![]() ![]() The electric company bills not for power but for energy, using units of kilowatt-hours. In some cases, however, Joule heating is exploited as a source of heat, such as in a toaster or an electric heater. In many cases, Joule heating is wasted energy. The power dissipated in a resistor goes into heating the resistor this is know as Joule heating. ![]() The power supplied to a circuit by a battery is calculated using P = VI.īatteries and power supplies supply power to a circuit, and this power is used up by motors as well as by anything that has resistance. 1 W = 1 J/sĮlectric power is given by the equations: The current can now be found from Ohm's Law: The area is the cross-sectional area of the wire. The resistivity can be found from the table on page 535 in the textbook. The first step, then, is to find the resistance of the wire: The V is the battery voltage, so if R can be determined then the current can be calculated. The current can be found from Ohm's Law, V = IR. If the wire is connected to a 1.5-volt battery, how much current flows through the wire? We'll focus mainly on ohmic materials for now, those obeying Ohm's Law.Ī copper wire has a length of 160 m and a diameter of 1.00 mm. The connection between voltage and resistance can be more complicated in some materials.These materials are called non-ohmic. In many materials, the voltage and resistance are connected by Ohm's Law: Resistance in wires produces a loss of energy (usually in the form of heat), so materials with no resistance produce no energy loss when currents pass through them. This is reflected in the equations:Īt low temperatures some materials, known as superconductors, have no resistance at all. For reasonably small changes in temperature, the change in resistivity, and therefore the change in resistance, is proportional to the temperature change. Resistance also depends on temperature, usually increasing as the temperature increases. Good conductors have low resistivity, while poor conductors (insulators) have resistivities that can be 20 orders of magnitude larger. The resistivity and conductivity are inversely related. ![]() The resistance (R) of a material depends on its length, cross-sectional area, and the resistivity (the Greek letter rho), a number that depends on the material: The same applies for flowing currents: long thin wires provide more resistance than do short thick wires. For water flowing through a pipe, a long narrow pipe provides more resistance to the flow than does a short fat pipe. Using the flow analogy, electrical resistance is similar to friction. Voltage can be thought of as the pressure pushing charges along a conductor, while the electrical resistance of a conductor is a measure of how difficult it is to push the charges along. Your bike is W (kg).Current and resistance Current and resistance The combined weight of you (the cyclist) and The heavier you and your bike are, the more energy you must spend Percentage grade G: rise divided by run, multiplied by 100. This page measures the steepness of a hill in terms of
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