Electricity

What is electricity?

⬜ Charges that could be either positive or negative and that they could be transferred from one object to another.

What is electrical charge

⬜ Protons carry positive charges

⬜ Electrons carry negative charges

⬜ Electrical charge is a property of protons and electrons.

⬜ A negatively charged body contains fewer protons than electrons.

⬜ A positively charged body contains fewer electrons than protons.

How to measure charge

⬜ The unit: the coulomb (C)

⬜ 1 C = 6.25 X1018 electrons or protons

⬜ 1 proton or 1 electron contains 1.602 X 10 -19 C of charge

⬜ This is called the elementary charge

Attraction and Repulsion

⬜ Opposites attract and likes repel!

⬜ Attraction and repulsion between charges is a force

The Law of Conservation of Charge

⬜ Charges are never created or destroyed, just transferred from one place to another.

Objects that are electrically neutral

⬜ These are objects that contain the same amount of positive and negative charges

⬜ Protons = Electrons

Charging an object

⬜ The transfer of electrons from one object to another requires energy.

⬜ Transferring electrons from one atom to another causes an imbalance of the positive and negative charges in an object.

⬜ As a result, the object can become electrically charged.

Static electricity

• Electrical charges at rest.

• Usually with insulated charges.

⬜ A substance that permits the free flow of electrical charges.

⬜ Usually metals and electrolytic solutions

⬜ Electrolytic solutions are solutions in which ions are dissolved.

⬜ Ex: salt dissolved in water

Conductors

Insulators

⬜ Substances that block the free flow of charges

⬜ When an insulator is charged, the charges in the object do not move.

⬜ Mostly nonmetals

⬜ Other substances include: wood, plastic, glass, paper, ceramics, rubber, silk and air.

Semiconductors

⬜ Are only conductors in certain conditions.

⬜ Metalloids and Carbon are examples of semiconductors.

Charging an Object

⬜ Friction:

⬜ 2 neutral bodies are rubbed together and electrons are transferred from the atoms of one body to the atoms of the other body.

⬜ The objects become oppositely charged.

Friction

⬜ Triboelectric series: list of substances that can take on a negative or positive charge through friction.

PlasticSulphur Gold Nickel, copperHard rubber Wood, yellow amber, resin Cotton PaperSilk Lead Wool/ Fur Glass

More likely to capture electrons (become negatively charged)

More likely to give up electrons (become positively charged)

Friction

Charging by Conduction

⬜ Putting an uncharged object in contact with another object that is already charged.

⬜ The result is two objects of the same charge

⬜ The charge will flow through the uncharged object (conductor)

⬜ Both objects will now have a weaker charge than the original single charged object.

Conduction

Charging by Induction

⬜ There is no direct contact between the objects.

⬜ When the objects are brought close to one another the neutral object becomes partially charged.

⬜ Electrons within the object move so that opposite charges are brought close together

⬜ Like charges move as far from each other as possible.

Induction- with an insulator

Induction with a conductor

EST-SE

Electrical Fields

When two charged objects come together

⬜ Any electrically charged body placed near another charged body is subjected to an electrical force.

⬜ This is the force of attraction or repulsion between objects.

Coulomb’s Law

⬜ The magnitude of the force of one particle over the other depends on their charges and the distance separating them.

⬜ The greater the charge, the greater the electric force

⬜ The greater the distance, the weaker the electric force

EST

What is an electrical field?

⬜ The area of space in which the electrical force of a charged body can act on another charged body.

⬜ It is actually invisible

⬜ A “field” explains how a force can act over a certain distance.

How to draw field lines around a single charge

Field lines point towards a negative charge

Field lines point away from a positive charge.

What would the field lines look like between these two metal plates?

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Coulomb’s Law

⬜ Fe

= kq1q

2

r2

EST

Fe = electric force

Q1 = charge of the first particle

Q2 = charge of the second particle

R = the distance between the particles

Let’s work through the example on page 149!

p. 172 # 9

⬜ Take a few minutes to complete the problem.

