# Current Electricity http://www.physicsclassroom.com/class/circuits.

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Current Electricity

Producing Electric Energy
Electric energy provides the means to transfer large quantities of energy over great distances with little loss.

Producing Electric Energy
Because electric energy can so easily be changed into other forms, it has become indispensable in our daily lives.

Potential Energy High PE Low PE
If you do work against gravity, the gravitational field stores that energy as Gravitational Potential Energy, GPE If you do work against an electrostatic force, the electric field stores that energy as electric Potential Energy, EPE

Electric Potential Energy
Electrical potential energy is the energy contained in a configuration of charges. Like all potential energies, when it goes up the configuration is less stable; when it goes down, the configuration is more stable. The unit is the Joule.

Electric Potential Energy
Electrical potential energy increases when charges are brought into more unstable configurations. Lower PE Higher PE Fe + + + + d Stable Unstable Moving q1 closer to q2 requires work and that will increase the PE of the charge. Work against electric force increases electric PE

Electric Potential Energy
Electrical potential energy decreases when charges are brought into more stable configurations. Higher PE Lower PE Fe - - + + d Unstable Stable q1 will naturally move or fall towards q2 in the direction of E. No work is required and the PE of the charge will decrease. Work with the electric force decreases electric PE

Electric Potential Energy
Electrical potential energy, EPE, is stored or lost as charges, q, move in an electric field, E. EPE is dependent on both the location in the E field and the amount of charge, q moved. E is not the same for every situation and it changes in space. It depends on the configuration of the source charges

Electric Potential Difference
Electric Potential greatest at? + - Moving a charge in an electric field requires work or energy input A B + + Fe A t every location, a charge has a position-dependent potential Potential difference is simply the difference in potential at any 2 points Higher V Lower V

Circuit A B High V Lower V
Charges flow from high to low V through conducting wire High V A This flow of positive charge is called conventional current Lower V B The flow stops when the potential difference between A and B is zero.

Potential Difference and Current
High V To be a circuit, charges must flow continuously thru a loop, returning to their original position and cycling thru again. To do so requires energy input, a charge pump that raises the electric potential of the charge Lower V

Electric Circuits A circuit is simply a closed loop through which charges can continuously move Flow of charge is CURRENT

Producing Electric Current (Charge Pump)
1. Voltaic or galvanic cell converts chemical E to electric E. A battery is made up of several galvanic cells connected together. 2. Photovoltaic cell, or solar cell— changes light energy into electric energy.

Requirements of a Circuit

What do the 4 successful arrangements have in common?
Requirements of a Circuit 1. Closed conducting loop that extends from the positive to negative terminal What do the 4 successful arrangements have in common?

What do the 4 successful arrangements have in common?
Requirements of a Circuit 1. Closed conducting loop that extends from the + to - terminal 2. There must be an energy source that maintains an electric potential difference across the ends of the circuit. What do the 4 successful arrangements have in common?

Electric Circuits A circuit is simply a closed loop through which charges can continuously move

Current Once the two requirements of a circuit are met, charge will flow. The flow or movement of charge is called CURRENT

Unit of current: Ampere (A)
The flow or movement of charge is called CURRENT. Electric current is represented by I It is the overall rate of flow of electric charge, q/t. I = q/t Unit of current: Ampere (A) 1 A = 1 C/s

Current A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s. I = _____________ A 20C/40s = 0.5

Conventional Current Direction
The particles that carry charge through a wire are mobile electrons which move in a direction opposite the electric field. Ben Franklin, who conducted extensive scientific studies in both static and current electricity, envisioned positive charges as the carriers of charge. The convention has stuck and is still used today. The direction of an electric current is by convention the direction in which a positive charge would move. Electrons would actually move through the wires in the opposite direction.

Opposite, right to left Problem
A long wire is connected to the terminals of a battery.  In 5.0 sec, 5.8 x 1020 electrons pass a cross section along the wire. a) Determine the current in the wire (if you need any extra information, ask your classmates). b) If the electrons flow from left to right, in which direction is the current? Opposite, right to left

A typical flashlight battery will produce a 0
A typical flashlight battery will produce a 0.5-A current for about 3 h before losing its charge. Determine the total number of electrons that have moved past a cross section of wire connecting the battery and light bulb.

