law allows us to define resistance R and to write

the following forms of the law:


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Ohm's law

Current in a circuit is directly proportional to the applied

voltage.

Current is inversely proportional to the amount circuit

resistance.

This means that if the voltage goes up, the current flow

will go up, and vice versa. As the resistance goes up, the current goes down.


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Ohms Triangle


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Quiz 1a

For a voltage of 12V, calculate the current

for a resistant of:

1 =

2 =

4 =

24 =

1 k =


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Quiz 1b

A 75 ohm circuit is powered by a 110-Volt energy

source. What is the current flowing in this circuit?

First we identify the information that we are given in

the problem:

resistance R = 75 ohms

voltage V = 110 volts

Need to know I amps 110 V = I x 75 ohms (rearrange formula)

Amps = 110 volts / 75 ohms (W) = 1.5 A


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Quiz 1c

What is the voltage of a circuit with a resistance of 250

ohms and a current of 0.95 amps?

First we identify the information that we are given in

the problem:

resistance = 250 ohms

current = 0.95 amps

Need to know volts V= 250 ohms x 0.95 amps

Volts = 250 ohms x 0.95 amps = 237.5 V


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Quiz 1d

In this circuit voltage source

is 12 volts and resistance is2 Ohms.

What is total amperage.

I = V / R

12 volts / 2 Ohms = 6 amps


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Electr ical Charge

Electrons are the charge carriers that flow in an

electrical circuitfrom the negative to positive

terminals.

Electric charge is given the symbol Q

Charge is measured in Coulombs (C)

The charge on an electron is

Is the same size as the charge on a proton.


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Electr ically Neutral

Objects with no net charge are said to be electrically

neutral, has equal amounts (+) and (-) charges

Most objects around you are electrically neutral, they

have no net electrical charge.


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Quiz 2a

Question: The electric current in a wire is 6 A.

How many electrons flow past a

given point in a time of 3 s?

Answer:

q= (6 A)(3 s) = 18 C

Recall that: 1 e-= 1.6 x 10-19C

In 3 s: 1.12 x 1020

electrons


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Quiz 2b


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Quiz 2c


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Direct Current (DC)

Direct current (DC) is the unidirectional flow of

electric charge.

The current (or voltage) does not change withtime.


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Alternating Current (AC)

An alternating current

(AC) is continuallychanging direction.

The alternating current

(or voltage) has adistinctive waveform.


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Peak and Effective Voltage of

Alternating Current

Using the oscilloscope, we can measure the

peak voltage of the AC supply.

The effectivevoltageis always less than the

peak voltage.

The effective voltage is ~ 0.7 x peak voltage.


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What is the dif ference between

AC and DC?

The difference between alternating current (a.c.) and

direct current (d.c.) can be seen using an oscilloscope.

For each current, the oscilloscope trace is a graphshowing how the voltage of an electricity supply varies

with time.


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Frequency of Alternating Cur rent

The frequency of AC electricity is the number of complete

cycles per second, which is measured in hertz (Hz).

If the frequency is increased, the number of complete

waves shown on the screen increases.

For example, if the frequency is doubled, the number ofwaves doubles.


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Mains Electr ici ty I n I ndonesia

In Indonesia, the frequency of

mains electricity is 50 Hz: this

alternating current flows

backwards and forwards 50

times per second.

The voltage of mains electricity

supplied to Indonesia homes is220-240 V and this is an

effective voltage.


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Electr ic Power

Electric Power: The rate at which electrical

energy is converted to another form of energy.

Electric power = voltage x current

P = V x I

The unit is watts (W)


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Electr ic Energy

This is how electric companies charge for

energy.

Electric energy = electric power x time

E = P x t

The unit is kWh


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Home Work

Diketahui biaya listrik

PLN adalah

Rp.1000/kWh.

Hitung biaya per hari,

biaya per minggu dan

biaya per bulan (30 hari)

pemakaian peralatanrumah tangga pada tabel

sebelah.


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Quiz 3

Question: When a 3-V battery is connected to a

light, a current of 6 mA is observed.

What is the resistance of the light

filament?

Answer:


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Resistors

The symbol for a resistor is


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Electr ic Resistance

Suppose we apply a constant potential difference of 4V to the ends of geometrically similar rods of, say:

steel, copper, and glass.

The current in glass is much less than for steel or iron,suggesting a property of materials called electrical

resistance R.


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As seen in the table, there is a

broad range of resistivity

values for various materials. Those materials with lower

resistivities offer less

resistance to the flow of

charge; they are betterconductors.

The materials shown in the

last four rows of the table

have such high resistivity that

they would not even be

considered to be conductors.


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Varying Resistance

The resistance of a material (measured in )

depends on:

Type of material (the better the conductor, the lower

the resistance)

Length of material (the longer the material, the higher

the resistance)

Thickness of material (the thinner the material, thehigher the resistance)

Temperature of material (the higher the temperature,

the higher the resistance)


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Variable Resistor

The relationship between length of the material

and resistance allows us to make a variable

resistor (or rheostat).


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Variable Resistors

If the slider is moved in the direction AB the

resistance will increase because the length


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Variable Resistors

Variable Resistor

(Potentiometer & Rheostat)


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Potential Dif ference

The potential difference of the supply is divided

between the two resistors.

When the two resistors are identical (i.e. have the same

value of resistance), the potential difference is split

equally.


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Application of Var iable Resistors

Volume or brightness controls on televisions.

Volume control on MP3 players.

Light dimmer switches.


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Digital Mul timeters

This is an electrical

instrument designed to

measure voltage, current,resistance, and often other

variables as well.

Multimeters are

manufactured in bothdigital and analog form.


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Measur ing Voltage

The symbol for a

battery in a circuit

diagram.

M i C


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Measur ing Current

Electric current is measured

in amps (A) using an

ammeter connected in series

in the circuit.


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Conductor and I nsulator

Conductor:

Material that allows electrons to move easily

through it. Copper, zinc, tin, iron, and aluminumare conductors.

Insulator:

Material that does not allow electrons to movethrough it. Glass, plastic, and wood are insulators.


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Calculate The Electr icity Costs

Question:

If Karen uses her clothes dryer (5000 W) for 1 hour a day

every day of the year, what is her kWh consumption?

How much money does it cost Karen to use her dryereach year? (Electricity costs Rp.1000 per kWh.)

Answer:

kWh consumption = (wattage X hours used per day Xdays per year) / 1000

1,825 kWh

Cost: 1,825 kWh X Rp.1000 = Rp. 1.825.000,-

C l l t Th H t Pl t V lt


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Calculate The Hot Plate Voltage

Question:

How many Volts does the hot plate require if the

Current is 0.5 amps and the Resistance is 440 Ohms?

Answer:

Voltage = Current X Resistance

Voltage = (0.5 Amps) (220 Ohms)

Voltage = 220 Volts

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