Dr. Jie ZouPHY Chapter 21 Electric Current and Direct- Current Circuits

Dr. Jie ZouPHY Outline Electric current Batteries Electromotive force and the direction of current flow Resistance and ohm’s law Energy and power in electric circuits Resistors in series and parallel, and combination circuits

Dr. Jie ZouPHY Electric Current Electric current, I: A flow of electric charge from one place to another. I =  Q/  t SI unit: coulomb per second (C/s) = ampere, or amp (A) When charge flows through a closed path and returns to its starting point, we refer to the closed path as an electric circuit. Example 21-1: The disk drive in a portable CD player is connected to a battery that supplies it with a current of 0.22 A. How many electrons pass through the drive in 4.5 s?

Dr. Jie ZouPHY Batteries Battery: A battery uses chemical reactions to produce a difference in electric potential between its two ends, or two terminals. Water flow as an analogy for electric current When a battery is connected to a circuit, electrons move in a closed path from the negative terminal of the battery to the positive terminal. The flashlight: a simple electrical circuit. A mechanical analog to the flashlight circuit.

Dr. Jie ZouPHY Electromotive Force and the Direction of Current Flow Electromotive force, emf or  : The electric potential difference between the terminals of a battery in an open circuit. SI unit: volts (V) – emf is not a force. For an ideal battery, the potential difference between its terminals = its emf, even in a closed circuit. Direction of the current flow: The direction in which a positive test charge would move. The flow of electrons and the current flow point in opposite directions. The average speed of electrons in a wire is ~ m/s – rather slow, due to repeated collisions with atoms in the wire.

Dr. Jie ZouPHY Resistance and Ohm’s Law Ohm’s law: V = IR V: potential difference or voltage (V); I: Current (A); R: Resistance (  ) R =  (L/A)  : the resistivity of the material of the wire; L: length of the wire; A: cross- sectional area of the wire. Table 21-1: values of  of different materials. Example 21-2: A current of 1.82 A flows through a copper wire 1.75 m long and 1.10 mm in diameter. Find the potential difference between the ends of the wire. (For copper,  = 1.68 x  m) Symbol for a resistor

Dr. Jie ZouPHY Energy and Power in Electric Circuits Electrical power: P = IV SI units: watts (W) In the case of a resistor, the electrical power is dissipated in the form of heat. The power dissipated in a resistor is: P = IV = I (IR) = I 2 R, or equivalently P = IV = (V/R) V = V 2 /R. Conceptual checkpoint 21-2: A battery that produces a potential difference V is connected to a 5-W light bulb. Later, the 5-W light bulb is replaced with a 10-W light bulb. (a) In which case does the battery supply the greatest current? (b) Which light bulb has the greater resistance?

Dr. Jie ZouPHY Resistors in Series and Parallel A series circuit A parallel circuit R eq = R 1 + R 2 + R 3 1/R eq = 1/R 1 + 1/R 2 + 1/R 3

Dr. Jie ZouPHY Combination Circuits Example 21-7: In the circuit shown, the emf of the battery is 12.0 V, and all the resistors have a resistance of . Find the current supplied by the battery to this circuit. Find the current in each resistor.

Dr. Jie ZouPHY Another Example Find the current I in this circuit and the current in each resistor.

Dr. Jie ZouPHY Homework #4 Chapter 21, P , Problems: #7, 19, 30, 52 (Physics, Walker, 4 th edition).