Transcript Resistance - Effingham County Schools

Electricity
Current Electricity
Current Electricity

Producing Electric Current

Conventional current – where charges flow from positive to negative

Flow stops when potential difference zero

Any closed loop or conducting path of electric flow is electric circuit
Current Electricity

Producing Electric Current

Voltaic or galvanic cell (common dry cell), converts chemical energy
to electric energy

Battery - several galvanic cells connected together

Photovoltaic or solar cell—changes light energy into electric energy
Current Electricity

Conservation of Charge

Charges cannot be created or destroyed, but can be separated. Energy
also conserved

Energy carried by electric current depends on charge transferred, q,
and potential difference across which it moves, V. Thus, E = qV

Power measures rate at which energy transferred in watts, W

If generator transfers 1 J kinetic energy to electric energy each second,
rate is 1 J/s, or 1 W
Current Electricity

Rate of Charge Flow

Rate of flow of electric charge, q/t, (electric current) measured in
coulombs per second
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I = q/t. So 1 C/s = 1 ampere, A
Current Electricity

Rates of Charge Flow and Energy Transfer

Energy carried by electric current related to voltage, E = qV

Power of electric device found by multiplying voltage (E) and current
(I)
P=IE
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Energy equal to product of power and time
E = Pt
Current Electricity

Resistance and Ohm’s Law
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Resistance determines how much current will flow
Current Electricity
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Resistance and Ohm’s Law
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Resistance, R = ratio of electric potential difference, V, to current, I
Resistance

Measured in ohms. One ohm (1 Ω) is
resistance permitting
electric charge of 1 A to
flow when potential
difference of 1 V applied
Current Electricity

Resistance and Ohm’s Law

Named for German scientist Georg Simon Ohm, who found ratio of
potential difference to current constant for given conductor
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Resistance for most conductors does not vary
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Device having constant resistance obeys Ohm’s law
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Most metallic conductors obey Ohm’s law, over limited range of
voltages
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Wires used to connect electric devices have low resistance
Current Electricity

Resistance and Ohm’s Law

Wires have little resistance
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Resistor - device having specific resistance. Made of graphite,
semiconductors, or thin, long wires
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Two ways to control current in a circuit:
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Voltage or resistance
I =V/R
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When V = 6 V and R = 30 Ω, I = 0.2 A
Current Electricity

Resistance and Ohm’s Law

In first figure, voltage applied across resistor reduced from 6 V to 3 V
to reduce current to 0.1 A.
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Can also replace 30-Ω resistor with 60-Ω resistor
Current Electricity

Resistance and Ohm’s Law
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Smooth, continuous variation of current is sometimes desired
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To achieve control, variable resistor, potentiometer, used
Current Electricity

Resistance and Ohm’s Law
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Variable resistors can consist of coil of resistance wire and sliding
contact point
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Moving contact point varies amount of wire in circuit
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More wire = increased resistance = decreased current
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Adjusts speed of motor from fast to slow
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Other examples: volume, brightness, contrast, tone, and hue controls
on televisions
Current Electricity

Diagramming Circuits

Standard symbols for circuit elements, used in drawings called circuit
schematics
Current Electricity

Diagramming Circuits

When connected across component, called parallel connection because
circuit component and voltmeter aligned parallel to each other
Current Electricity
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Diagramming Circuits
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Ammeter measures current through circuit component
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Same current going through component must go through ammeter, one
current path
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A connection with only one current path is called a series connection
Current Electricity

Diagramming Circuits

To add an ammeter to a circuit, the wire connected to the circuit
component must be removed and connected to the ammeter instead
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Then, another wire is connected from the second terminal of the
ammeter to the circuit component
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In a series connection, there can be only a single path through the
connection
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Always associate the words current through with a series connection
Using Electrical Energy

Energy Transfer in Electric Circuits

Motor converts electric energy to mechanical energy
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Lamp changes electric energy into light
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Some energy converted into thermal energy
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Some devices designed to convert electrical into thermal energy by
acting as resistors in circuit
Using Electrical Energy

Heating a Resistor
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Power dissipated in resistor is P = IV
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Voltage in resistor, V = IR, so
Power
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P = I 2R
Can substitute I = V/R into P = IV to obtain the following equation
Power
Using Electrical Energy

Heating a Resistor
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Because P = I2R and P = V2/R, total energy converted to thermal
energy can be written:
Thermal Energy
E = Pt
E = I2Rt
E=
Using Electrical Energy

Superconductors

Superconductor is material with zero resistance

No restriction of current in superconductors, so no potential difference,
V, across them
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Power dissipated in conductor = IV, superconductor can conduct
electricity without loss of energy
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Almost all superconductors must be kept at temperatures below 100 K
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Practical uses include MRI magnets and in synchrotrons
Using Electrical Energy

Transmission of Electric Energy
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Hydroelectric facilities capable of producing a great deal of energy

Energy often transmitted over long distances to reach homes and
industries
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How can the transmission occur with as
little loss to thermal energy as possible?
Using Electrical Energy

Transmission of Electric Energy
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Thermal energy produced represented by P = I2R
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Unwanted thermal energy called joule heating loss, or I2R loss
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To reduce, either I, or R must be reduced
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Cables of high conductivity and large diameter available, but such
cables expensive and heavy
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Because loss of energy also proportional to square of the current in the
conductors, more important to keep current in transmission lines low
Using Electrical Energy

Transmission of Electric Energy
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Cables of high conductivity and large diameter (and therefore low
resistance) are available, but such cables are expensive and heavy
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Because the loss of energy is also proportional to the square of the
current in the conductors, it is even more important to keep the current
in the transmission lines low
Using Electrical Energy
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Transmission of Electric Energy
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How can the current in the transmission lines be kept low?
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The current is reduced without the power being reduced by an increase
in the voltage
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Some long-distance lines use voltages of more than 500,000 V
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The resulting lower current reduces the I2R loss in the lines by keeping
the I2 factor low
Using Electrical Energy

Transmission of Electric Energy
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Electric companies often called power companies, but provide energy
rather than power

Amount of electric energy used by device is rate of energy
consumption, (W), times seconds operated
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Electric companies measure energy sales in kilowatt-hours, kWh

Kilowatt-hour equal to 1000 watts delivered continuously for 3600 s
(1 h), or 3.6×106 J
Using Electrical Energy

Transmission of Electric Energy

A 60 W lightbulb is connected to a 115 V power source. What is the
current through the lightbulb? What is the resistance of the lightbulb?
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A microwave draws a 15.0 A current from a 120 V power source. How
much power is delivered to the microwave? How much energy does
the microwave use to heat food for 1 min?
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A current of 0.85 A is measured through a 27 Ω resistor for 1.00 hr.
How much heat does the resistor generate?
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A 60 W bulb powered by a 120.0 V source has a resistance of 5.0 Ω at
room temperature and 100.0 Ω at operating temperature. What is the
current and power of the bulb at room temperature? What is the
current and power of the bulb at operating temperature?
Using Electrical Energy

Transmission of Electric Energy
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A 100 W lightbulb is turned on for two-fifths of the time for 30 days.
At a price of $0.090 per kWh, how much does the lightbulb cost to run
during the 30 days? (1 kWh = 3.6 x 106 J)
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A 240 V water heater has a resistance of 15 Ω. What is the power of
the heater? What thermal energy is supplied by the heater in 15
minutes?
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A long distance high-tension wire uses 500,000 V. What is the power
output of these wires in W, kW, MW if the current is 20 A? What is the
power dissipated in the wires if the resistance is 0.015 Ω?