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Electricity Current Electricity

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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

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)

Energy equal to product of power and time

Current Electricity

P = I E

E = Pt

Resistance and Ohm’s Law

Resistance determines how much current will flow

Current Electricity

Resistance and Ohm’s Law

Resistance, R = ratio of electric potential difference, V, to current, I

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

Resistance and Ohm’s Law

Named for German scientist Georg Simon Ohm, who found ratio of potential difference to current constant for given conductor

Resistance for most conductors does not vary

Device having constant resistance obeys Ohm’s law

Most metallic conductors obey Ohm’s law, over limited range of voltages

Wires used to connect electric devices have low resistance

Current Electricity

Resistance and Ohm’s Law

Wires have little resistance

Resistor - device having specific resistance. Made of graphite, semiconductors, or thin, long wires

Two ways to control current in a circuit:

Voltage or resistance

I =V/R

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.

Can also replace 30-Ω resistor with 60-Ω resistor

Current Electricity

Resistance and Ohm’s Law

Smooth, continuous variation of current is sometimes desired

To achieve control, variable resistor, potentiometer, used

Current Electricity

Resistance and Ohm’s Law

Variable resistors can consist of coil of resistance wire and sliding contact point

Moving contact point varies amount of wire in circuit

More wire = increased resistance = decreased current

Adjusts speed of motor from fast to slow

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

Diagramming Circuits

Ammeter measures current through circuit component

Same current going through component must go through ammeter, one current path

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

Then, another wire is connected from the second terminal of the ammeter to the circuit component

In a series connection, there can be only a single path through the connection

Always associate the words current through with a series connection

Current Electricity

Energy Transfer in Electric Circuits

Motor converts electric energy to mechanical energy

Lamp changes electric energy into light

Some energy converted into thermal energy

Some devices designed to convert electrical into thermal energy by acting as resistors in circuit

Using Electrical Energy

Heating a Resistor

Power dissipated in resistor is P = IV

Voltage in resistor, V = IR, so

Can substitute I = V/R into P = IV to obtain the following equation

Using Electrical Energy

Power P = I2R

Power

Heating a Resistor

Because P = I2R and P = V2/R, total energy converted to thermal energy can be written:

Using Electrical Energy

Thermal Energy E = I2RtE = Pt

E =

Superconductors

Superconductor is material with zero resistance

No restriction of current in superconductors, so no potential difference, V, across them

Power dissipated in conductor = IV, superconductor can conduct electricity without loss of energy

Almost all superconductors must be kept at temperatures below 100 K

Practical uses include MRI magnets and in synchrotrons

Using Electrical Energy

Transmission of Electric Energy

Hydroelectric facilities capable of producing a great deal of energy

Energy often transmitted over long distances to reach homes and industries

How can the transmission occur with as

little loss to thermal energy as possible?

Using Electrical Energy

Transmission of Electric Energy

Thermal energy produced represented by P = I2R

Unwanted thermal energy called joule heating loss, or I2R loss

To reduce, either I, or R must be reduced

Cables of high conductivity and large diameter available, but such cables expensive and heavy

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

Cables of high conductivity and large diameter (and therefore low resistance) are available, but such cables are expensive and heavy

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

Transmission of Electric Energy

How can the current in the transmission lines be kept low?

The current is reduced without the power being reduced by an increase in the voltage

Some long-distance lines use voltages of more than 500,000 V

The resulting lower current reduces the I2R loss in the lines by keeping the I2 factor low

Using Electrical Energy

Transmission of Electric Energy

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

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?

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?

A current of 0.85 A is measured through a 27 Ω resistor for 1.00 hr. How much heat does the resistor generate?

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

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)

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?

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?

Using Electrical Energy