Notes on Circuits - Problems with Solution - Physics II | PHY 2054, Study notes of Physics

Download Notes on Circuits - Problems with Solution - Physics II | PHY 2054 and more Study notes Physics in PDF only on Docsity! PHY2054: Chapter 18 21 Circuits The light bulbs in the circuits below are identical. Which configuration produces more light? (a) circuit I (b) circuit II (c) both the same Circuit II has ½ current of each branch of circuit I, so each bulb is ¼ as bright. The total light in circuit I is thus 4x that of circuit II. PHY2054: Chapter 18 22 Circuits The three light bulbs in the circuit are identical. The current flowing through bulb B, compared to the current flowing through bulb A, is a) 4 times as much b) twice as much c) the same d) half as much e) 1/4 as much Branch of circuit A has ½ resistance, thus it has 2x current. PHY2054: Chapter 18 25 Problem Solving Using Kirchhoff’s Rules Label the current in each branch of the circuit Choice of direction is arbitrary (signs work out in the end) Apply junction rule at each junction: Apply loop rule to each loop (follow in one direction) Resistors: if loop direction in current direction, voltage drop Batteries: if loop direction in “normal” direction, voltage gain Sum of all voltages = 0 around loop Solve equations simultaneously You need as many equations as you have unknowns See example next slide I I1 I2 I = I1 + I2 +V - IR PHY2054: Chapter 18 26 9 V 22 Ω 15 Ω 6 V Example 1: Applying Kirchhoff’s Rules Determine magnitudes and directions of all currents Take two loops, 1 and 2, as shown Define currents: I1, I2, I3 I1 I2 I3 Use I1 = I2 + I3 3 2 3 6 15 0 22 9 15 0 I I I + − = − + + = 2 3 1 2 3 15/ 22 0.68 6 /15 0.40 1.08 I I I I I = = = = = + = 1 2 1 2 I1 PHY2054: Chapter 18 27 Example 2: Applying Kirchhoff’s Rules Use I3 = I1 + I2 1 2 1 3 10 6 14 4 0 10 6 2 0 I I I I + − + + = + − − = 1 2 1 2 3 1 2 2 3 1 I I I I I = = − = + = − PHY2054: Chapter 18 30 Circuit Problem (2) The light bulbs in the circuit shown below are identical. When the switch is closed, what happens to the intensity of the light bulbs? a) bulb A increases b) bulb A decreases c) bulb B increases d) bulb B decreases e) nothing changes Before: Potential at (a) is 12V, but so is potential at (b) because equal resistance divides 24V in half. When the switch is closed, nothing will change since (a) and (b) are still at same potential. (b) (a) PHY2054: Chapter 18 31 (a) (b) Circuit Problem (3) The bulbs A and B have the same R. What happens when the switch is closed? a) nothing happens b) A gets brighter, B dimmer c) B gets brighter, A dimmer d) both go out Before: Bulb A and bulb B both have 18V across them. After: Bulb A has 12V across it and bulb B has 24V across it (these potentials are forced by the batteries). 24V PHY2054: Chapter 18 32 Wheatstone Bridge An ammeter A is connected between points a and b in the circuit below, in which the four resistors are identical. What is the current through the ammeter? a) I / 2 b) I / 4 c) zero d) need more information The parallel branches have the same resistance, so equal currents flow in each branch. Thus (a) and (b) are at the same potential and there is no current flow across the ammeter.