IB Physics 12

Mr. Jean

November 12th, 2014

The plan:

• Video clip of the day

• Series & Parallel Circuits

Ways to Wire CircuitsThere are 2 basic ways to wire a circuit. Keep in

mind that a resistor could be ANYTHING ( bulb, toaster, ceramic material…etc)

Series – One after anotherParallel – between a set of junctions and parallel to each other

Simple Circuit

When you are drawing a circuit it may be a wise thing to start by drawing the battery first, then follow along the loop (closed) starting with positive and drawing what you see.

Series Parallel

Series Circuit In a series circuit, the

resistors are wired one after another. Since they are all part of the SAME LOOP they each experience the SAME AMOUNT of current. In figure, however, you see that they all exist BETWEEN the terminals of the battery, meaning they SHARE the potential (voltage).

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Resistors in Series

• http://www.youtube.com/watch?v=yKbCSE7GGBQ

Circuits in Series:

• When a circuit is in series it means that each component has the same current.

Resistance in Series:

• Simply sum all resistance together. The total amount of the resistance is equal to the sum of all the resistance in the system.

• Req = R1 + R2 + … + Rn

Example question:• There are 4 resistors in series. They have

the resistance of 3.0Ω , 5.0Ω, 7.0Ω and 9.0Ω. The power supply is a 9.0V battery.

• 1) Draw the circuit with all resistance.

• 2) Draw the simplified circuit.

• 3) What is the total current in the system?

• 4) What is the potential difference across each individual resistor?

Simplifying Resistance in Series:

Vs = IsReq

• Use Ohm’s Law:

Potential difference across each:

• V1 = IsR1

• V2 = IsR2

• V3 = IsR3

• V4 = IsR4

Parallel CircuitIn a parallel circuit, we have

multiple loops. So the current splits up among the loops with the individual loop currents adding to the total current

It is important to understand that parallel circuits will all have some position where the current splits and comes back together. We call these JUNCTIONS.

The current going IN to a junction will always equal the current going OUT of a junction.

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Circuits in Parallel

• A circuit in parallel means that each component has the same potential difference.

• In this example all 4 lamps have the full battery voltage across them; however the current is divided between the 4 lamps.

Parallel Circuit:

Parallel Circuits:• Vs = V1 = V2 = … = Vn

• Skiing example: The ski hill represents the battery and the runs represent the resistors. Each skier has the same gravitational potential difference, likewise each individual load in a parallel circuit must have the same total potential difference.

• This is a Mr. Jean original drawing if you couldn’t tell....

Current in parallel circuits

• Is = summation of current for circuit.

• Is = I1 + I2 + I3 + …. + In

Therefore:

Resistors in Parallel

• For resistors in parallel the inverse of the equivalent resistance is the sum of the inverses of the individual resistances.

• Example: There is a 12 volt battery which is connect to three resistors 3.0Ω, 12.0Ω, and 6.0Ω in parallel.

– 1) Draw the circuit.– 2) Find the equivalent resistance for the

situation.– 3) Find the total current leaving the battery.– 4) Find the current through the 12.0 Ω

resistor.

1) The circuit:

2) Req

3) Current leaving the battery:

4) Find the current through 12Ω

Parallel CircuitNotice that the JUNCTIONS both touch the POSTIVE and NEGATIVE terminals of the battery. That means you have the SAME potential difference down EACH individual branch of the parallel circuit. This means that the individual voltages drops are equal.

This junction touches the POSITIVE terminal

This junction touches the NEGATIVE terminal

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