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- 1. IB Physics Power Points<br />Topic 5.0<br />Electric Currents<br />www.pedagogics.ca<br />5.2 Electric Current<br />
- 2. Electric Current<br />Electric current is the rate that charge flows through a conductor:<br />Charge Q is measured in coulombs C<br />Unit of electric current: the ampere, or A<br />1 A = 1 C s-1<br />
- 3. Electric Current<br />Formal definition of the ampere:<br />
- 4. Charge Movement in Conductors<br />Current is the movement of charge. A substance that allows charges to move freely is called a conductor.<br />Solid Conductors<br />What we are most concerned with i.e.. metal wires.<br />Structural model for a metal consists of positive ions surrounded by a “sea” of loosely held valence electrons. It is these electrons that transfer charge.<br />
- 5. Charge Movement in Conductors<br />If an electric field is established in the conductor, the field force accelerates the electrons, but positive ions remain fixed.<br />Free electrons move in field (from low potential to high potential), in the process interacting with positive ions and other electrons. Movement is called “drift”.<br />
- 6. Interactions of drift electrons increase KE of fixed ions. Current produces a heating effect in a conductor.<br />The “drift” velocity in an average metal conductor is approximately 10-4 ms-1. Question: if electrons move so slowly in a wire, why when you turn a switch, a light bulb lights instantly (regardless of the length of wire)?<br />Answer: electrons are already in wire. Closing the switch establishes the electric field (instantly). All electrons in wire feel electrical force instantly.<br />
- 7. Charge Movement in Conductors<br />Liquids<br />Liquids can also act as conductors. A liquid that conducts charge is called an electrolyte.<br />In electrolytic liquids, charge transfer is bi-directional. Positive ions move one way, negative ions move the opposite.<br />A classic example of an electrolytic solution is salt (NaCl) in water. Molten (melted) NaCl is also an electrolyte.<br />
- 8. Charge Movement in Conductors<br />Gases<br />Ionized gases (plasma) are also conductors. Think charged particles that are free to move.<br />
- 9. Current in Electrical Circuits<br />By convention, current is defined as flowing from high potential (positive) to low potential (negative). Electrons actually flow in the opposite direction in a typical circuit.<br />
- 10. Current in Electrical Circuits<br />For charge to move through a circuit, a continuous path must be supplied from the point of high potential to the point of low potential (i.e. the terminals on this battery)<br />Note the schematic drawing that represents the circuit in the photo.<br />
- 11. Resistance and Resistors<br />Experiments indicate that the current in a wire is proportional to the potential difference between its ends:<br />The ratio of voltage to current is called the resistance:<br />
- 12. Resistance and Resistors<br />Resistance can be thought of as opposition to current flow. Resistance is a property of all devices requiring electrical energy ex. toasters, light bulbs etc.<br />Resistance is measured in ohms (Ω). <br />What does 1 Ωequal in fundamental units?<br />
- 13. The EPE decrease per C of charge across the bulb is measured by the voltmeter.<br />The current through the circuit (and the bulb) is measured by the ammeter.<br />resistor<br />V<br />voltmeter<br />ammeter<br />A<br />battery<br />When charge encounters resistance there is a decrease of electrical potential energy. This decrease is measured by a voltmeter as the potential difference across the resistor.<br />
- 14. The graph shows the EPE changes as a coulomb of charge moves through the circuit. <br />What happens to the EPE in the bulb?<br />How does the EPE get restored in the battery?<br />Understanding the energy transfer in resistors is critical to an understanding of electric circuits.<br />
- 15. Demonstration – potential difference in a simple circuit<br />
- 16. Ohm’s Law<br />The electrical resistance of a material depends on its internal structure. <br />Under normal operating voltages, the resistance of many conductors is constant.<br />These conductors are said to obey Ohm’s Law i.e. resistance is independent of the normal operating voltage<br />Materials that do not follow Ohm’s law are called non-ohmic. <br />
- 17. Mathematically<br />Resistivity and Other Factors Affecting Resistance<br />The resistance of a wire is <br /><ul><li> directly proportional to its length
- 18. inversely proportional to its cross-sectional area
- 19. related to the type of material</li></ul>The constant ρ, the resistivity, is characteristic of the material.<br />
- 20. Resistivity and other factors affecting Resistance<br />The resistance of a wire also depends on temperature: <br /><ul><li> resistivity of conductors increases with temperature
- 21. some semi-conductors exhibit the opposite effect</li></ul>Resistivity of common conductors at 20oC. <br />(values would increase at higher temperatures)<br />
- 22. Electric Power and Energy Dissipation<br />Watt does this mean?<br />
- 23. Electric Power and Energy Dissipation<br />Power, as in kinematics, is the energy transformed by a device per unit time:<br />Power as before is measured in WATTS (W)<br />
- 24. Express power in terms of current and resistance, and voltage and resistance. USEFUL equations<br />KEY CONCEPT: when applying these equations, the values used are specific to a particular device.<br />
- 25. Demonstration<br />
- 26. What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. Instead of measuring energy in joules, the electric company measures it in kilowatt-hours (kWh).<br />
- 27. Refrigerator problem<br />

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