Electrostatics

What happens when you rub a balloon against your hair? Does this create electric charges?

No! The charges were already there!

Since everything is made up of matter, and all atoms have charges, we can conclude that all matter is charged!

What exactly happens when you rub a balloon against your hair? Both the balloon and your hair have positive and

negative charges. Both items are said to be neutral because the number of positive charges

is equal to the number of negative charges.

Since the balloon and your hair are made up of different material, one item will attract the

positive charges and the other will attract the negative charges.

Now each item is charged!

You have probably experienced a shock as a result of walking across carpet and reaching for

a doorknob. This shock is the result of static electricity!

Walking across the carpet allows your body and the carpet to exchange charges, similar to the balloon and your hair! Your body has actually become charged but the change does not move...yet!

The charge waits for a chance to jump onto a neutral or opposite charge from something else (like a

doorknob!)

The Law of Electric Charges!

The law of electric charges states that:

"like charges repel one another, and unlike charges attract one another." Therefore charged objects will attract ________________ objects and objects with ______________ charges.

On the other hand, charged objects will only repel objects with __________________ charge.

A quick Matter review which will help our understanding of electricity!

1. All matter is made up of particles called atoms. 2. At the centre of each atom is a nucleus, with two kinds of particles: the positively charged proton and the uncharged neutron. Protons do not move from the nucleus when an atom becomes charged. 3. A cloud of negatively charged particles called electrons surrounds the nucleus. When atoms become charged, only the electrons move from atom to atom. 4. Like charges repel each other; unlike charges attract each other. 5. Some elements have a weaker attraction for its electrons than others and the electrons are able to move freely from atom to atom. A good example is copper 6. A single atom is always electrically neutral. 7. If an atom gains an extra electron, the net charge on the atom is negative and it is called a negative ion. If an atom loses an electron, the net charge on the atom is positive and it is called a positive ion.

Why use the term static electricity? Clothing, such as nylon shirts and wool sweaters often become charged with electricity. Charges can

last for minutes, days, weeks, months or even longer!

Because of this we say that the charge remains static, in other words, it stays on that particular object!

This has become known as static electricity, and the study of static electric charge is called

electrostatics.

It is important to remember that when electrons are transferred from one object to another both objects become electrically charged.

There are 3 ways in which an

object becomes electrically charged.

1. By friction 2. By contact 3. By induction

Charging by Friction Examples of charging by Friction 1. Walking across a carpet 3. Clothes in a dryer 2. Pumping gasoline through a hose 4. Air rushing over an airplane or car Charging by friction happens with the positively charged nuclei of one object attracts the electrons from the other object. Some types of matter have a stronger hold on their electrons which means

they may not give them up as easily.

The Electrostatic Series is helpful for finding out what type of electric charge will be produced on each substance when two objects are rubbed together.

Acetate

Glass

Wool

Cat’s Fur, Human Hair

Calcium, lead

Silk

Aluminum, Zinc

Cotton

Paraffin Wax

Ebonite

Plastic

Carbon, Copper, Nickel

Rubber

Sulfur

Platinum, Gold

Charging by Contact

Charging by contact occurs as a result of one object already being charged. For example: After you have crossed a carpeted floor, your body may be negatively charged. The door knob is usually neutral.

Your body then shares the electric charge with the doorknob through “contact”. Actually, your hand does not even have the chance to touch the doorknob before the charge begins to transfer in the form of a spark.

Charging by Induction What happens when a piece of dust comes close to a television screen after it has been on for

a while?

The television screen is charged and therefore acts on the neutral dust particle. The charges on the screen induce the electrons on the dust particle to change position. Whether an object is charged positively or negatively the dust particle is still attracted to it, but its electrons will move to suit the law of electric charges. Another example of this is our demonstration with the fur, rod, and pith ball!

Insulators and Conductors

In order to use electricity to our advantage, we need both insulators and conductors.

An insulator is a substance which slows the movement of electrons between atoms.

A conductor is a substance where electrons can move freely between atoms.

Since insulators do not allow the free movement of electrons, they can have a build up of static charge on the surface. On the other hand, since conductors such as copper allow the free movement

of electrons, a static charge will not build up.

Good Conductors Fair Conductors Insulators Copper

Gold Iron

Carbon Human Body

Earth

Fur Rubber Wood

What are some common uses for insulators and conductors?

Pesky static electricity in the winter...

Why is it that our problems with static electricity seem so much worse in the winter?

This has to do with the cold and dry air that we experience in the winter!

Dry air acts as an insulator and does not easily pick up charges as air molecules constantly bang into us. This means that any static charge that builds up on our clothes tends to stay there.

