Lecture 7 - Electric Circuits

Loop Rule = conservation of energy

net potential changes = 0 (in closed loop)

OR
net voltage gains = net voltage losses

Junction Rule = conservation of charge

net currents entering = net currents leaving (at any junction)

Resistors in a series

Req = R1 + R2

Resistors in parallel

I/Req = 1/R1 + 1/R2

globes connected in parallel give…

more light

what is brightness based on?

Amount of power

When calculating the multiple resistors, treat the parallel circuits…

as one entity, calculate the resistance of the parallel part and then add with the series

Current from a battery flows steading in one direction…

direct current (DC)

current from a power plant varies sinusoidally…

alternating current (AC)

• the electrons in a wire first move in one direction and then in the other, thus polarity changes every half a cycle

Average power in AC circuit

Average power = VI/2

max power in AC circuit

Max power = VI

electric hazard

• faulty wiring and improper grounding can be hazardous

• make sure electrical work is done by a professional

• appliance connected to a wall socket via a 3 prong plug

  • 3rd prong connects the metal casing directly to the ground wire, in this case the current will go directly into the ground instead of flowing into the person’s body

How to find the value of one parallel resistor

I = VR

I = 8/10 + 8/15 + 8/R

2 = 8/10 + 8/15 + 8/R

R = 12 ohms

Rank the largest to smallest currents

I1 = I2 + I3

I2/I3 = R3/R2 = 2R/R → I2 = 2I3

I1 = 2I3 + I3 = 3I3

I1 > I2 > I3

In a series circuit current is the…

and voltage is….

same everywhere

shared

In a parallel circuit, voltage is…

• the same for all branches

In a parallel circuit, the current in the two branches must be equal to…

the overall charge