Circuit Components + - + - Light bulb Cell Battery Resistor Wire
Switch

Sample Problem – Draw a single loop circuit that contains a cell, a light bulb and a switch. Name the components bulb + - switch cell

Series Circuit–How do the brightnesses compare?
Which circuit has the greater current flow? Does the charge get used up? dimmer reasoning Voltage across left bulb, current, which value determines brightness? 1 2 and 3 equal brightness, what does that say about current thru each bulb What does that way about conservation of charge What does a battery do for a circuit? Does the battery supply a constant current to all circuits? Does the battery supply a constant energy to all circuits? Does the battery supply a constant potential difference to all circuits? Use your observations about bulb brightness to explain your answers. Predominant equation used to study circuits So something with a large resistance – small current (obstacle to current) Large R large DV ie looses energy going thru resistor The drop in potential occurs as electrical energy is transformed to other forms (heat, light) and work is done. Same current in each lightbulb, equal brightness brighter 1 2 3 + - + -

Increases, There are more pathways so less resistance
Parallel Connections As the number of light bulbs increases, what happens to the current through the circuit? Increases, There are more pathways so less resistance less I More I + - + - reasoning Voltage across left bulb, current, which value determines brightness? Compare the brightness of bulbs D and E. What can you conclude from this observation about the amount of current through each bulb? Describe the flow of current around the entire circuit. What do your observations suggest about the way the current through the battery divides and recombines at junctions where the circuit splits into two branches? How does the current at point 1 compare with the currents at points 2 and 3? How is this related to the conservation of charge? How does the brightness of bulb A (Circuit I) compare to the brightness of bulbs D and E (Circuit III)? What can you infer about the current at point 1 in Circuit III compared to the current at point 1 in Circuit I? Predominant equation used to study circuits So something with a large resistance – small current (obstacle to current) Large R large DV ie looses energy going thru resistor The drop in potential occurs as electrical energy is transformed to other forms (heat, light) and work is done. I 3I

Circuit Connections

Series Connections If one resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit? If one resistor goes out, there is no longer a closed loop for current flow and all other devices in series will go out. There is an OPEN CIRCUIT

Parallel Connections If on resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit? If one resistor goes out, there is still a closed loop for current flow and so the other devices in series will stay on

Series or Parallel? The light bulbs are identical and have identical resistance, R. Which configuration produces more light? Which way do you think the headlights of a car are wired? More pathways, Less resistance, More current More light

Circuit Components + - + - Light bulb Cell Battery Resistor Wire
Switch

Circuit Components V Voltmeter A Ammeter W Ohmmeter

Measuring Current Ammeter – measures current
Current to be measured must pass through the ammeter, so it must be placed in SERIES mode in the circuit. Ideally ammeters have ZERO resistance so that they do not affect the energy of the circuit While ammeters are used to measure the currents, the voltmeter is used to measure the voltage. Both the devices are different in terms of functionality and circuit placement Ammeter - In order to get a reading with an ammeter, the circuit must be unplugged in order to attach the ammeter to the circuit.  In order for an ammeter to measure a current, the current must pass through the ammeter and hence it must be placed in a series mode inside the circuit. The polarities must correspond, the positive and negative polarity must match up with the positive and negative on the circuit. Though ideally, ammeters should have zero resistance, in actuality it has relatively low resistance compared to voltmeters. If the resistance is too high, it may block too much current and affect the currents in the circuit and alter the readings. If an ammeter is accidently wired in parallel with a voltage source, it could cause a short circuit and result in blowing a fuse.

Measuring Voltage Voltmeter – measures voltage Does NOT require the current to pass through it. It must be placed in parallel to the circuit element. Ideally voltmeters have INFINITE resistance so that they do not draw current away from circuit. While ammeters are used to measure the currents, the voltmeter is used to measure the voltage. Both the devices are different in terms of functionality and circuit placement A voltmeter is an instrument that is used to measure the voltage between two points in a circuit. Unlike ammeters, the circuit does not need to be unplugged in order to attach a voltmeter. In order to for a voltmeter to measure the voltages, it does not require current to be passed through it. It is placed parallel to the circuits they are expected to measure. Polarity is also expected to be observed in the placement of voltmeters. Ideally voltmeters should have infinite impedance, but this is not the case with actual voltmeter; they have a finite resistance value. While voltmeters should not draw any current from the circuit, they require currents in order produce the repulsive magnetic field. The currents drawn from the circuit can be minimized by employing amplifiers for a more accurate read. If the internal resistance of a voltmeter is too small it will not block enough current and provide a faulty reading. Voltmeter’s accuracy is affected by many factors, including temperature and supply voltage variations. Voltmeters are easily and much safer to install and also provide a more accurate reading compared to ammeters.