During warmer seasons the air contains lots of water molecules which will redistribute the static charges on our clothes, meaning less static electricity buildup!

Discharging Electrically Charged Objects All matter wants to reach a neutral state, which means that it is neither positive nor negative. This means that if an object is charged, negatively or positively, it wants to lose the charge to return to a neutral state.

If an object has all the excess electric charges removed, it is said to be discharged or neutralized.

Example: Think about a plane flying through the air at very high speeds. It develops a huge amount of static charge on the outside of the plane. If it didn't have a way to get rid of this charge, the communications systems in the plane would not work properly and could cause it to crash.

Grounding!

The easiest way to discharge an object is to connect it to the Earth! You can do this by using a wire connected to a metal rod buried in the ground.

When a charged object is grounded, it shares its charge with the Earth (which has lots of damp soil =

good conductor) and effectively removes the charge from the object.

From our previous example, all the parts of a gas pump are carefully grounded in order to avoid any charge build up.

Discharge at a Point!

In some situations having a grounding wire is not very practical... For examples airplanes and cars are not connected to the ground with a wire at all times.

Airplanes use "static wicks" to discharge their huge build up of charges. These static wicks are pointed and will push electrons off the tip and into a continuous stream behind the aircraft.

Static Wick Diagram

Electrons Build Up… Electrons Pushed into the Air

The Control of Electricity in Circuits

Recall our discussion about static electricity and what happens when you touch a doorknob? The shock that you feel is caused by the flow of electric charges between you and the doorknob.

This flow of electric charges is known as... ELECTRIC CURRENT! There is a very important difference between an electrical static discharge and the electric current flowing through a light bulb... The current flowing through a light bulb is traveling in a controlled path, known as an electric circuit. Electric Circuits are made up of 4 parts: 1. Power Source All circuits require a power source. These power sources could be many things, but include:

1. Battery 2. Power outlet in your home 3. Car battery 4. Generator 5. Photoelectric Cell

2. Electrical Load

This is actually the reason the electric circuit exists as it is the thing that requires the electrical energy. Examples: Light bulbs, televisions, computers, toasters, DVD players

3. Electric Circuit Control Device This device (as the name states) controls electric circuits. You may recognize them as "switches" such as a light switch or a switch on a computer. Some switches are more complicated than others, such as timers on a microwave or television. 4. Connectors Connectors are what allow the electric current to flow from one point to another. They create a controlled path for the current to flow through.

The most common connectors are wires!

Name the components in this circuit.

1. Power Source 2. Electric Load 3. Switch 4. Connectors

Types of Circuits: Circuits can exist in 2 forms, closed and open. Closed circuits are those circuits where current is flowing. Open circuits are those circuits where a current is not flowing because the switch is not closed.

Electric Potential Why is it safe to touch a 1.5 volt battery at both ends but it would be a VERY bad idea to touch two wires coming out of a 120 volt outlet? In both cases you are completing a circuit, but in the case of the 1.5 volt battery, the amount of energy each electron has is much less than that of the electrons leaving the outlet.

The amount of potential energy is higher in the 120 volt outlet!

Electric Potential

The potential energy of a power source is very important to consider. The greater the potential energy of a power source, the greater the force applied to the electrical load in the circuit. This is why a 1.5 volt battery will not light a 60 watt bulb. The electrons leaving the battery do not have enough energy to power the light. The energy each electron has is called its electric potential. Electric potential is commonly referred to as voltage, and is measured in volts (V). Electrochemical Cells (AKA Batteries!) Cells are another name for batteries and are considered to be either primary or secondary. In a primary cell, chemical reactions will use up some of the materials in the cell as electrons flow through it. When these materials have been used up, the cell is discharged and cannot be used again. Secondary cells can be discharged and recharged over and over again. Secondary cells are often referred to as rechargeable batteries. This means that two chemical processes must be occurring in the cell, one to discharge it, and one to charge it again. A car battery consists of a group of secondary cells. Different electrical appliances operate on different voltages. Remember that voltage refers to the electric potential of an object, or the amount of energy that an electron has. Examples: Wristwatch = 1.5 V Flashlight = 3.0 V Telephone = 48 V (72 V when ringing) Household outlets = 120 V Clothes dryer = 240 V

Electric Current Comparison In your house you have a water pump that moves water to the upper levels of your house. The bigger the pump, the further the water can be pushed.

Think of a battery as a pump that moves electrons to a higher energy. The larger the battery, the higher the electrons can be pushed. In a circuit the same electrons are pushed around the circuit over and over again.

