10/14/2018. Current. Current. QuickCheck 30.3

Transcription

1 Current If QCurrent is the total amount of charge that has moved past a point in a wire, we define the current I in the wire to be the rate of charge flow: The SI unit for current is the coulomb per second, which is called the ampere. 1 ampere = 1 A = 1 C/s. current is the rate at which charge flows The conventional current I and the electron current i e are related by Slide QuickCheck 30.3 Every minute, 120 C of charge flow through this cross section of the wire. The wire s current is A. 240 A. B. 120 A. C. 60 A. D. 2 A. Slide Current Note that the direction of the current I in a metal is opposite to the direction of the electron current i e. Slide

2 QuickCheck 30.5 A and B are identical lightbulbs connected to a battery as shown. Which is brighter? A. Bulb A. B. Bulb B. C. The bulbs are equally bright. Slide Conservation of Current The figure shows two lightbulbs in the wire connecting two charged capacitor plates. As the capacitor discharges, the current through both bulbs is exactly the same! The rate of electrons leaving a lightbulb (or any other device) is exactly the same as the rate of electrons entering the lightbulb. Slide Conservation of Current Slide

3 The Current Density in a Wire The Current Density in a Wire The current density J in a wire is the current per square meter of cross section: The current density has units of A/m 2. Slide QuickCheck 30.4 The current density in this wire is A A/m 2. B A/m 2. C A/m 2. D A/m 2. Slide QuickCheck 30.7 Both segments of the wire are made of the same metal. Current I 1 flows into segment 1 from the left. How does current I 1 in segment 1 compare to current I 2 in segment 2? A. I 1 > I 2. B. I 1 = I 2. C. I 1 < I 2. D. There s not enough information to compare them. Slide

4 QuickCheck 30.8 Both segments of the wire are made of the same metal. Current I 1 flows into segment 1 from the left. How does current density J 1 in segment 1 compare to current density J 2 in segment 2? A. J 1 J 2. B. J 1 J 2. C. J 1 J 2. D. There s not enough information to compare them. Slide Conductivity and Resistivity The conductivity of a material is Conductivity, like density, characterizes a material as a whole. The current density J is related to the electric field E by: The resistivity tells us how reluctantly the electrons move in response to an electric field: Slide Conductivity and Resistivity Slide

5 QuickCheck 30.9 Both segments of the wire are made of the same metal. Current I 1 flows into segment 1 from the left. How does the electric field E 1 in segment 1 compare to the electric field E 2 in segment 2? A. E 1 > E 2. B. E 1 = E 2 but not zero. C. E 1 < E 2. D. Both are zero because metal is a conductor. Slide #1 - A 3.00-mm-diameter wire carries 12 A current when the electric field is V/m. What is the wire s resistivity? #2 - A 0.50-mm-diameter silver wire carries a 20 ma current. What are the electric field and the electron drift speed in the wire? Kirchhoff s Junction Law For a junction, the law of conservation of current requires that where the symbol means summation. This basic conservation statement is called Kirchhoff s junction law. Slide

6 QuickCheck 30.6 The current in the fourth wire is A. 16 A to the right. B. 4 A to the left. C. 2 A to the right. D. 2 A to the left. Slide Resistance and Ohm s Law The figure shows a section of a conductor in which an electric field E is creating current I by pushing the charge carriers. The field strength is The current density is J I/A E/. So the current is related to V by Slide Resistance and Ohm s Law The current through a conductor is proportional to the potential difference between its ends. We define the resistance R of a long, thin conductor of length L and cross-sectional area A to be: The SI unit of resistance is the ohm. 1 ohm 1 1 V/A. The current through a conductor is determined by the potential difference V along its length: Slide

7 QuickCheck Wire 2 is twice the length and twice the diameter of wire 1. What is the ratio R 2 /R 1 of their resistances? A. 1/4. B. 1/2. C. 2. D. 4. Slide Batteries and Current A battery is a source of potential difference V bat. The battery creates a potential difference between the ends of the wire. The potential difference in the wire creates an electric field in the wire. The electric field pushes a current I through the wire. The current in the wire is: I V wire /R Slide Ohm s Law Ohm s law is limited to those materials whose resistance R remains constant or very nearly so during use. The materials to which Ohm s law applies are called ohmic. The current through an ohmic material is directly proportional to the potential difference; doubling the potential difference doubles the current. Metal and other conductors are ohmic devices. Slide

8 QuickCheck The current through a wire is measured as the potential difference V is varied. What is the wire s resistance? A B C. 50. D Slide Battery-Wire-Resistor-Wire Circuit The figure shows a resistor connected to a battery with currentcarrying wires. Current must be conserved; hence the current I through the resistor is the same as the current in each wire. The next two slides show how the electric potential varies through the circuit. Slide Battery-Wire-Resistor-Wire Circuit Slide

9 Battery-Wire-Resistor-Wire Circuit Slide #3 - The electric field inside a 30-cm-long copper wire is 5.0 mv/m. What is the potential difference between the ends of the wire? #4 - A 10-m-long wire with a diameter of 0.80 mm has a resistance of 1.1 Ω. What material is the wire made of? 9