Electric Current and Resistance

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Electric Current and Resistance
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(No Transcript)
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English/ Italian glossary
Current corrente
Circuit circuito
Charge carica
Potential potenziale
Voltage Voltaggio differenza di potenziale
Battery batteria
Switch Interruttore
Net rete
Wire Filo (conduttore)
Copper Rame
Ammeter/Voltmeter Amperometro/voltmetro
Drift deriva
Plate piastra
Positive/negative terminal Polo positivo/negativo
electron elettrone
Electric field Campo elettrico
Resistor resistore
Resistance resistenza
Series/parallel Serie/parallelo
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Which of these connections could work to light
the bulb?
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The Central Concept Closed Circuit
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circuit diagram
Scientists usually draw electric circuits using
symbols
lamp
cell
switch
wires
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Electric Circuit
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Diagram of Electric Circuit
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Remember Electric Potential Energy- Two Unlike
Charges

Higher Potential Energy
-
Lower Potential Energy

• To cause movement of a charge, there must be a
  potential difference.

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While the switch is open

• Free electrons (conducting electrons) are always
  moving in random motion.
• The random speeds are at an order of
• 106 m/s.
• There is no net movement of charge across a cross
  section of a wire.

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What occurs in a wire when the circuit switch is
closed?
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What occurs in a wire when the circuit switch is
closed?

• An electric field is established instantaneously
  (at almost the speed of light, 3x108 m/s).
• Free electrons, while still randomly moving,
  immediately begin drifting due to the electric
  field, resulting in a net flow of charge.
• Average drift velocity is about 0.01cm/s.

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Closing the switch establishes a potential
difference (voltage) and an electric field in the
circuit.
High Potential
Low Potential

• Electrons flow in a net direction away from the
  (-) terminal.

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Question

• If the drift velocity is about 0.01cm/s, why do
  the lights turn on instantaneously when the
  circuit switch is closed?

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Conventional Current

• By tradition, direction in which positive
  charges would flow.
• Direction is opposite of electron flow.

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Question

• What is required in order to have an electric
  current flow in a circuit?
• Answers
• A voltage source.
• The circuit must be closed.

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Battery (Chemical Cell)

• A device that converts chemical energy to
  electricity.
• A battery provides a potential energy difference
  (voltage source).

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Electric Current

• The flow of electric charges.

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Electric Current, I

• I Dq
• Dt
• Rate
• Unit Coulomb / sec Ampere (A)
• Andre Ampere (1775-1836)

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Conventional current has the direction that the
() charges would have in the circuit.
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TWO TYPES OF CURRENT

• Direct Current
• DC
• Provided by batteries

• Alternating Current
• AC
• Provided by power companies

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MEASUREMENT DEVICES1) Ammeter

• Measures electric current.
• Must be placed in series.

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2) Voltmeter

• Measures the voltage between two points in an
  electric circuit.
• Must be connected in parallel.

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Example

• What charge flows through a cross sectional area
  of a wire in 10min, if the ammeter measures a
  current of 5mA?
• Answer 3C

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RESISTANCE

• It is known from experiment that the current
  flowing in a conductor is directly proportional
  to the potential difference across it.
• The constant of proportionality, R, is called the
  RESISTANCE
• R DV / I
• (resistance equals the ratio of voltage to
  current)
• Unit Ohm (O) 1 O 1 V/A

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Ohms Law
I V / R
I Current (Amperes) (amps) V Voltage
(Volts) R Resistance (ohms)
I
V
Georg Simon Ohm (1787-1854)
A device that obeys Ohms Law is called a ohmic
resistor (its resistance does not depend on the
voltage)
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• The current through an ohmic conductor is
• directly proportional to the voltage across it
  (over a limited range of V)
• (more voltage? more current)
• inversely proportional to the resistance of the
  conductor
• (the greater the resistance ? the less the
  current)

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But what is resistance?

• It is a measure of how much a conductor impedes
  the flow of current
• At the molecular level, electrons undergo
  frequent collisions with the ions of materials.
• The higher the number of collisions, the higher
  the resistance of the material is.
• ? Its a sort of friction, an opposition to the
  flow of current

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Resistor

• An object that has a given resistance.

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RESISTORS
Colour codes are used to identify resistance value
The four colour code bands are at one end of the
component. Counting from the end, the first three
(or sometimes four or five) bands give the
resistance value and the last the tolerance
TOLERANCES
BROWN 1
RED 2
GOLD 5
SILVER 10
NONE 20
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Example

• Calculate the current through a 3 O resistor when
  a voltage of 12V is applied across it.
• Answer 4 A

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Example

• A 6 O resistor has a power source of 20V across
  it. What will happen to the resistance if the
  voltage doubles?
• Answer
• resistance doesnt change!

