Dudaie 1
Physics II­ Chapter 21
Chapter 21­ Electric Current & Direct­Current Circuits 1. Electric Current a. The flow of electric charge from one place to another is known as electric current
i. I =∆ Q/∆ t
1. 1 A= 1C/s} ampere or amp
b. Often charge is carried by electrons moving through a metal wire
i. Do example 21­1 c. When the charge flows through a closed path and returns to its starting
point the closed path is referred to as electric circuit
i. In this chapter we consider direct current circuits­ also known
dc circuit­ is a current in which always flows in the same direction ii. **when circuits with currents that periodically reverse their directions are AC circuits (chapter 24)**
d. Batteries and electromotive force
i. A battery performs a similar functions in an electric circuits
1. To put it simply, a battery uses chemical reactions to produce a difference in electric potential between 2 ends (terminals)
a. The terminal that corresponding to a high electric potential is denoted by +
b. The terminal that corresponding to a low electric potential is denoted by –
2. The switch is open
creating an open circuit; no closed path
Dudaie 3
i.
∆ U=( ∆ Q ) V
1. The rate at which energy changes a. P=∆ U /∆ t = ( ∆ Q ) V / ( ∆ t )
ii. Power is measured by multiplying amps and current 1. P=IV
a. Watts, W
iii. Applying ohm’s law to this case we can write the power dissipated in a resistor 2
1. P=IV =I R
iv. We can also modify ohm’s law to solve for current 2
1. P=IV =V / R a. Conceptual check point 21­2
b. Example 21­4 4. Circuits Contains Capacitors a. Voltage is the difference in charge between two points
b. Current is the rate at which charge is flowing
i. Series of capacitors, ae when resistors connected one after another 1. A capacitor is a device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator
c. Capacitor in parallel
i. In a series circuit, the current through each of the components is the same, and the voltage across the circuit is the sum of the voltages across each component
ii. The simplest way to combine capacitors is connecting them in parallel Dudaie 5
a. Voltmeter measures voltage
i. A voltmeter measures
the potential drop
between any two point in
a circuit ii. A real voltmeter always
allow some current to
flow through it 1. An ideal voltmeter would be one in which resistance is infinite so that the current it draws from the circuit is negligible b. Ammeter measures currents i. It is designed to measure the flow of current through a particular portion of a circuit
ii. If an ammeter has a finite resistance the presence of the meter in the presence of the meter in the circuit will alter the current
1. An ideal ammeter would be one with zero resistance