More on circuits: power and short circuits

2 Combinations of resistors Resistors in parallel I = I 1 +I 2 +I 3 = V/ R 1 + V/ R 2 + V/R 3 I = V/R eq 1/R eq = 1/ R 1 + 1/ R 2 + 1/R 3 R3R3 V R2R2 R1R1

3 The equivalent resistance for a group of parallel resistors is 1. less than any resistor in the group. 2. equal to the smallest resistance in the group. 3. equal to the average resistance of the group. 4. equal to the largest resistance in the group. 5. larger than any resistor in the group. Q8Q8

Electrical Power P =  V I Unit: Watt (W) = V A kW, MW, GW In circuits, charges do work on light bulbs, appliances, etc. and electric energy is transformed into other forms of energy (food blender example).

Q1Q1 When you connect a light bulb to a battery and a current flows through the circuit, which is true? A) Electric energy is conserved. B) Charges are absorbed in the thin wire inside the bulb causing the wire to glow. C) All of the electric energy is transformed into light. D) All statements above are true. E) All statements are above false.

Q2Q2 If the four light bulbs in the figure are identical, which circuit emits more light (more power)? 1) Circuit I 2) Circuit II 3) Both emit the same amount of light. + - II + - I

Rank in order, from brightest to dimmest, the identical bulbs A to D. 1. A = B = C = D 2. A > B > C = D 3. A > C > B > D 4. A > C = D > B 5. C = D > B > A Q3Q3

8 Q4Q4 Consider two light bulbs designed for 120 V. Is the filament resistance lower or higher in a 500 W light bulb than in a 100 W light bulb? 1. Higher in the 500 W light bulb. 2. Lower in the 500 W light bulb. 3. Same in both bulbs. 4. Answer depends on the current.

9 Q5Q5 Consider two light bulbs in series. The rated power for each bulb is indicated. Which bulb emits more light in this circuit? 1.The 40 W bulb. 2.The 60 W bulb. 3.Both emit the same amount of light. 4.I am so confused…. + - “120V,60 W”“120V, 40 W”

10 Power Bars Good power bars have a build-in breaker as a safety feature limiting the current to a maximum of ~ 15 A. You are working at your computer, which consumes ~ 200 W of power. You are cold and you also want to connect your space heater that consumes 500 W of electric power. Will your power bar be able to handle both appliances? A)yes; B)No; C) I don’t know.

Short Circuits Sometimes faulty appliances can lead to short circuits (often due to overheating, moisture buildup on circuit boards, etc.) Short circuit: Positive and negative terminal connected by very small resistance leading to large currents. Household wiring and can only handle current of fixed amount ~ 15A. Larger currents can damage the wire.

12 Q7

13 Q8Q8 The circuit below consists of two identical light bulbs burning with equal brightness and a single 12 V battery. When the switch S is closed, bulb a 1. burns more brightly. 2. burns as brightly. 3. burns more dimly. 4. goes out. ab + - S