Electric Charge and

Static Electricity

Notes

Electric Charge

•Electric charge is a property of protons and electrons.

•Protons have a positive charge. Electrons have a negative charge.

ProtonElectro

n

Electric Charge•Two charges that are the same push away

from each other. Two charges that are different pull toward each other.

•If a proton and an electron come close together, they attract each other.

•Attraction (pull) and repulsion (push) between electric charges is known as interaction between charges. The interaction between charges is called electricity.

Repulsion Attraction

Electric Charge•This is different from the

interaction between magnetic poles, which is known as magnetism.

•The terms “positive” and “negative” were given to charges by Benjamin Franklin in the 1700s.

Electric Charge

•Balloon + Aluminum Can

•Comb + Paper

•Balloon + Water

Electric Force•Electric force is the attraction or

repulsion between electric charges.

•A magnetic field surrounds a charged object. An electric field is a region around a charged object in which electric force occurs.

Electric Force•Suppose one charged object is placed in

the electric field of a second charged object. The first charged object is either pushed or pulled -- repelled or attracted.

•The strength of an electric field depends on how far away the charged object is. The farther away a charged object is, the weaker the electric field is.

The strength of an electric field is represented by how close the electric field lines are to each

other.

Static Electricity•Most objects usually have no charge.

However, objects can become charged.

•If an object loses electrons, it has more protons than electrons. Therefore, it has a positive charge.

•If an object gains electrons, it has more electrons than protons. Therefore is has a negative charge.

++

+

+

+

+

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-

-

--

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Before Rubbing(uncharged)

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--

--

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--

-+

+

+

+-

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After Rubbing(Negatively Charged)

•Static Electricity is the buildup of charges on an object. Static means “not moving.” In static electricity, the charges do not flow or move.

Static Electricity

Transferring Charge•An object becomes charged when

electrons move from one place to another place.

•Charging by friction is when electrons move from one uncharged object to another object by rubbing. For example, a girl charges by friction when she runs her socks on the carpet.

Transferring Charge•Charging by conduction is when

electrons move from a charged object to another object by direct contact. You can charge yourself by conduction when you touch a charged object.

•Think: carpet > socks = friction

•Then: socks > feet = conduction

Transferring Charge

•Charging by induction is when electrons move to one part of an object due to the electric field of another object. There is no touching in charging by induction.

Transferring Charge

•You can find out if an object is charged by using an instrument called an electroscope.

When the object is uncharged, the leaves hang down, but if a charged object gets close, the leaves repel each other and

spread apart.

Static Discharge•Charges may build up as static

electricity on an object. But the charges do not stay on that object forever.

•The loss of static electricity as charges move from one object to another is called static discharge.

Static Discharge

•A static discharge often produces a spark. For example, there may be a tiny spark when you touch a metal doorknob. Lightning is another example of static discharge.

Section 6:2

Electric Current

Flow of Electric Charges

• Electric current is the flow of electric charges through a material. The charges must flow continuously, or without stopping.

• The rate of electric current through a wire is how much charge passes a place in a certain amount of time (One coulomb per second = 1 amp).

• The unit used for the rate of electric current is the ampere. The name can be shortened to amp or A.

Flow of Electric Charges

•A current needs a path to follow. An electric circuit is an unbroken path through which electric charges flow. An electric circuit is always a complete loop with no breaks in the loop.

•If an electric circuit is complete, charges can flow continuously. If an electric circuit is broken, charges will stop flowing.

Open Circuit

Closed Circuit

Conductors and Insulators•Any material that an electric charge can

go through easily is called a conductor. Metals are good conductors. Silver, copper, aluminum, and iron.

•In a conductor, atoms contain electrons that are bound loosely. These electrons, called conduction electrons, are able to move throughout the conductor.

Conductors and Insulators•Any material that an electric charge

has a hard time going through is called an insulator. Rubber, glass, plastic, and wood.

•The rubber coating on an electric cord is an example of an insulator; allowing electrons to flow though the copper wiring inside, but preventing the electrons from flowing into your hand.

Voltage•Charges need energy to flow through a

wire. The energy that makes charges flow is called electrical potential energy. A battery for example, creates an electrical potential energy in an electric circuit.

Voltage•Voltage is the difference in electrical

potential energy between two places in a circuit. Another name for voltage is potential difference. The unit of measure of voltage is volt, which is abbreviated as V.

Voltage•An electric circuit needs a source of

energy to have voltage. A voltage source creates a potential difference, or voltage, in an electric circuit. A battery is an example of a voltage source. An electric generator is also a voltage source.

Resistance•Resistance is the measure of how

hard it is for charges to flow through a material. The unit for resistance is the ohm the symbol Ωstands for “ohms.”

•How much current there is through a circuit depends on how much resistance there is. The more resistance there is, the less current there will be.

