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Download IB 5.2 Reisistance Jan 19 Agenda
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Physics 4 – Jan 19, 2017 P3 Challenge – What is the electrostatic force (magnitude and direction) between a positive charge of 1.3 x 10-3 C and a negative charge of 6.2 x 10-3 C when they are separated by a distance of 15.0 cm? Objectives/Agenda/Assignment Objective: 5.2 Resistance Assignment: Agenda: Homework Review Potential difference p205 #14 Ohm’s Law p225 #15-22 Resistance Power Electric Potential Energy The electric force is a conservative force so there is a potential energy that corresponds to the opposite of the work done by the conservative force. The state for 0 electrical potential energy is if a test charge is infinitely far away from an isolated positive charge. As the charge moves closer to the charge, it is more and more repelled and the electric force is doing negative work. The charge therefore is experiencing a positive potential energy change. Positive charges are at a high potential energy The work done by the electric force is equal to r times the force or W= −𝒌 The potential energy is the U = 𝒌 𝑸𝒒 𝒓 𝑸𝒒 𝒓 Electric potential The potential energy per unit charge is known as the electric 𝑸 potential or V = 𝒌 and is measured in volts. 𝒓 W = QV 1 V = 1 J/C Electric potential is to electrical potential energy as height is to gravitational potential energy. The test charge is akin to the mass. Work is done moving a charge across a potential difference, just like work is done to raise a mass across a height. Electric potential The potential energy per unit charge is known as the electric 𝑸 potential or V = 𝒌 and is measured in volts. 𝒓 W = QV 1 V = 1 J/C Electric potential is to electrical potential energy as height is to gravitational potential energy. The test charge is akin to the mass. Electric potential Positive charges create a relative high potential state. Negative charges create a relative low potential state. Moving from a lower to a high potential is a positive voltage. Just like height, the word potential can refer to both a fixed state and a difference in states. Wherever there is an electric field there will be a potential difference. The electron Volt (eV) Because electrons are so small, the associated energies are very small, much smaller than a J. 1 eV is the energy of one electron charge over 1 volt 1 eV = (1.6 x 10-19 C) (1 J/C) 1 eV = 1.6 x 10-19 J If you get a very small amount of energy, convert to eV. Or you may be given information in eV. Ohm’s Law The current through a conductor is proportional to the potential difference across it. I V So a plot of I vs V is a line with a positive slope. V = IR R is the resistance of the conductor. A plot of V vs I is a line with a slope = R Many conductors obey Ohm’s law but not all do. We will only be dealing with those that obey Ohm’s law. In data booklet as R = V/I Graphs of V vs I are not always linear. Unit: Ohm, Resistance Source of resistance are collisions of moving electrons with the atoms of the conductor. The kinetic energy of the electrons moving (current) gets transferred to the wiggling of the conductor atoms (kinetic energy of atoms = temperature). Resistance creates a heating in the conductor. Factors effecting resistance: R = L/A Nature of material = resistivity of the material (tables exist) Length (proportional) Cross-sectional area (inversely proportional) Sample Problems The resistivity of copper is 1.68 x 10-8 m. What is the length of a copper wire with a diameter of 2.00 mm with a resistance of 2.00 ? What is the voltage across a 5.00 resistor when a 1.50 A current passes through it? What is the current through a 25.0 resistor when a 12 V voltage is placed across it? Electric power Recall the work done when moving a charge through a potential difference: W = QV Power = W/t = QV/t I = Q/t P= IV All three forms are included in the data booklet. P= IV Sub V= IR P = I2R P=IV P= V2/R Sub I = V/R Recall Unit: Watt, W = J/s Exit slip and homework Exit Slip – What is the current through a copper 2.00 mm diameter 15.0 cm in length wire when connected to a 9V battery? (Resistivity of copper is 1.68 x 10-8 m) What’s due? (homework for a homework check next class) P205 #14 and p225 #15-21 What’s next? (What to read to prepare for the next class) Read 5.2 p 212-226
