Unit 7: Electric Current & Circuits Electric current is relatedtovoltage that produces it, and resistance that opposes it.
Flow of Charge Heatflows through a conductor only when a temperaturedifference exists. Heat flows fromhigherto lower temperature. heat
Flow of Charge difference in potential of water pressure no potential difference at equal water pressure Chargeflows from a potential difference(voltage), between the ends of a conductor. The flowstops whenboth ends reach the samepotential. To continue the flow of charge, oneend must remain at a higherpotential than the other.
Electric Current electric current: flow of charge Electrons carry the charge through a conductor because they are free to move (loosely held).
The netcharge of a wire carrying a current is zero. Electric Current WHY?
Electric Current amperes (A). Current is measured in ___________ 1 A = 1 C/s (1 coulomb of charge per second) 6 billion billion electrons (1 C) per second is a current of 1 A.
Quick Quiz! • Electric charge will flow in an electric circuit when… • electrical resistance is low enough. • a potential difference exists. • the circuit is grounded. • the circuit has resistance.
Quick Quiz. • The electric current in a copper wire is normally composed of… • electrons. • protons. • ions. • amperes.
Quick Quiz. • Which statement is correct? • Voltage flows in a circuit. • Charge flows in a circuit. • A battery is the source of electrons in a circuit. • A generator is the source of electrons in a circuit.
Voltage Sources Chargeflows from a potential difference(voltage), between the ends of a conductor. voltage source: batteries and generatorsprovide a potentialdifference (for a continuous flow of electrons) (a ski-lift provides grav. PE diff) Batteries release electrical energy from a chemical reaction. Generators—like alternators in cars or turbines in power plants—convert mechanical energy to electrical energy.
Voltage Sources • Consider a long pipe filled with water. • Water flows through a pipe if there is a difference in pressure across the pipe between its ends. • Water flows from high pressure to low pressure. • Current(charges) flows througha circuit b/c of a voltage(difference) acrossa circuit. • Voltage does NOTflow through a circuit. • Voltage doesn’t go anywhere, the charges move. • Voltage causes current.
Electric Resistance The amount of charge (current) that flows depends on the voltage source, and on… electric resistance: resistance a conductor offers tochargeflow
Electric Resistance • Resistance of a wire depends on the conductivity, thickness, and length of a wire. • Thick wires have less resistance. • Longer wires have more resistance. Certain metals acquire superconductivity (zero resistance to the flow of charge) at temperatures near absolute zero.
Electric Resistance Resistance Series Rtotal = R1 + R2 + R3 Resistance Parallel 1/Rtotal = 1/R1 + 1/R2 + 1/R3 Rtotal = Less than any single R in parallel
Quick Quiz! • How do batteries and generators cause current to flow through a wire? • by producing a charge • by adding electrons • by creating a potential difference • by increasing electrical forces
Quick Quiz. • Which of the following type of copper wire would you expect to have the least electric resistance? • a thick long wire • a thick short wire • a thin long wire • a thin short wire
Ohm’s Law coulomb (C) 6 x 1018 e– volt (V) 1 J/C ampere (A) 1 C/s ohm (Ω) V = IR ∆PE q q t V = I = Ohm’s Law
Ohm’s Law V R V I V = IR I = R = double Double the voltage to ________ the current. more voltage, more current Double the resistance to ________ the current. more resistance, less current halve
Ohm’s Law The resistance of a typical lamp cord is much less than 1 ohm, while a typical light bulb has a resistance of about 100 ohms. Toasters have a resistance of 15-20 ohms. Resistanceconverts electrical energyintolight and/or heat energy.
Ohm’s Law think! How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it? Answer: V R I = V = IR
Ohm’s Law and Electric Shock Voltage Resistance V = IR Current
Ohm’s Law and Electric Shock Your body’s resistance ranges from about 100 Ω if soaked with salt water to about 500,000 Ω if your skin is very dry. Touch a battery’s electrodes with dry fingers and your resistance would be about 100,000 Ω. You would not feel 12 V, and 24 V would just barely tingle. With moist skin, however, 24 V could be quite uncomfortable. The Body’s Resistance
Ohm’s Law and Electric Shock Many people are killed each year by current from common 120 V electric circuits.
Ohm’s Law and Electric Shock think! If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12-volt battery? Answer: V = IR
Ohm’s Law and Electric Shock think! If a current of 0.050 A flows through a circuit with resistance 10 Ω, what is the voltage? V = IR V = (0.050)(10) V = 0.50 V If a 1.5 V battery produces a current of 0.002 A through a circuit, what is the resistance? V I 1.5 0.002 R = R = R = 750 Ω
Quick Quiz! • When you double the voltage in a simple electric circuit, you double the … • current. • resistance. • ohms. • resistors.
Quick Quiz. • To receive an electric shock there must be… • current in one direction. • moisture in an electrical device being used. • high voltage and low body resistance. • a difference in potential across part or all of your body.
The Source of Electrons in a Circuit You can buy a water hose that is empty of water, but you can’t buy a piece of wire, an “electron pipe,” that is empty of electrons. The source of electrons in a circuit is the conducting circuit material itself.
The Source of Electrons in a Circuit When plug into an outlet, energy flows from the outlet, not electrons. Energy is carried by the electric field which vibrateselectrons already there. Electrical energy is converted to mostly heat and some light. Power utilities do not sell electrons. They sell energy. When you get shocked, your electrons vibrate. Small vibrations tingle; large vibrations can be fatal.
Electric Power A charge moving in a circuit expends energy, which may cause motion, heat, or light. electric power: the rate at which electrical energy is converted into another form. watts (W) electric power = current × voltage P = IV W t P =
Electric Power The power and voltage on a light bulb is read as “60 W 120 V.” What is the current that would flow through the bulb? P = IV I = P V = (60 W) (120 V) = 0.5 A
Electric Power The power is also equal to 2 other equations. These are found by simply substituting Ohm’s Law equation. P = I2R P = V2 R P = IV V = IR
Electric Power A kilowattis 1000 watts, and a kilowatt-hour represents the amount of energy consumed in 1 hour at the rate of 1 kilowatt. If electrical energy costs 10 cents per kilowatt-hour, a 100-watt light bulb burns for 10 hours for 10 cents. A toaster or iron draws more current and more power costing more to operate for the same time.
Electric Power think! How much power is used by a calculator that operates on 8 volts and 0.1 ampere? Answer: P = (0.1 A) × (8 V) = P = IV 0.8 W
Quick Quiz! • When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons… • must be supplied by you, just as water must be supplied for a water pipe. • are already in the wire. • may fall out, which is why wires are insulated. • enter it from the electric outlet.
Quick Quiz. • If you double both the current and the voltage in a circuit, the power… • remains unchanged if resistance remains constant. • halves. • doubles. • quadruples.