W10D2 DC Circuits Today’s Reading Assignment W10D2 DC Circuits & Kirchhoff’s L

W10D2 DC Circuits Today’s Reading Assignment W10D2 DC Circuits & Kirchhoff’s L www.phwiki.com

W10D2 DC Circuits Today’s Reading Assignment W10D2 DC Circuits & Kirchhoff’s L

Greenlaw, Bruce, Contributing Editor has reference to this Academic Journal, PHwiki organized this Journal W10D2 DC Circuits Today’s Reading Assignment W10D2 DC Circuits & Kirchhoff’s Loop Rules Course Notes: Sections 7.1-7.5 Announcements PS 8 due Week 11 Tuesday at 9 pm in boxes outside 32-082 or 26-152 Next Reading Assignment W10D3 PS07: PhET: Building a Circuit 7.1-7.5, 7.10 Exam 3 Thursday April 18 7:30 pm –9:30 pm Outline DC Circuits Kirchoff’s Laws Electrical Power Measuring Voltage in addition to Current

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DC Circuits Examples of Circuits Symbols as long as Circuit Elements

Electromotive Force (EMF) The work per unit charge around a closed path done is called electromotive as long as ce EMF. This is a bad name because it is not a as long as ce. Let denote the as long as ce per unit charge, then the EMF is If a conducting closed path is present then Ideal Battery Inside Battery: chemical as long as ce , non-zero inside battery (zero outside), moves charges through a region in which static electric field opposes motion 2) Ideal battery: net as long as ce on charges is zero 3) Potential difference between its terminals 4) Extend path through external circuit where Concept Question The electric field inside the battery points to The left The Right

Concept Question Answer The electric field inside the battery points to The left Goes from positive to negative charge Sign Conventions – Battery Moving from the negative to positive terminal of a battery increases your potential Moving from the positive to negative terminal of a battery decreases your potential Concept Question The electric field inside the resistor points to The left The Right

Concept Question Answer The electric field inside the resistor points to The right Electric field pushes charge in the same direction, so current in direction of E This is cause in addition to effect! Sign Conventions – Resistor Moving across a resistor in the direction of current decreases your potential Direction of current Is the same as direction of electric field which points to lower electric potential Moving across a resistor in the direction of current decreases your potential Internal Resistance Real batteries have an internal resistance, r, which is small but non-zero Terminal voltage: (Even if you short the leads you don’t get infinite current)

Series vs. Parallel Series Parallel Resistors In Series The same current I must flow through both resistors Current Conservation Sum of currents entering any junction in a circuit must equal sum of currents leaving that junction.

Resistors In Parallel Voltage drop across the resistors must be the same Concept Q: Resistors In Parallel Suppose that . Then the equivalent resistance 2. 3. 4. Concept Q: Resistors In Parallel Answer 1. When Then the equivalent resistance is the smaller resistance. This is an important circuit design principle.

Group Problem: Four Resistors Four resistors are connected to a battery as shown in the figure. The current in the battery is I, the battery emf is , in addition to the resistor values are R1 = R, R2 = 2R, R3 = 4R, R4 = 3R. Determine the current in each resistor in terms of I. Measuring Voltage & Current Measuring Potential Difference A voltmeter must be hooked in parallel across the element you want to measure the potential difference across Voltmeters have a very large resistance, so that they don’t affect the circuit too much

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Measuring Current An ammeter must be hooked in series with the element you want to measure the current through Ammeters have a very low resistance, so that they don’t affect the circuit too much Concept Question: Measuring Current If R1 > R2, compare the currents measured by the three ammeters: I1 > I2 > I3 I2 > I1 > I3 I3 > I1 > I2 I3 > I2 > I1 I3 > I1 = I2 None of the above Not enough in as long as mation is given. Concept Question Answer: Measuring Current Answer: 4. I3 > I2 > I1 The total current must add to the two individual currents, so I3 must be largest. Most current prefers to go through the smaller resistor so I2 > I1 .

Measuring Resistance An ohmmeter must be hooked in parallel across the element you want to measure the resistance of Here we are measuring R1 Ohmmeters apply a voltage in addition to measure the current that flows. They typically won’t work if the resistor is powered (connected to a battery) Concept Question: Bulbs & Batteries An ideal battery is hooked to a light bulb with wires. A second identical light bulb is connected in parallel to the first light bulb. After the second light bulb is connected, the current from the battery compared to when only one bulb was connected. Is Higher Is Lower Is The Same Don’t know Concept Question Answer: Bulbs & Batteries There are several ways to see this: (A) The equivalent resistance of the two light bulbs in parallel is half that of one of the bulbs, in addition to since the resistance is lower the current is higher, as long as a given voltage. (B) The battery must keep two resistances at the same potential I doubles. Answer: 1. More current flows from the battery

Group Problem: Wheatstone Bridge A circuit consisting of two resistors with R1=6.0 ohms in addition to R2=1.5 ohms, a variable resistor, with resistance Rvar, a resistor of unknown value Ru, in addition to 9.0 volt battery, are connected as shown in the figure. When Rvar is adjusted to 12 ohms, there is zero current through the ammeter. What is the unknown resistance Ru

Greenlaw, Bruce Contributing Editor

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