Resistance An electron traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the flow of charge. The amount of current in a circuit depends on BOTH the potential difference across the circuit, DV, AND the total resistance in the circuit, R. LOAD I I Reistance analogy – flow of water thru pipes Wires have pretty small resistance so not much energy lost in wites. Energy Source DV

Resistance The table lists some of the factors that impact resistance.
Do Ohms Law activity. Discuss results as a class Students discover Ohms law experimentally

Resistors Resistors are devices designed to have a specific resistance. Resistors are devices put in circuits to reduce the current flow Resistors may be made of graphite, semiconductors, or wires that are long and thin. In many circuits and electronic devices, resistors are used to control the amount of current

To produce electric current, I, a potential difference, DV, is required. Simon Ohm established experimentally that the current in a metal wire is proportional to the potential difference applied to its ends. Current flow does NOT depend only on voltage. Charge traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the current. The higher the resistance, the smaller the current. To produce electric currrent, a difference in potential is required. One way to produce DV is a battery. It was Simon Ohm who established experimentally that the current in a metal wire is proportional to the potential difference applied to its ends. (for eg. If connect wire to 6V battery, the current flow would be twice that from 3 v battery Helpful to compare elec current to water flow in river or pipe acted on by gravity. It the pipe or river were nearly level (not much GPE), the flow rate would be small. But if one end were higher than the other, the flow rate or current would be greater. The greater the difference in height, the greater the current. Electric potential is analogous to the gravitational case of height. Just as inc in height causes increased water flow, so a greater elec potential difference or voltage causes a greated current flow. Exactly how much current flows in a wire depends not only on the voltage, but also the resistance the wire offers to the flow of electrons. The walls of a pipe or the banks of a river and rocks in the middle offer resistance to the flow of current. Similarly electrons are slowed down because of interactions with the atoms of the wire. The higher the resistance, the less the current for a given voltage, V. Resistance is defined so that the current is inversely proportional to the resistance. In many circuits and electronic devices, resistors are used to control the amount of current

Ohm’s Law Every element in a circuit obeys Ohm’s Law Resistance
Units: Ohms (W) Current Units: Amperes (A) Electric potential Units: Volts (v) Predominant equation used to study circuits To produce electric currrent, a difference in potential is required. One way to produce DV is a battery. It was Simon Ohm who established experimentally that the current in a metal wire is proportional to the potential difference applied to its ends. (for eg. If connect wire to 6V battery, the current flow would be twice that from 3 v battery Helpful to compare elec current to water flow in river or pipe acted on by gravity. It the pipe or river were nearly level (not much GPE), the flow rate would be small. But if one end were higher than the other, the flow rate or current would be greater. The greater the difference in height, the greater the current. Electric potential is analogous to the gravitational case of height. Just as inc in height causes increased water flow, so a greater elec potential difference or voltage causes a greated current flow. Exactly how much current flows in a wire depends not only on the voltage, but also the resistance the wire offers to the flow of electrons. The walls of a pipe or the banks of a river and rocks in the middle offer resistance to the flow of current. Similarly electrons are slowed down because of interactions with the atoms of the wire. The higher the resistance, the less the current for a given voltage, V. Resistance is defined so that the current is inversely proportional to the resistance Every element in a circuit obeys Ohm’s Law

In which circuit does the light bulb have highest resistance?

Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. a. decrease the voltage b. decrease the resistance c. increase the voltage d. increase the resistance

a. increase the voltage of the battery (add another cell)
A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply. a. increase the voltage of the battery (add another cell) b. decrease the voltage of the battery (remove a cell) c. decrease the resistance of the circuit d. increase the resistance of the circuit , R

If the resistance of a circuit were tripled, then the current through the circuit would be ____.
a. one-third as much b. three times as much c. unchanged d. ... nonsense! There would be no way to make such a prediction. + - I + - 1/3 I

If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.
a. one-fourth as much b. four times as much c. unchanged d. ... nonsense! There would be no way to make such a prediction. + - I 4I + -

If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.
a. one-fourth as much b. four times as much c. unchanged d. ... nonsense! There would be no way to make such a prediction. + - I 4I + -

Use the Ohm's law equation to provide numerical answers to the following questions:
a. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of ________ Volts is impressed across the device. b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________ Ω. c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω will have a current of ________ Amps.

Use the Ohm's law equation to determine the missing values in the following circuits.

Conservation of Charge in a Circuit
Charges cannot be created or destroyed, but they can be separated. Thus, the total amount of charge—the number of negative electrons and positive ions—in the circuit does not change. If one coulomb flows through the generator in 1 s, then one coulomb also will flow through the motor in 1 s. Charge is a conserved quantity.

