Circuit info

Some information about circuits, in general.

voltage (V) = energy(work)/charge

V = W/q

The unit is the volt (joule/coulomb).


current (I) = charge/time

I = q/t

The unit is the ampere (coulomb/second).


Resistance = voltage/current

R = V/I

The unit is the ohm (volt/ampere).


The last relationship is often referred to as Ohm's Law, typically written as:

V = I R

Furthermore, power (used or radiated) in a circuit can be expressed by:

P = I V = I^2 R = V^2 / R

The unit is the joule/second, also called a watt (W).



Series circuit reminders

In a simple series circuit:

The current is the same in each resistor
The voltages ("over" each resistor) add to the total voltage (battery) available
The total resistance of the circuit is equal to the sum of the individual resistances -
Rs = R1 + R2 + R3 + .....


Parallel circuit reminders

In a simple parallel circuit:

The voltage is the same over each resistor
The currents ("through" each resistor) add to the total current (battery) available
The total resistance of the circuit is equal to the inverse of the sum of the inverted individual resistances. That is -

1/Rp = 1/R1 + 1/R2 + 1/R3 + ......


Combination circuits

Solving combination circuits (series and parallel together) is not too difficult, IF you break the circuit down to a simpler one first.

Determine what resistors are in series (and add them appropriately) and what resistors are in parallel (and add them appropriately). You'll have a simpler circuit that should be able to be simplified even further. We will examine this in class with several examples. Here is one to consider:

Imagine having 2 resistors (10 ohms and 20 ohms) in series with each other. These two are in series with a pair of resistor (3 ohms and 6 ohms) in parallel with each other. The combinaton is powered by a 12-V battery. What is the total resistance of this circuit?

The 2 series resistors make 30 ohms. The two parallel resistors make 2 ohms (do the math). The total combination makes 32 ohms.

The next step would be to find the total current. Take the total voltage (12-V) and divide it by the total resistance (32 ohms). This will give you the battery current. And since the battery is directly in series with the 10 and 20 ohm resistors, THEY TOO have that same current.

More to see in class this evening.