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Electric Current Chapter 34 ELECTRIC CURRENT A charged object has charges with potential energy. A difference in potential energy causes the charges to flow from places of higher potential energy to those of lower potential energy. POTENTIAL DIFFERENCE The difference in potential between two different places is the potential difference. POTENTIAL DIFFERENCE Potential Difference is measured in volts (V). Potential difference is often called voltage. It is measured with a voltmeter. CIRCUIT A circuit is a closed path through which charges can flow. Charges in a circuit will continue to flow as long as there is a potential difference. CURRENT The flow of charges in a circuit per unit time is called current. I = Current [amperes (A)] Q = Charge [Coulombs (C)] t = time [second (s)] CURRENT The SI unit for current is amperes (A) 1 ampere = 1 Coulomb/second One ampere of current is said to flow if 1 coulomb of charge passes a certain point in one second. REMEMBER electron = -1.6 x 10-19 C one coulomb of charge has 6.25 x 1018 electrons Then, one ampere of current is the movement of 1 C/s OR 6.25 x 1018 electrons per second CONVENTIONAL CURRENT Conventional current is defined as the movement of positive charges (from positive to negative). This direction is opposite the direction that negative charges move. VOLTAGE SOURCE A voltage source provides a potential difference. It supplies the energy for charges to flow. Examples: dry & wet cells, electric generators and photovoltaic cells VOLTAGE SOURCE In dry cells and wet cells, energy from a chemical reaction is converted into electrical energy. dry cell wet cell VOLTAGE SOURCE In electrical generators mechanical energy is converted into electrical energy. VOLTAGE SOURCE In photovoltaic cells light energy is converted into electrical energy. HOUSE VOLTAGE SOURCE In the US the potential difference across the two slots in a wall socket is 120V. Electrical energy is provided by the electric generator at the power plant. Mr. Charge, Starbucks & the Stairmaster STARBUCKS A 12 V battery performs 12 J of work on each coulomb of charge that moves through it. Charges are transferred from the low potential side (negative) to a high potential side (positive) of the battery. THE STAIRMASTER Resistance is the tendency of a conductor to oppose the flow of current (charges) through it. This will result in changing electrical energy into thermal energy and light. Example: Light Bulb RESISTANCE The resistance of a conductor at a given temperature is directly proportional to its length, and inversely proportional to its crosssectional area, and dependent on the material from which it is made. WHAT!!! RESISTANCE DEPENDS UPON: 1. The type of material 2. The thickness of the material 3. The length of the material 4. The temperature of the material THICKNESS (CROSS-SECTION) The greater the thickness (crosssectional area) of the wire the smaller the resistance. LENGTH The shorter the wire the smaller the resistance. TYPE OF MATERIAL Copper is an excellent conductor (low resistance to the flow of electrons). Used in household wiring because little electrical energy is converted into thermal energy when current passes through it. TEMPERATURE Every material has a characteristic resistivity that depends on its electronic structure and temperature. For most materials resistance increases with temperature. SUPERCONDUCTOR a material that has zero resistance below its critical temperature. It can conduct electricity without energy loss (I2R). Less than 100 K OHM’S LAW States that the amount of current in the circuit is directly proportional to the voltage impressed across the circuit, and inversely proportional to the resistance of the circuit. OHM’S LAW I = Current (amperes) (A) V= Voltage (volts) (V) R = Resistance (ohms) () OHM’S LAW ELECTRICAL POWER Is the rate at which electrical energy is converted to another form of energy. This rate is different for different appliances. ELECTRICAL POWER SI unit is Watts (W). 