Electric Current Safety

- Small voltages (below 28 V) are considered safe. - Dry skin cannot detect voltages under approximately 50 V. - The 120 V in your household outlets can be lethal.

Measuring Current

We have discussed Voltage and its importance to electricity, but we must also consider another unit of measure. Current is measured in units called "Amps" (symbol = A).

Where voltage measures how hard the electrons are being pushed around the circuit, current measures how many electrons are flowing. The larger the current that is flowing, the bigger the wire that you need to contain it. Some common measures of current: CD Player = 0.2 A Microwave oven = 12 A 60W light bulb = 0.5 A Standard Oven = 20 A Toaster = 8 A

Electric Current...In you! Humans can detect very small electric currents in the body (0.0001 A). Above 0.0016 A and muscle control is lost. Think about it: Why is this a problem...? If 0.05A passes through the chest, the heart muscles stop their regular pumping action and flutter. A defibrillator is used to restart the heart. Currents above 0.200 A can cause severe burns. The current necessary to light a 100W lamp would be sufficient to suffocate 50 adult humans... Before helping a victim of electric shock make sure you cannot receive a shock yourself. The electric current must be turned off, or the victim removed with a non-conducting object (wood, glass).

Electricity in our Lives Think about all the different ways we use electricity in a typical day. It is changed into heat, sound, light, or mechanical energy by different types of electrical loads. As we discussed earlier, electrons are able to move easily through the atoms of a conductor. In a good conductor, electrons lose very little energy when they collide with the atoms from the conductor. In other materials like tungsten filament (found in a light bulb), the electrons lose the majority of

their energy. When the electrons collide with the tungsten atoms, the electric potential energy is

converted to thermal energy and the filaments gets to warm that it glows! And we have light!

Resistance From our discussion about light bulbs slowing down electrons in order to produce light, we come to our third measure of electricity. The resistance of a piece of wire determines how easily electrons will flow through it. Good conductors have very little resistance, allowing electrons to flow easily. The resistance for any device can be found if the voltage and current are known.

Voltage = Current x Resistance

V = IR

Application Problems:

Example: What is the voltage drop across a toaster that has a resistance of 10, and the current flowing through it is 11 A.

Solution: V = IR

= 11 A x 10

= 110 V

Electric Circuits with Multiple Electrical Loads

Some electric devices, such as calculators, simple cameras, and flashlights, operate using only one electric load at a time. However, when strings of lights or the lights in a car are turned on, several

loads are operating simultaneously. There are two kinds of circuits used to connect multiple loads: 1. the series circuit 2. the parallel circuit

Series Circuits A series circuit is when the elements in a circuit are connected so that there is only a single pathway for the current to flow. The three light bulbs are connected in a series circuit. When the switch is closed, all three bulbs produce light.

When a bulb burns out, the current is unable to flow through the circuit, just as if a switch were open. Have you had set of lights that wouldn't light up when you plugged them in? Parallel Circuit A parallel circuit is when the elements in a circuit are connected in such a way that there is more than one pathway for the current to flow.

Electrical Power Electrical power is the measure of how fast electrical energy is being used. Power is measured in Watts (W). Large objects use power measured in kilowatts (kW). Note: 1 kW = 1000 W

Power = Voltage x Current OR P = VI

Example: Find the power rating of a light bulb which draws 0.5 A of electric current when plugged into a 120 V circuit.

I = 0.5 A P = VI V = 120 V P = 120 V x 0.5 A P = ? P = 60 W

Small Scale Electrical Generation To generate electrical energy, we need to "use" another energy. These other types of energy include: Mechanical Energy: By moving an object we can generate electricity. (Ex: the fur and the rod) Thermal Energy: By digging deep into the Earth, we can use the heat from the Earth's core to produce electricity. Solar Power: Photocells are becoming cheaper and more effective all the time. They convert light into electrical energy. (Ex: used in small devices like calculators, and bigger objects such as solar panels, satellites) Chemical Energy: Batteries store energy (Ex: disposables, lead acid rechargeable, nickel cadmium, and lithium ion) Generators: Used in areas without power (Ex: construction sites, fishing shacks) Large Scale Electrical Generation

In New Brunswick there are 3 main sources for electrical energy. 1. Oil and Coal fired generation stations: Fossil fuels

are burned which produces heat that boils water. The steam from the water spins a turbine and a generator which produces electricity. This type of electricity is produced at Colson Cove in Saint John.

2. Nuclear Generation: Nuclear reactions generate

heat, which boils water. The steam spins turbines and generators. This type of power is produced at Point Lepreau.

3. Hydroelectric Generation: Water which has been dammed flows past a turbine which spins it and produces electricity. This is done at Mactaquac, in Fredericton.