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, 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