Dynamic Electricity

STATIC ELECTRICITY DYNAMIC ELECTRICITY

Describes all phenomena related to electrical charges

in motion.

Current

⬜ The number of charges that flow past a given point in an electrical circuit every second.

⬜ The symbol for current is I.

⬜ The unit for current is the ampere or amp (A).

⬜ 1A = 1C/s

Calculating current

⬜ I = q/t

⬜ Example: The data sheet for a car headlight indicates that the light requires a current of 15A. What is the charge needed for one minute?

Calculating current

⬜ I = 15A

⬜ Q =?

⬜ T = 1 min = 60 sec

⬜ I = Q/T

An ammeter

⬜ A device for measuring the current intensity.

⬜ It acts as a checkpoint that counts the number of charges that flow past a given point in a circuit in one second.

• Ammeters must be placed in the path that the charge will take.

Understanding your ammeter readings

⬜ 1000 mA = 1 A

⬜ 50mA =

⬜ 500mA =

A Voltmeter

⬜ An instrument to measure potential difference.

⬜ A checkpoint for measuring the energy each of the charges transfers to a circuit element.

• Voltmeters must be placed at the points where the circuit enters or exits an element (light bulb, resistor, etc.)

Placing an Ammeter and Voltmeter

Drawing Circuit Diagrams

⬜ Dynamic electricity is the phenomenon of electrical charges in motion.

⬜ Circuits: closed loops that allow the free flow of charges.

⬜ There cannot be a break in the circuit for electrons to flow freely.

FLOWING CHARGES⬜ When a battery is attached to a circuit, the freely moving electrons

can flow.

⬜ Electrons flow freely in and out of the battery.

⬜ Since electrons are negative, they flow out from the negative terminal of the battery and towards the positive terminal of the battery.

Conventional Current

⬜ Because scientists did not know this when they discovered electric current, by convention we say that current flows from the positive to the negative terminal. (Conventional current direction)

⬜ This type of flowing current is called direct current.

⬜ Batteries provide the energy for electrons to flow.

Plugging into a socket

⬜ When a socket is attached to a circuit, the electrons move back and forth.

⬜ This type of current is called alternating current.

⬜ The socket = the source of alternating current.

⬜ Symbol:

Drawing more symbols:

Circuit wires are made of conductors such as copper

(a metal).

⬜ More symbols for drawing circuits:

Wire Battery Switch Light bulb

Resistors

⬜ Resistors: transform electrical energy into heat.

⬜ Resistors reduce/hinder the flow of current

⬜ Symbol:

⬜ In comparison, light bulbs transform electrical energy into light and heat.

Build one with a resistor and one without

Protection: Fuses and Breakers

⬜ Protection fuses/breakers: If electrons flow too quickly, the fuse or breaker actually breaks the circuit.

⬜ The filament inside melts or the switch needs to be closed again.

⬜ Symbol:

Potential Difference

⬜ The amount of energy transferred between two points in an electrical circuit.

⬜ The greater the potential difference, the more energy transferred between two points.

⬜ The unit of measurement is the volt (V)

⬜ 1V = 1J/C

*The amount of energy transferred per coulomb of charge.

Calculating Potential Difference

⬜ V = E/Q (energy/coulomb)

⬜ Example: The electrical circuits in our homes usually supply a potential difference of 120V. What amount of energy is provided by a charge of 200 C?

Calculating Potential Difference

⬜ V = 120V

⬜ E =?

⬜ Q = 200C

V= E/Q

A Voltmeter

⬜ An instrument to measure potential difference.

⬜ A checkpoint for measuring the energy each of the charges transfers to a circuit element.

• Voltmeters must be placed at the points where the circuit enters or exits an element (light bulb, resistor, etc.)

Placing an Ammeter and Voltmeter

Resistance

⬜ Resistance is a force that hinders the flow of electric current.

⬜ Resistors transform electrical energy into another form of energy (thermal, mechanical, etc.)

Factors Increasing the Resistance

⬜ Nature of the material in the circuit:

⬜ A poor conductor will increase resistance to flow.