1 Kilowatt = 1000 watts Power = Current x Voltage P = I x V ELECTRICAL POWER (Power loss) ELECTRICAL ENERGY ELECTRICAL ENERGY Electrical energy is used by electrical companies to calculate energy consumption in kilowatthour (kWh). One kilowatthour is the amount of work (energy) done by 1 kilowatt for 1 hour. ELECTRICAL ENERGY E = P x t 1 kWhr = 1 kW x 1 hr = 1000 W x 3600 s = 3.6 x 106 J TYPES OF ELECTRIC CURRENT Direct current (DC) – charges flow in one direction, forward. Alternating current (AC) – charges repeatedly change direction between relatively fixed positions, forward and backward. AC vs DC AC DC Use Can travel over Short distance; longer distances can’t travel very far with more power before it loses energy Current changes direction and varies magnitude AC generator Source Inventor Nikola Tesla travels in one direction and constant magnitude Battery Thomas Edison USING AC TO OPERATE DC DEVICE The current in a laptop computer or cell phone is DC. You use an AC-DC converter to operate (or charge) these devices. The converter has a diode that only allows electrons to flow in one direction (changes AC to DC). It also has a capacitor to keep the current continuous. OTHER HOUSEHOLD DEVICES These devices can be used in your home to protect it or you from the harm of a short or overloaded circuit. Fuse Circuit breaker Ground-fault interrupter THE BODY AND ELECTRIC SHOCK Very dry skin 500,000 ohms Skin soaked in salt water 100 ohms THE BODY AND ELECTRIC SHOCK Current Effect (Amperes) 0.001 Can be felt 0.005 Painful 0.010 Involuntary muscle contractions 0.015 Loss of muscle control 0.070 If through the heart, serious disruption; probably fatal if current last more than 1 second THE BODY AND ELECTRIC SHOCK Note: Charges move VERY slowly. Like the particles in a wave, charges transfer their energy to other charges nearby. The energy transferred to your body causes your tissues to overheat and disrupts normal nerve functions. WHAT DO YOU THINK? (Introduction to circuits) WIRE THE LIGHT BULB BATTERY LIGHT BULB Which of the following drawings is correct? A? B? C? D? WHY? WE HAVE LIGHT!!!! Our simple circuit CIRCUIT PARTS Component Wire Resistor or Load Battery or voltage Switch Capacitor Item Schematic Symbol Schematic representation of our simple circuit Light bulb Battery Wires Batteries have positive and negative terminals (nodes) + - or + - + or - The light bulb does not have positive and negative connections (nodes) but its connections must be wired to the different terminals of the battery. We wire the positive terminal to one node on the light bulb. We wire negative terminal to the other node of the light. WHAT DO YOU THINK? (Introduction to conventional current) HOW DOES CURRENT FLOW? A? or B? or C? A? Current flows from the positive and negative nodes of the battery to the light bulb. B? Current flows from the positive node of the battery thru the light bulb into the negative node of the battery. C? Current flows from the positive node of the battery thru the light bulb into the negative node of the battery and some current is lost. WHY? B Current flows from the positive node of the battery thru the light bulb into the negative node of the battery. Schematic of our circuit with resistance and current I Resistance in light bulb I Battery I I NOTE Excess charges on the positive terminal (node) of the battery follow a path that takes them through the light bulb filament to the negative terminal (node) of the battery. The charges move in a steady stream and uniformly, therefore current exists. BATTERY — —— — — + + + + + + + + + — — — — The charges in the battery want to balance. However, there is no way for this to happen at this point. Add conducting wire and resistance The animation shows negative charges moving from the negative side of the battery to the positive side. However, conventional current defines the path the charges take as from positive to negative CONVENTIONAL CURRENT I + I We make the charges do the work of lighting the light bulb. The charges have no choice but to travel through the bulb to balance (from positive side of battery to negative side). In the process they give up their energy to the light bulb. Schematic of our circuit using Mr. Charge I I I I What is the mathematical representation of our circuit? I R I V I V = I x R I With numbers 5 A 8 40 V 40 = 5 x 8 What happens to the value of the current and voltage? 5 A 40 V 8 40 V 5 A 0 V The current does not change. If 5 charges leave the battery every second, 5 charges must return to the battery every second. The charges will give up their energy in the light bulb. SOURCE OF EMF Electromotive Force (EMF) maintains constant current in a closed circuit. Example: battery, generator. E emf of the battery r internal resistance of the battery R resistance of resistor V terminal voltage of battery r E R E Vacross R Vacross r E IR Ir E I (R r ) E I Rr r E R