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Unformatted text preview: PHYS2020: General Physics II Course Lecture Notes Section III Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3 Abstract These class notes are designed for use of the instructor and students of the course PHYS2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille. III. Current & Resistance A. Electric Current. 1. Current is defined as the rate at which charge flows through a surface. a) Mathematically: I = Δ Q Δ t , (III1) where I is the current, Δ Q is the amount of charge passing through an area of wire, and Δ t is the time interval in which Δ Q is measured. b) Current is measured in amperes in the SI unit system: 1 A ≡ 1 C/s . (III2) 2. The direction of current is defined to be the direction at which a positive charge would flow through a wire. a18a17 a19a16 a18a17 a19a16 + + a45 a45 a18a17 a19a16 a18a17 a19a16 a18a17 a19a16 + + + a45 a45 a45 A a45 I a) In metals, it is electrons that flow and not positive charges = ⇒ the electrons flow in the opposite direction of the current! b) Moving charge (whether positive or negative) through a conductor is known as a mobile charge carrier . III–1 III–2 PHYS2020: General Physics II 3. Electrons flow in the opposite direction of the vector Efield. a) As an electron (or any charged particle) moves through a conductor, it collides with atoms (and/or molecules) in the conductor = ⇒ causes a zigzag motion through the conductor. b) The amount of charge passing through a wire can be de termined as follows: i) Let A be the crosssectional area of a wire and Δ x be a small slice along the length of the wire. ii) The volume of this small segment of the wire is then V = A Δ x ( note that V here is volume not voltage). iii) Let N be the number of charge carriers contained in this volume and q be the charge per carrier. Then, n = N V = N A Δ x , represents the number of carriers per unit volume. iv) The total charge contained in this volume is thus Δ Q = Nq = ( n A Δ x ) q . (III3) c) Although the electron makes a zigzag path through the wire, on average , it continues to move down the electric field (remember in the opposite sense) at an average speed called the drift speed v d : v d = Δ x Δ t = ⇒ Δ x = v d Δ t . We can then substitute this into Eq. (III3) giving Δ Q = ( n A v d Δ t ) q . (III4) Donald G. Luttermoser, ETSU III–3 d) Dividing both sides by Δ t gives Δ Q Δ t = I = n q v d A (III5) or v d = I n q A ....
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 Fall '08
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 Physics, Electric charge

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