CHAPTER 17

SECTION 1

All atoms are made up of very small particles called atoms. Atoms are made up of even smaller particles – protons, neutrons, and electrons. The charged particles of atoms are protons and electrons – protons are positively charged and electrons are negatively charged.

Charges Exert Forces

1. Charge is a physical property – an object can have a positive charge, a negative, or no charge.

2. Charged objects exert a force – a push or a pull – on other charged objects.

3. The Law of Electric Charge – states that like charges repel, or push away and opposite charges attract.

Because protons are positive and electrons have opposite charges, they are attracted to each other. Without this attraction, electrons could not be held in atoms.

4. The electric force is the force between charged objects.

5. There are two factors that determine the size of the force are: 1) the amount of the charge on the object – the greater the charge – the greater the electric force and 2) the distance between the charges – the closer together the charges are – the greater the electric force.

6. An electric field - the region around a charged object in which an electric force is exerted on another charged object.

Three ways to charge an object

1. There are three ways to charge an object: friction, conduction, and induction. Atoms become charge when they gain or lose electrons.

2. Friction – charging by friction happens when electrons are “wiped” from one object onto another.

E.g.Using a cloth and rubbing a plastic ruler - the electrons move from the cloth to the ruler. Also, rubbing a balloon on your hair causes electrons to move from your hair to the balloon.

3. Conduction – when electrons move from one object to another by direct contact. Electrons can be transferred from a positively charged object to an uncharged object.

E.g. A negatively charged plastic ruler touches an uncharged metal rod and transfers electrons to the uncharged metal rod. The rod becomes negatively charged by conduction.

4. Induction – happens when charges in an uncharged metal object are rearranged without direct contact with a charged object.

E.g. If you hold a metal object near a positively charged object, the electrons in the metal are attracted to and move toward the positively charged object.

CONSERVATION OF CHARGE - When an object is charged by any method, NO charges are created or destroyed. Because charges are not created or destroyed, charge is said to be conserved.

5. Electroscope–used to detect whether an object is charged.

Moving Charges

1. Electrical conductor – is a metal material in which charges can move easily. Most metal are good conductors because some of their electrons are free to move. E.g. copper, aluminum, and mercury

2. Electrical insulators–materials in which charges to do not move easily. They do not conduct charges very well because electrons cannot flow freely. E.g. plastic, rubber, glass, wood, and air

Static Electricity

1. Static electricity – is the electric charge at rest on an object; generally produced by friction or induction. The term static means “not moving” so the charges do not move away from the object they are in.

2. Electric Discharge – is the release of electricity stored in a source or the loss of static electricity as charges move off an object. E.g. clothes stuck together by static electricity – this happens slowly.

3. Sometimes electric discharge happens quickly. One of the most dramatic examples of electric discharge is lightning.

See figure 6 How lightning Forms on page 479 in the text.

4. A lightning rod is a pointed rod connected to the ground by a wire. They are mounted so that they are the tallest point on a building. When lightning strikes a lightning rod, the electric charges are carried safely to Earth through the rod’s wire which prevents lightning from damaging buildings.

SECTION 2

Electric Current

1.Electric current - is the rate at which charges pass a given point.

2. The higher the current is, the greater the number of charges that pass the point each second.

3. Electric current is expressed in units called amperes – which are often shortened to amps.

4. The symbol for ampere is A. The symbol for current is the letter I.

Making Charges Move

1. When you flip a switch, an electric field is set up in a wire at the speed of light.

2. The electric field causes the free electrons in the wire to move. The energy of each electron is transferred instantly to the next electron.

3. The electric field is created so quickly that all electrons start moving through the wire at the same instant.

AC AND DC

1. There are two kinds of electric current – direct current (DC) and alternating current (AC).

2. In direct current the charges always flow in the same direction. E.g. the electric current from the batteries used in a camera is DC

3. In alternating current, the charges continually shift from flowing in one direction to flowing in the reverse direction. E.g. the electric current from outlets in your home is AC

Voltage

1. Voltage – is the potential difference between two points in a circuit.

2. Voltage is expressed in volts (V). In equations, the symbol for voltage is the letter V.

3. Voltage is a measure of how much work is needed to move a charge between two points.

4. Think of voltage as the amount of energy released as a charge moves between two points in the path of a current. The higher the voltage is, the more energy is released per charge.

5. As long as there is a voltage between two points on a wire, charges will flow in the wire.

Resistance

1. Resistance – is the opposition to the flow of electric charge.

2. Resistance is expressed in ohms (Ω, the Greek letter omega). In equations, the symbol for resistance is the letter R.

3. Resistance can be thought of as “electrical friction.” The higher the resistance of a material – the lower the current in the material is.

4. If voltage doesn’t change, as resistance goes up, current goes down.

5. An objects resistance varies depending on the object’s material, thickness, length, and temperature. As resistance increases, current decreases.