⬜ Length: The longer the element or wire, the greater the resistance to flow.

⬜ Diameter: a smaller diameter resists current more than a larger one.

⬜ Temperature: The hotter the element the more the resistance.

What if you want a good conductor?

⬜ Short, Fat, and Cold!

⬜ You want a good conducting metal with these characteristics.

⬜ This scenario presents the least resistance to flow.

Calculating Resistance

⬜ The symbol for resistance is R.

⬜ The unit for resistance is the ohm (Ω)

⬜ 1Ω= 1V/1A

⬜ To calculate resistance, you need Ohm’s Law.

Ohm’s Law

⬜ V = IR

⬜ The law states that for a given resistance, the potential difference in an electrical circuit is directly proportional the current intensity.

Try p. 173 # 16

Electrical Power

⬜ The amount of energy transformed in a given amount of time.

⬜ The amount of work an electrical device can perform per second.

⬜ The symbol for electrical power is P.

⬜ The unit is the watt (W).

⬜ 1W = 1J/s J= joule

s = second

Power Rating

Calculating Electrical Power

⬜P = E/t

P = electrical power in watts

E = Energy or work (joules)

t = time in seconds

Example: If a light bulb consumes 2400J of energy in 60 seconds. What is the power of this light bulb?

Calculating Power in terms of Potential Difference

⬜P = VI ⬜ P = electrical power in watts

⬜ V= potential difference in volts

⬜ I = current intensity in amps

Example: If a light bulb has a potential difference of 10 V and a current of 4A what is the power of this light bulb?

Let’s look at why there are 2 formulas.

⬜ 1W = 1V X 1A = 1 J/s

⬜ 1V =

⬜ 1A =

Power and Energy

What is the relationship between power and electrical energy?

⬜ Remember... Electrical Power indicates the amount of energy a device can transform in a certain period of time.

⬜ Electrical Energy is the amount of energy a device ACTUALLY uses over an ACTUAL period of time.

Power and Energy

We can calculate Electrical Energy three different ways!

Energy= P( in Watts) x t in seconds

Energy= P (in kW) x t (in hours)

Energy= P(in Watts) x t(in hours)

We can calculate Electrical Energy three different ways!

Energy= P( in Watts) x t in seconds

Energy= P (in kW) x t (in hours)

Energy= P(in Watts) x t(in hours)

Example: A 1000 W microwave oven operates for 6 minutes. What is the amount of energy used? Let’s calculate the Electrical Energy used all three ways!!!

Power and Energy Example

A 1000 W microwave oven operates for 6 minutes. What is the amount of energy used? Let’s calculate the Electrical Energy used all three ways!!!

The KWh

⬜ When calculating the power consumed in our homes, Hydro Quebec refers to the KWh.

⬜ 1 KW = 1000W

⬜ 1KWh = the amount of energy consumed in 1 hour.

1000W X 3600s = 3 600 000 J

And so,

1KWh = 3 600 000J

Cost of Using Electricity

We use ALWAYS kWh to calculate the cost of using something.

Example: If your house used 800 KWh of electricity in one month how much would it cost if Hydro Quebec charges 0.08 $/KWh.

Electrical Circuits

⬜ Network in which electrical charges can flow continuously in a closed loop.

⬜ Necessary components:

⬜ A power supply (battery) – Creates the potential difference

⬜ Resistors or elements (light bulbs) – use up electrical energy

⬜ Wires that carry the charge between the battery and the resistors and back again – current intensity is measured here.

Series circuit

⬜ Elements are connected end to end.

⬜ Characteristics:

⬜ If one component is defective, the entire circuit stops working

⬜ The energy used by the resistors adds up with each new resistor in the circuit.

Rtotal = R1 + R2 + R3

Parallel Circuits

⬜ This type of circuit contains at least 1 branch.

⬜ Characteristics:

⬜ If an element of the circuit is defective, the elements in the other branches can still function.

⬜ The effect of each resistor is shared among the pathways.

⬜ It doesn’t add up

⬜ Current intensity is shared among the different resistors.