Electric CircuitsLevel 1 Physics

A Basic Circuit

All circuits have three (3) main parts: Source of NRG Closed Path Device which uses the NRG

*If not a closed path (open circuit), the device will not work

Potential Difference

Within a battery, a chemical reaction occurs. Electrons are transferred from one terminal (leaving it positively charged) to another (leaving it negatively charged). A potential difference (voltage) exists between the two terminals.

Electromotive Force (EMF) – maximum potential difference a power source can have. Not actually a force but an amount of

NRG per charge V = J/C (Joules/Coulomb)

Battery example

Chemical reaction in a battery maintains a difference of 12 volts (12 J/C) of between positive/negative terminals

Symbol for battery

Reason for charge movementBattery creates an electric field within (parallel

to) wireDirected toward negative terminal

It is this field that exerts a force on the free electrons in the wireTherefore, electrons will move

N.S.L. F = ma

Comparison with water flow

A pump works due to 2 principles Pressure difference Certain amount of flow

that passes each second

Circuit works due to 2 principles Potential difference -

Voltage Certain amount of charge

that flows each second - Current

Current

Current is defined as the rate at which charge flows through a surface (imaginary)

Current is the same direction as the flow of positive charge.€

I =Δq

Δt=Coulombs

second= Ampere = Amps = A

Resistance

Resistance (R) – is defined as the restriction of electron flow. It is due to interactions that occur at the atomic scale. For example, as electron move through a conductor they are attracted to the protons on the nucleus of the conductor itself. This attraction doesn’t stop the electrons, just slow them down a bit and cause the system to waste energy.

The unit for resistance is the OHM,

Electric PowerPower is the rate at which work is done or

energy is dissipated

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P =ΔEPE

t=

ΔqV

t= IV

As charge moves through a device, the charge loses electric potential energy.L.O.C.O.E. tells us that the decreases in EPE must be accompanied by a transferof energy to some other form (or forms).

The charge in a circuit can also gain electrical energy. As it moves through the battery, the charge goes from a lower potential to a higher potential. The oppositehappens in the electrical device. L.O.C.O.E says this increase in NRG must comefrom somewhere – chemical energy stored in the battery

Other Power equations

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P = IV

Use Ohm's Law V = IR

P = I2R

P =V 2

R

Ways to Wire a CircuitThere are two (basic) ways to wire a circuit. Keep

in mind that a resistor can be anything; light bulb, toaster, etc.

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

Schematic SymbolsFor the battery symbol, the LONG line is considered to be the POSITIVE terminal and the SHORT line is considered to be the NEGATIVE terminal

The VOLTMETER and AMMETER are special devices you place in or around the circuit to measure VOLTAGE and CURRENT

Series Circuit

In a series circuit, the resistorsare wired one after another.Since they are all part of the SAME LOOP they each experience the SAME amount of CURRENT. Notice in the figure, they all exist BETWEENthe terminals of the battery – meaningthey SHARE the same POTENTIAL(voltage).

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IIII

Totalseries

Totalseries

Series Circuit

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Totalseries

Totalseries

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V(series)Total =V1 +V2 +V3; ΔV = IR

(ITReq )series = I1R1 + I2R2 + I3R3

Req = R1 + R2 + R3

Req = Ri∑

As the current goes through the circuit, the charges must USE ENERGY to get through the resistor. So each individual resistor will get its own individual potential voltage). We call this VOLTAGE DROP.

Note: They may use the terms “effective” or “equivalent” to mean TOTAL!

ExampleA series circuit is shown to the left.

a) What is the total resistance?

b) What is the total current?

c) What is the current across EACH resistor?  

d) What is the voltage drop across each resistor?( Apply Ohm's law to each resistor separately)

Req = 1 + 2 + 3 = 6

V=IR 12=I(6) I = 2A

They EACH get 2 amps!

V1 2 V V3=(2)(3)= 6VV2=(2)(2)= 4V

Notice that the individual VOLTAGE DROPS add up to the TOTAL!!

Parallel Circuit

In 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

OUTIN

Totalparallel

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:Junctions Regarding

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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.

Junctions

Parallel Circuit

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V(parallel )Total =V1 =V2 =V3

I(parallel )Total = I1 + I2 + I3; ΔV = IR

(VTReq

)Parallel =V1

R1

+V2

R2

+V3

R3

1

Req=

1

R1

+1

R2

+1

R3

1

Req=

1

Ri∑

Notice 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

V

Example

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1454.0

1

9

1

7

1

5

11

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P

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9

8

7

8

5

8975 III

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To the left is an example of a parallel circuit. a) What is the total resistance?  

b) What is the total current?  

c) What is the voltage across EACH resistor?  

d) What is the current drop across each resistor? (Apply Ohm's law to each resistor separately)

2.20

3.64 A

8 V each!

1.6 A 1.14 A 0.90 A

Notice that the individual currents ADD to the total.

Compound (Complex) Circuits

Many times you will have series and parallel in the SAME circuit.

Solve this type of circuit from the inside out.

WHAT IS THE TOTAL RESISTANCE?

3.1133.3380

3.33;50

1

100

11

s

PP

R

RR

Compound (Complex) Circuits

3.1133.3380

3.33;50

1

100

11

s

PP

R

RR

T

T

TTT

I

I

RIV

)3.113(120

Suppose the potential difference (voltage) is equal to 120V. What is the total current?

1.06 A

What is the VOLTAGE DROP across the 80 resistor?

80

80

808080

)80)(06.1(

V

V

RIV

84.8 V

Compound (Complex) Circuits

AI

VV

AI

VV

R

T

T

T

06.1

8.84

06.1

120

3.113

80

80

3&2

3&2

3&21)(

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8.84120

V

V

VVV

VVV

seriesT

parallelT

What is the VOLTAGE DROP across the 100 and 50 resistor?

35.2 V Each!

50

2.35100

2.35

50

100

3&21)(

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I

I

III

III

seriesT

parallelT

What is the current across the 100 and 50 resistor?

0.352 A

0.704 A

Add to 1.06A