Generating Electrical Energy

1. Energy cannot be created or destroyed. It can only be transformed or changed into other kinds of energy.

2. Many things change different kinds of energy into electrical energy. E.g. Generators convert mechanical energy to electrical.

3. Cells and batteries convert chemical or radiant energy into electrical energy.

4. The two kinds of cells are wet cells and dry cells.

5. Wet cells have liquid electrolytes. A car battery is made of several wet cells that use sulfuric acids as electrolytes.

6. Dry cells work similar to wet cells, but the electrolytes are solid or pastelike and are used in small radios and flashlights.

7. Thermocouples – a device that converts thermal energy into electrical energy.

8. Photocells – a device that converts light energy in electrical energy.

SECTION 3

Electrical Calculations

1. Ohm studied the resistance of materials. He measured the current that resulted from different voltages applied to a piece of metal wire.

Ohm’s Law

1. Ohm found that the ratio of voltage (V) to current (I) is a constant for each material. This ratio is the resistance (R) of the material.

2. The equation for Ohm’s Law is

R= V/I

Or

V=I x R

As the resistance goes up, the current goes down. And as resistance decreases, the current increases.

Electric Power

1. Electric power – the rate at which electrical energy is converted into other forms of energy.

2. The unit for power is the watt (W), and the symbol for power is the letter P.

3. Electric power is expressed in watts when the voltage is in volts and the current is in amperes.

4. Electric power is calculated by using the following equation:

Power = voltage x current

P = V x I

Watt: The unit of Power

1. As electrical energy is supplied to a light bulb, the light bulb glows. As power increases, the bulb burns brighter because more electrical energy is converted into light energy.

2. The higher power rating of 120 W bulb indicates that it burns brighter than a 60 W bulb.

3. Kilowatt is another common unit for power – one kilowatt is equal to 1000W.

4. Kilowatts are used to express high values of power. E.g. the power needed to heat a house

Measuring Electrical Energy

1. The amount of electrical energy used in a home depends of the power of the electrical devices in the house and the length of time that those devices are on.

2. The equation for electrical energy is:

Electrical energy = power x time

Or

E = P x t

The unit of electrical energy is usually kilowatt-hours (kWh).

SECTION 4Electric Circuits

1. An electric circuit is a closed pathway – in other words the parts form a complete path through which electric charges flow.

Three parts of an electric circuit

  • Energy source – The energy source can be a battery, a photocell, a thermocouple, or an electric generator at a power plant.
  • Wires – The wires connect the other parts of a circuit. Wires are made of conducting materials that have low resistance, such as copper.
  • Loads – Loads change electrical energy into other forms of energy which include:
  • Thermal energy
  • Light energy
  • Mechanical energy

2. When loads change electrical energy into other forms, they create some resistance of electric currents.

  • Examples of loads include light bulbs, appliances, TV, & motors

3. Circuits can also contain a switch.

  • A switch is used to open (turn off) and close (turn on) a circuit.
  • Charges flow through a circuit when the switch is closed, or “turned on.” The closed circuit allows the electric charges to flow freely through the circuit.
  • Charges cannot flow through a circuit when the switch is open or “off.” When the switch is open, the gap between the two pieces of conducting material prevents the electric charges from traveling through the circuit.

Two Types of Electric Circuits

1. Series Circuit

  • A circuit in which the parts are connected in a single loop.
  • In a series circuit there is only one path for charges to follow so the charges must flow through each part of the circuit.
  • All loads in a series circuit share the same current.
  • Because the current in each bulb is the same, the lights in the circuit glow with the same brightness (power), if they have the same resistance.
  • When you add more bulbs, the resistance would increase so the current would decrease and the bulbs would be dimmer(less power).
  • To find the total resistance of a series circuit: add the resistance of all the items.
  • (R1+R2+R3+. . . =Rtotal)

2. Parallel Circuit

  • Is a circuit in which loads are connected side by side.
  • Charges have more than one pathon which they can travel.
  • In parallel circuits the parts are joined in branches such that the “potential difference” (voltage) across each part is the same.
  • The loads in a parallel circuit do not have the same current.
  • Branches with lower resistance get more current.
  • “Electricity takes the path of least resistance”
  • Instead of having the same current, each load in a parallel circuit uses the same voltage.
  • Because each bulb uses the full voltage, each bulb glows at full brightness, if they have the same resistance, no matter how many bulbs are connected.
  • The total resistance of a parallel circuit gets smaller as you add more branches.
  • (1/R1 + 1/R2 + 1/R3 + . . . = 1/Rtotal)

Advantages/Disadvantages of Series and Parallel Circuits

Create a table showing all of the uses and advantages and disadvantages for series and parallel circuits.

Series Circuit / Parallel Circuit
Current through each bulb
Voltage across each bulb
Total Voltage of circuit
Total resistance as you add more bulbs/branches
Power of bulbs compared to other type of circuit
Power as you add more bulbs
Common Uses/Examples of
Pros (advantages)
Cons (Disadvantages)