Warm Up: Battery and Bulb

• Can you make a bulb light using only a bulb, a D-cell battery and one piece of wire?

• Draw a picture of all the ways you try to light the bulb.

• Identify which ones worked and which did not.

• Why do you think the methods that worked behaved the way they did?

What you need: At the front of the room, take a bulb, a single wire and a single D-cell battery

Chapter 18

Electric Circuits

What Methods Worked?

Why did this circuit work?What is Electricity?

Electricity is the flow (current) of electric charges from a high potential to a low potential.

Two Things are needed to make current flow.

1)A closed path of conducting material.

2)A difference in electric potential.

What We Already Know:Just like how a mass inside a gravitational field will move from an area of high gravitational potential energy to an area of low gravitational potential

energy, Charges will flow from an area of high electrical potential energy to an area of low electrical potential energy.

-The amount of Potential energy a charge has divided by the amount of charge present is called Voltage

Therefore:

Voltage (V)

V PE

qWhere,

V = Voltage: measured in Volts (V)

&

q = Charge: measured in Coulombs (C)

Other name for Voltage:

Potential Difference

Electromotive Force (EMF)

What we already know continued:

Conductor: Allows Electricity to flow easily (Metals, water with dissolved materials in it)

Insulator: Does NOT allow electricity to flow easily (Glass, rubber, plastic, air, pure water)

Current ( I ): The net flow of electrons through a conductor

Current ( I )Current: The number of electrons that pass a specific point in a circuit in one second

• Current is measured in amperes (A) (amps)

• I = q/t… so the units are 1A = 1C/s

I q

t

Voltage does not go anywhere,Voltage does not go anywhere,

the charges are the only things that move.the charges are the only things that move.

Current ( I )The way we say current flows through a circuit is actually in the opposite direction (called Conventional current) that electrons move… I know… its weird

Conventional Current, just like in the electrostatics chapter, flows from positive to

negative.

Types of Current

Direct Current: Current in a circuit flows in only one direction.

-Positive and negative ends are fixed and current flows from + to -.

-Batteries are DC

Alternating Current: Current in a circuit oscillates back and forth.

-Positive and negative end switch back and forth so current’s direction switches back and forth.

-Wall Socket has AC

-Many motors are AC motors

Voltage and Current

Voltage is what “pushes” the electricity through the circuit. That is why it is called the EMF. It causes the current to flow.

Voltage Source

Current

Voltage and Current

• Think of the Current as water. Water flows from Higher PE to lower PE

• Eventually the water settles and “loses” its energy

• The pump acts like a battery, because it brings the water from Low Energy to High Energy

Pump

Flow

You built this…

It is known as a circuit.

- A circuit is the path in which current can flow between a potential difference

Refresh: What two things are needed to make a circuit work (for current to flow)?

What if I wanted to give you a diagram of how to build a circuit?

Without special symbols we would be forced to draw complicated pictures like this:

So, you built this…

BUT…we would draw it like this…

Symbollogy…How to draw circuit diagrams

Resistance

•The amount of current flowing through a circuit depends on 2 things:1.How big the voltage is (how hard the electrons are being pushed)2.How big the resistance is.

What is resistance?

All of the components we have just talked about affect how much current runs through them

Resistance

• Resistance- degree of opposition (sort of like friction) that charges encounter as they move through a conductor.

• Measured in Ohms (Ω)

•What factors affect the resistance of a wire?

Factors that change Resistance

1) Length:

Longer wires have _________ Resistance.

2) Thickness (gauge):

Thicker wires have _________ Resistance.

3) Temperature:

Hotter wires have _________ Resistance.

4) Material

We will fill these in after the lab.

Ohm’s LawOhm’s Law shows the relationship between the 3 major values we have discussed (voltage, current, and resistance)

V = IR

Voltage = Current x Resistance

Power LawThe Power Law shows the relationship between the voltage, current, and power)

P = IV

Power = Current x Voltage

Power LawWe have discussed power in this class already in an earlier unit. Is this the same power? Can you prove that electrical power is the same as mechanical power?

Building Circuits

Building Circuits

Closed circuit Vs Open Circuit

Closed Circuit Open Circuit

Circuit Rules: Voltage

Voltage Drop: The portion of the total voltage in a circuit used up by a resisting component.

-Resistors in a circuit will reduce the voltage as current flows

Series Circuit: Voltage

VT = 9V

V1 = 4V

V2 = 5V

As current flows through a circuit voltage is lost across each component.

The total voltage is equal to the sum of the voltage drop across each resistor.

VT V1 V2 ...Vn

Series Circuit: Current

IT= 0.2A

I1= 0.2A

I2= 0.2A

The current does not change as it flows through the circuit.

IT I1 I2 ...In

Series Circuit: Resistance

The Equivalent (total) resistance is equal to the sum of the individual resistors.

R1 = 10

R2 = 20

Req R1 R2 ... Rn

Req 10 20 30

Parallel Circuit: Voltage

VT = 120V V1 = 120V V2 = 120V

The voltage through each branch is the same. Each branch needs to use up all of the available voltage.

VT V1 V2 ...Vn

Parallel Circuit: Current

I1 = 1A

IT = 4A

I2 = 3A

As current flows through a circuit, it splits and distributes its parts through each branch. How much in each branch depends on the resistance in that branch.

The total current is equal to the sum of the currents through each branch of the circuit.

IT I1 I2 ... In

Parallel Circuit: Resistance

R1 = 10 R2 = 20

The inverse of the Equivalent resistance is equal to the sum of the inverse of the individual resistors.

1

Req

1

R1

1

R2

...1

Rn

1

Req

1

10

1

20

3

20

Req 20

36.67

Series Circuits Parallel Circuits

IT I1 I2 ...In

VT V1 V2 ...Vn

Req R1 R2 ... Rn

IT I1 I2 ... In

VT V1 V2 ...Vn

1

Req

1

R1

1

R2

...1

Rn

Circuit Rules(To sum it all up)

Examples of Series and Parallel

Series Circuits Parallel Circuits

Fuses in your house

-Houses

-Christmas Lights

What is a Short Circuit?

Generally caused when electricity is allowed to flow in a path that was unintended. Like when a resistance free wire closes a path between hot wires and ground wires.

Can cause circuits or appliances to break, or even heat wires enough to start fire.

The danger from a short circuit comes from a current through a wire that is way too high.

Example: Battery and steel wool

More on FusesA parallel circuit can act like a short.

As branches are added to a parallel circuit, resistance decreases and current increases.

A fuse in your house will break when you have too many things on, and the current through your house becomes too great.

Ohm’s Law and Electric Shock

• Damaging effects of electric shock are from current traveling through the body. The initial cause of electric shock is the voltage, however the current does the damage.

• Your body typically offers 100,000 Ohms of resistance. A current of 0.01 Amperes could cause muscle spasms.

The faster the current passes through you, the more damage it does (pg 537 in “Conceptual Physics”)

• increases or decreases AC voltage• based on induction• insulated current-carrying wire coiled on one

side of iron ring (primary)• second wire coiled on other side of iron ring

(secondary)

Transformers

secondary voltage # turns on secondary------------------------- = ---------------------primary

voltage # turns on primary

Vs/Vp = Ns/Np

Vp Ip = Vs Is

Is/Ip = Vp/Vs = Np/Ns

(ideal transformer power is conserved)

Math of Transformers