Resistance•How much resistance there is in a wire

depends on these four factors:

- material the wire is made of

- length of the wire

- diameter of the wire

- temperature of the wire

Resistance

•Material

- Conductor - Low Resistance

- Insulator - High Resistance

•Length

- Short - Low Resistance

- Long - High Resistance

Resistance, cont.•Diameter

- Wide - Low Resistance

- Narrow - High Resistance

•Temperature

- Hot - High Resistance

- Cold - Low Resistance

Resistance•If an electric charge can flow through

one or more paths with varying resistances, it will flow through the path with the least resistance.

Bird on an Electric WireThe bird has more

resistance than the wire.

Section 6:4

Electric Circuits

Ohm’s Law•Current, voltage, and resistance are

related to one another. The relationship among resistance, voltage, and current is summed up in Ohm’s Law.

Ohm’s Law• Ohm’s Law states that the resistance of most

conductors does not depend on the voltage across them.

• Changing the voltage in a circuit changes the current but will not change the resistance.

• Ohm concluded that conductors and most other devices have a constant resistance regardless of the applied voltage.

Ohm’s Law•The equation for Ohm’s Law is:

•The units in the equation are:

•Ohms (Ω) = Volts (V) ÷ Amps (A)

CurrentResistance =

Voltageor R=V/I

Ohm’s Law•You can rearrange Ohm’s Law to find voltage with this equation:

•Voltage = Current X Resistance

•or

•V=IR

Ohm’s Law•If a circuit has a resistance of 30.5

ohms and a current of .05 amps, what is it’s voltage?

1.52 volts

Ohm’s Law•The brake light on an automobile is

connected to a 12-volt battery. If the resulting current is 0.40 amps, what is the resistance of the brake light?

30 Ω

Features of a Circuit•Circuits have devices that are run by

electrical energy.

•A circuit has a source of electrical energy.

•Electric circuits are connected by conducting wires.

Features of a Circuit•Radios, appliances, and light bulbs are

examples of electrical devices. All these devices resist the flow of electrical energy. As a result, electrical devices are known as resistors.

Features of a Circuit•Sources of electrical energy in a circuit

include batteries, generators, and electric power plants. When you plug a radio into a wall socket, the source of electrical energy is your local electric plant.

Features of a Circuit•Conducting wires complete the path of

an electric circuit. Wires allow charges to flow from the energy source to the electric device and back to the energy source.

Features of a Circuit•Often, a switch is placed in an electric

circuit. With a switch, you can turn a device on or off by opening or closing the circuit.

Circuit Diagram

Circuit Diagram

Electric Circuits Part 2

Series Circuits•A series circuit has all the parts of the

circuit connected on one path.

•An example of a series circuit is a circuit with a battery and two light bulbs connected by a single wire.

Series Circuits•Suppose a series circuit has two light

bulbs. If one burns out, the second bulb goes out because the circuit is broken when the first bulb burns out.

Series Circuits•If you add resistors to a series circuit,

the resistance of the circuit increases.

•This can be shown as:

•RTotal=R1+R2+R3...

•Remember, Resistance is measured in Ohms and uses the symbol Ω.

Series Circuits• In a series circuit, the current throughout the

circuit is constant.

• This can be shown as:

• ITotal = I1 = I2 = I3...

• Where current is represented by the letter I in the equation, but is measured using the units amperes or amps, and uses the units A.

Series Circuits•Also, in a series circuit, as bulbs are

added in line, all of the bulbs become dimmer.

•Each bulb adds more resistance to the circuit, therefore, for a constant voltage, when resistance increases, current decreases.

Series Circuits•An ammeter is an instrument used to

measure current.

Parallel Circuits•A parallel circuit has more than one

path for current to take. There are separate branches in a parallel circuit. Each resistor may be on its own branch.

Parallel Circuits•Suppose a parallel circuit has two light

bulbs, each on its own branch. If one light bulb burns out, the other light bulb will stay lit. The second bulb stays lit because it is on its own branch.

Parallel Circuits•Because the voltage source (battery)

provides constant voltage, the only factors that can change in a circuit are resistance and current.

Parallel Circuits•In a parallel circuit, when more

branches are added, the overall resistance decreases, meaning that the current will increase.

•Think blowing through one straw versus blowing through 2 straws. Two straws allow twice as much air through, but your lungs are only capable of blowing so much at a time.

Parallel Circuits• The major disadvantage of a parallel circuit is

that as you add more things in parallel, the current draw on the source goes up with each new branch. If the source cannot supply the current that is demanded by the multiple resistors of the circuit, the voltage will (must!) decrease. This could be bad, as some devices, notably motors, do not like to run at low voltage and can actually be damaged if voltage decreases too much.

Parallel Circuits•A voltmeter is a device used to measure voltage, or electrical potential energy difference.

Parallel Circuits•The circuits in your home are parallel.

•Electrical energy enters your home through heavy-duty, low resistance wires, and then branch out in parallel to your wall sockets, then to appliances and lights in each room.

•The voltage in most household circuits is 120 V.