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Factors that affect resistance

• Type of material
• Size and shape
• Temperature

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Resistance

• R ? L (a.k.a. Ohms 2nd law)
• A
• L length of the wire
• A cross-sectional area
• ? resistivity (inherent to material)
• Unit Wm

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RESISTIVITIES FOR CERTAIN MATERIALS AT
20C (measured in Wm) Silver 1.6
10-8 Copper 1.7 10-8 Aluminium 2.8
10-8 Tungsten 5.6 10-8 Constantan (alloy of
copper and nickel) 49 10-8 Nichrome (alloy of
nickel, iron chromium) 100 10-8 Graphite (3
- 60) 10-5 Silicon 0.1 60 (semiconductor)
Germanium (1 - 500) 10-3 Pyrex glass
1012 (insulator)
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What is the best material?
The table of resistivity values shows that,
although silver has a low resistivity, it is
expensive to use in electrical circuits. Copper
is the preferred metal although aluminum is
commonly used in electricity transmission cables
because of its lower density.
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Example Determine the resistance of a piece of
copper wire that is 10.0 m long and 1.2 mm in
diameter. Solution The resistance, R, is given
by the formula R ?L / A, where A pr2. This
means that R (1.7 10-8 Om) (10.0 m) / p(6.0
10-4)2 m2 0.150 O. The resistance of the copper
wire is 0.15 O.
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Question

• What happens to the resistance when the length is
  doubled and the area is quadrupled?
• Answer It changes by 1/2

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Temperature Dependence of Resistance
The resistance of a material increases with
temperature because of the thermal agitation of
the atoms it contains, and this impedes the
movement of electrons that make up the
current. Resistance increases because resistivity
increases, as shown in this formula rt r201
a(t -20) where r20 equals the resistivity at
20 C, rt is the resistivity at some temperature,
t C, above the reference temperature, and a is
the temperature coefficient for the material
being used.
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SUPERCONDUCTIVITY
One interesting phenomenon of the effect of
temperature on resistance is superconductivity.
In 1911, H. Kammerlingh Onnes found that mercury
loses all its resistance abruptly at a critical
temperature of 4.1 K. When a material attains
zero resistance at some critical temperature, it
is called a superconductor. The possibility of
having a material that has an induced electric
current that lasts forever has become a topic for
research physicists. Just think of the energy
saving if the perfect superconductor is found
that can give zero resistance at room temperature.
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POWER DISSIPATION INRESISTORS

• Electric power is the rate at which energy is
  supplied to or used by a device.
• It is measured in J/ s called watts (W).
• When a steady current is flowing through a load
  such as a resistor, it dissipates energy in it.
• This energy is equal to the potential energy lost
  by the charge as it moves through the potential
  difference that exists between the terminals
• of the load.

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Some possible power ratings for household
appliances

• If a vacuum cleaner has a power rating of 1500 W,
  it means it is converting electrical energy to
  mechanical, sound and heat energy at the rate of
  1500 J/s.
• A 60 W light globe converts electrical energy to
  light and heat energy at the rate of 60 J/s .

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Electric power

• Basic definition of power P W/ t
• Work / time
• We know that W q V and q I t
• P q V/t I t V/ t ? P V I
• (V is the voltage)
• And applying Ohms law P R I2 V2 / R

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UNIT OF ELECTRIC ENERGY

• The commercial unit of electrical energy is the
  kilowatthour (kW h).
• It is the energy consumed when 1 kW of power is
  used for one hour.
• The consumer has to pay a certain cost per
  kilowatt-hour.
• --------------------------
• Question How many Joules in a kWh?
• 1 kWh 1000 W x 3600 s 3600000 J
• 3.6 x 106 J

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HEATING EFFECT OF A CURRENT(Effetto Joule)

• It was investigated in 1841 by James Joule.
• He was able to demonstrate that by supplying
  electrical energy to a high resistance coil of
  wire this energy could be converted to thermal
  energy.
• V I t m c ?T
• electric energy ? heat infrared radiation

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Example

• An electrical appliance is rated as 2.5 kW, 240
  V.
• (a) Determine the current needed for it to
  operate.
• (b) Calculate the energy it would consume in 2.0
  hours.
• Solution
• Given that P 2.5 103 and V 240 V, we use
  the formula, P IV ? I P/W 10.4
• So I 1.0 101 A.
• (b) Next, we use the formula W VIt, so that
• W (240 V) x(10.4 A)x 7.2 103 s 1.8 107 J
• The energy consumed is 1.8 107J.

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Exercises

• 1) The element of an electric jug has a
  resistance of 60 O and draws a current of 3.0 A.
  Determine by
• how much the temperature of 5.0 kg of water will
• rise if it is on for 6 minutes.
• 2) Calculate the cost to heat 200 kg of water
  from
• 12C to its boiling point if power costs 14 cents
• per kilowatt-hour.