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Transcript
10.10 Electrical Resistance & Ohm’s Law Resistance • The degree to which a substance opposes the flow of electric current through it. • All substances resist electron flow to some extent. Resistance • Conductors, such as metals, allow electrons to flow freely through them and have low resistance values. • Insulators resist electron flow greatly and have high resistance values. Resistance • Resistance is measured in ohms (Ω) using an ohmmeter. • An ohmmeter is a device for measuring resistance. Resistance • When a substance resists the flow of electrons, it slows down the current and converts the electrical energy into other forms of energy. Resistance in a Circuit • A resistor is any material that can slow current flow. • In a light bulb, the filament’s high resistance to the electron’s electrical energy causes it to heat up and produce light. Resistors and Potential Energy • Resistors can be used to control electric current or electric potential in a circuit. • In a circuit, electrons have a higher potential energy as they enter a resistor compared to when they leave the resistor because they use up some energy in passing through the resistor. Analogy • Imagine electrons entering a resistor as being at the high end of a ramp, where they have a lot of potential energy. In this analogy, electrons leaving the resistor are at the bottom end of the ramp, where their potential energy has been converted to another form of energy. Resistance in a wire – water analogy • Longer thinner pipes have more resistance to the flow of water than pipes with a larger diameter. The same idea applies to electricity. • The more resistance that you have in a circuit, the more it will decrease current at a given voltage. • Larger, shorter wires provide less resistance for electrons to travel. • Temperature and material can also affect resistance. Ohm`s Law • Ohm’s law established the relationship between voltage (V) and current (I). • Ohm`s law refers to the amount of resistance in a circuit. • The symbol for resistance is called the ohm (Ω) in honour of Georg Ohm’s work in this field. Ohm`s Law • According to Ohm`s law, the amount of voltage (or energy) in a circuit is equal to the current multiplied by the resistance. • Ohm’s law states that, as long as temperature stays the same, V = IR • if the resistance of a conductor remains constant, then the current is directly proportional to the voltage. Ohm`s Law and Temperature • Ohm’s law works for most circuits. However, temperature affects resistance. Generally, resistance is lower when a conductor is cooler. As the temperature increases, resistance increases. • For example, a filament in an incandescent light bulb often has 10 times more resistance when the bulb is warm. Calculating Resistance • Resistance = voltage current R=V I … and the formula can be rearranged Voltage V=IxR Current I=V R V I R Calculating Resistance Quantity Symbol Units of measurement Resistance R Ω (ohm) Current I A (Ampere) Voltage V V (Volt) Sample Problem #1 • A current of 4.0 A flows through a 40.0 Ω resistor in a circuit. What is the voltage? Use GRASP and sig. digits. Sample Problem #2 • A 30 V battery generates a current through a 15 Ω resistor. How much current does the battery generate? Use GRASP. Sample Problem #3 • An electric stove is connected to a 240-V outlet. If the current flowing through the stove is 22 A, what is the resistance of the heating element? Use GRASP and sig figs. Homework • Read pg. 316 – 319 • Answer #1 – 4, 6, 8 on pg. 319