Connections + - + - less I More I I3 I3 I2 I1 I__ I2
Which circuit draws more current (how are I1 and I2 related)? What is the order of bulb brightness? How does charge flow in these circuits (how are I2 and I3 related)? Does the charge get used up? same I3 dimmer brighter brighter + - I2 I3 I2 I1 I__ + - less I More I

Kirchoff’s Junction Rule Current into node= Current out of node
At circuit nodes (junctions), the current divides, and each path gets a fraction of it. No charge is lost. Kirchoff’s Junction Rule Current into node= Current out of node I = I1 + I2 + I3 I I2 I3 I1 I The lower resistance the path, the greater the current. I1 =V/R1 I2 =V/R2 I3 =V/R3

Kirchoff’s Loop Rule In a closed circuit, sum of all the voltage boosts = sum of all the voltage drops Voltage boosts =sum voltage drops Energy is conserved as charge flows around a closed loop

Conservation of Energy in a Circuit
The change in electric energy, ΔE, equals qV. Because q is conserved, the net change in potential energy of the charges going completely around the circuit must be zero. The increase in potential difference produced by the generator equals the decrease in potential difference across the motor.

Kirchoff’s Rules 1. Junction Rule At any junction point in a circuit, the sum of all the currents entering the junction must equal the sum of all currents leaving the junction Current into node= Current out of node (Conservation of charge) In many circuits and electronic devices, resistors are used to control the amount of current 2. Loop Rule The sum of the changes in potential around any closed path of a circuit must be zero. (Conservation of energy)

Two Types of Connections
When there are 2 or more electrical devices in a circuit with an energy source, there are a couple of ways to connect them. When there are 2 or more electrical devices present in a circuit with an energy source, there are a couple ways to connect them – in series or in parallel

+ - Rank the currents at points A-F from greatest to least 4 W
If the resistors were light bulbs, which would be brighter? 8 W D C + - this may seem surprising. but remember when you put resistors in parallel you are giving current additional paths to follow. Hence net resistance is less. B A 12 V

Energy Transfer and Power
Movement of charge from terminal to terminal is of little use if the energy possessed by the flowing charge is not transformed into another useful form. High current and high rate of energy consumption. Would heat wire and drain battery

Energy Transfer and Power
When a LOAD is put on the circuit (light bulb, beeper, motor…), electrical energy is transformed to other, useful forms of energy. LOAD An electrical circuit is simply an energy transformation tool. Rate of energy transformation/transfer is POWER When a circuit is equipped with a light bulb, beeper, or motor, the electrical energy supplied to the charge by the battery is transformed into other forms in the electrical device. A light bulb, beeper and motor are generally referred to as a load. In a light bulb, electrical energy is transformed into useful light energy (and some non-useful thermal energy). In a beeper, electrical energy is transformed into sound energy. And in a motor, electrical energy is transformed into mechanical energy. Electric energy is useful because it can be easily transformed to other forms of energy Electric heaters, stoves,, toasters, hair dryers – elec E to thermal energy Lightbulb – elec E heats wire filament which becomes so hot it glows. Only few % of the energy transformed into light, rest to thermal Lightbulb filaments and heating elements in household apploiances have resistances typically of a few ohms to few hundred ohms. Energy Source

Energy Transfer and Power
LOAD POWER, P, is the rate that energy is supplied to the load or the rate of work done on the charge. Energy Source Unit of Power: Watt (W) 1 W = 1 J/s

Energy transfer and Power
POWER, P, is the rate that energy is supplied to the load or the rate of work done on the charge. LOAD 60 Watt light bulb means 60 J of energy delivered to bulb every second OR 60 J of energy used by the bulb per second Energy Source

Electric heater. An electric heater draws 15. 0 A on a 120 V line
Electric heater. An electric heater draws 15.0 A on a 120 V line. How much power does it use and how much does it cost per month (30 days) if it operates 3.0 h per day and the electric company charges 10.5 cents per kW-h? To operate it for 30 days, 3 hr/day would total 90hrs and would use

Will a fuse blow? Determine the total current drawn by all the devices used at once. Will they blow a 20-A fuse? Fuse blows to prevnet wires from getting too hot and starting a fire

The Human Body The human body acts as a variable resistor.
When dry, skin’s resistance is high enough to keep currents that are produced by small and moderate voltages low. If skin becomes wet, however, its resistance is lower, and the electric current can rise to dangerous levels. A current as low as 1 mA can be felt as a mild shock, while currents of 15 mA can cause loss of muscle control, and currents of 100 mA can cause death.

Example: Lightning is a spectacular, natural example of electric current. There is much variability to lightning bolts, but a typical event can transfer 109 J of energy across a potential difference of 5 x 107 V during a time interval of 0.2 s. Estimate the total charge transferred, the current, and the average power over the 0.2 s.