 # Electric circuit power, circuit elements and series-parallel connections.

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Electric circuit power, circuit elements and series-parallel connections

Electrical Power P = VQ/t = VI (Why ?) Unit: Watt (W) = VA kW, MW, GW

Electrical power on an ohmic device P = VI V = RI (Ohm’s Law) P = V 2 /R = I 2 R

Q1: On a light bulb, you see “60W, 120V”. A) 60J electric energy is consumed every second by the light bulb at 120V; B) 60J heat (invisible) is produced every second; C) The radiation power (visible light) is 60W; D) 60W is total radiation power (including visible light, infrared heat etc). E) Both A) and D).

Q2 Two light bulbs. One is of “60W, 120V” and the other “100W, 120V”. A) 60W light bulb has a bigger resistance; B) 60W light bulb has a smaller resistance; C) The resistance of 60W light bulb is equal to that of 120W light bulb.

Rank in order, from largest to smallest, the powers P a to P d dissipated in resistors a to d. 1. P b > P a = P c = P d 2. P b = P c > P a > P c 3. P b = P d > P a > P c 4. P b > P c > P a > P d 5. P b > P d > P a > P c Q3Q3

Circuit elements Elements with negligible resistance---Wires. We assume that there is no potential difference across a wire.

Fuses and switch

Kirchhoff's rules At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction. ( Junction law). Along any loop, the sum of the potential difference of each component is equal to zero (Loop law).

Q4

Q5Q5 As more resistors R are added to a series circuit, the current drawn from the battery 1. increases. 2. remains the same. 3. decreases. R3R3 R2R2 R1R1

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

Terminal Voltage Ideal battery:  is the work is done by an electrochemical reaction in order to separate charges. Terminal voltage  V ab <  due to internal resistance R i in real batteries. The internal resistance is in series with the load resistance R, i.e., the resistance of the external electric circuit. ab R RiRi battery

Q6Q6 A light bulb having a resistance R is connected to a battery. If the light bulb is replaced with another light bulb having a larger resistance, the terminal voltage  V ab of the battery 1. increases with increasing R 2. decreases with increasing R 3. remains the same. ab R RiRi

Q7Q7 As more resistors R are added to a parallel circuit, the total current drawn from the battery 1. increases. 2. remains the same. 3. decreases. R3R3 R2R2 R1R1 I I1I1 I2I2 I3I3

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

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