Electric Charge and Force A. Electric Charge

I. Electric Charge and Force

A. Electric Charge

1. Type of Charges

a. positive

b. negative

c. neutral (lack of charge)

2. Source of charge: The Atom

All matter is made up of atoms. Each atom contain a central "nucleus".

The nucleus contains protons and neutrons, and the nucleus is considered to be at rest. Electrons move around the nucleus in the empty space of the atom.

In a nuetral atom, the number of protons equal the number of electrons

a. Atomic Structure

Protons and electrons have equal and opposite electric charge. By convention, electrons are said to have a negative charge. Similarly, protons are said to have a positive charge. Neutrons have no charge.

Atoms are electrically neutral...not because they contain no overall charge...they have equal numbers of protons and electrons...their total charge adds up to zero.

Electric Forces• Like charges repel, unlike charges attract• Two electrons will tend to repel each other because both have a negative electrical

charge. • Two protons will also tend to repel each other because they both have a positive charge. • Electrons and protons will be attracted to each other because of their unlike charges.• Since the electron is much smaller and lighter than a proton, when they are attracted to

each other due to their unlike charges, the electron usually does most of the moving. • This is because the protons have more mass and are harder to get moving. Although

electrons are very small, their negative electrical charges are still quite strong. • Another important fact about the electrical charges of protons and electrons is that the

farther away they are from each other, the less force their electric fields have on each other. Similarly, the closer they are to each other, the more force they will experience from each other due to this invisible force field called an electric field.

Like energy and momentum, charge is neither created nor destroyed, it is conserved.

Opposite charges attract and like charges repel. As a result negatively charged electrons are attracted to the positive nucleus

An electron is said to have a charge of - and a proton a charge of +.

1 An atom in it's normal state has no charge. This is due to the fact that atoms

A have only neutronsB have no protons or electronsC have equal numbers of protons and electrons

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2 What object moves freely around the atom?

A ElectronB NeutronC ProtonD Nucleus

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3 What symbol is used for the charge on a proton?

A +B -C o

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What the atom DOESNT look like:

This is NOT what an atom looks like!!!

If an atom was magnified so that the nucleus was the size of a dime on the 50 yard line of a football field, the first electrons would be at the edge of the stadium…nothing exists in between. The remaining electrons could be miles away (depending on the element/size of the atom)

And the electrons would STILL be too small to see. Atoms are almost all empty space.

Since everything (including us) is made of atoms, that means everything (including us) is mostly empty space.

More on Electrons

The nuclei of atoms are much more massive than electrons. Each proton or neutron is 2000 times more massive than an electron with each nucleus containing at least one proton.

That's one reason that when electric charge moves it's usually the result of electrons moving, not protons.

The other reason is that in solids, the nuclei are locked together so they can't move...regardless of their mass.

b. ions

Some atoms are charged because they have more protons or electrons-----

When that happens----the atom gains a charge and is called an “ion”

Ions are atoms with charges

b. ions

Some atoms are charged because they have more protons or electrons-----

When that happens----the atom gains a charge and is called an “ion”

Ions are atoms with charges

3. Build up of Charge

Solids are a form of matter whose nuclei form a fixed structure.

Nuclei, and their protons, are "locked" into position.Some electrons are bound more tightly to their atoms than other electrons.

It depends on the make up of the material. For example, human hair easily gives up electrons while plastic is good at “stealing” electrons.

Static electricity occurs when material obtains and holds onto a charge (either positive or negative)

4. Movement of Charge

A nucleus, and the protons, are "locked" into position. Protons are never removed or added to atoms (unless we are working in the field of nuclear physics—not this unit)

In solids….

Some materials have electrons that are bound more tightly to their atoms than other electrons. In other materials, electrons are free to move.

In conductors, some electrons are free to move through the solid. In insulators, no electrons are free to move

Current electricity is the movement of charges through a conductor in a particular direction (like a flow of water)

Static electricity is the build up of charges on material without “flowing”

In conductors, electrons move freely inside the solid. Like charges repel, therefore the electrons tend to want to spread as far apart as possible.

a. Conductors

Insulators are materials that have strongly bound electrons which cannot move within the solid. Different insulators have varying levels of insulation capabilities.

However, electrons can still be picked up or removed from the material (just can’t move with in it)

b. Insulators

Electrolytes are liquids that conduct electricity.

Most often, water has charged partilces dissolved in it. These charge can move through the water to conduct.

Water itself is an insulator. (pure water does not conduct, tap water does)

c. electrolytes

II. Electrical Potential Difference

A. Movement of Charges• Like charges repel and opposite charges attract• Repulsion and attraction are forces that cause charges to move• Force causing motion over a distance is known as ___________ which can be

converted into ___________.

• Which way will charges move?….

B. Electrical Potential

Placing a positive charge near a negative charge is a form of potential energy. They want to move toward each other. It takes work to keep them apart.

Like charges desperately want to move away from each other. It takes work to keep them together.

A force needs to separate opposite charges and a force needs to keep like charges together.

Charges that are forces to be apart are together contain potential energy (think of GPE or EPE).

The amount of potential energy per charge is called electrical potential

C. Potential Difference (voltage) Two charges separate from each other will move.

Like charges move away from each other

Opposite charges move towards each others

The difference in the potential energy they have from before and after the movement is the potential difference, which we call voltage

Charges are being forced to stay close to each other

They want to separate; right now they have potential energy

Each charge has high potential energy

When free to move the charges will separate and each have less potential

energy

When free to move the charges will separate and each have less potential

energy

Each charge has less potential energyThe difference is potential energy for each charge before and after the move is called Potential Difference, or Voltage and is measured in volts.

III. CurrentA. Definition

Current is the movement of charges in a single path; it measures the number of charges that flow past a point in a given time (C/s—renamed amperes)

This requires a conducting path (a metal wires)

III. CurrentB. Cause of current

1. Charges that are separated or put near each other have electrical potential energy

2. Charges move through a wire to go from high potential energy to low potential energy: which is voltage

III. CurrentC. Factors Affecting Current

1. Resistancea. Defined: resistance is hindrance to the flow of

electricityb. Factors affecting resistance

• Diameter of a wire (the greater the diameter, the lower the resistivity)

• Length (the longer the wires, the more the resistance)

• Temperature (generally, higher temp is lower conductivity)

• Type of material (varies)

III. CurrentC. Factors Affecting Current

2. Ohm’s Law

Current

The letter "I" is the symbol for current. It is defined as the amount of charge that flows past a location in a conductor per unit time.

Current flows through a conductor

It is measured in amperes (amps or A)

Voltage

The letter “V" is the symbol for voltage. It is defined as the is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit.

Batteries and outlets provide the force that pushes the electrons through the conductor

It is measured in volts (V)

D C AAA AA 9 V

1.5 V

D, C, AA, & AAA have the same voltage, however they differ in the amount of power they deliver. (meaning how fast)

For instance, D batteries can deliver more current and therefore more power.

Batteries

Resistance

The letter “R" is the symbol for resistance. Resistance determines how much current will flow through a component. Resistors are used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow

Appliances are filled with resistors

It is measured in ohms (Ω)

What is Ohm's Law: Ohm's shows the relationship between electric voltage, current and resistance.

What is voltage? An analogy would be a huge water tank filled with thousands of gallons of water high on a hill. The difference between the pressure of water in the tank and the water that comes out of a pipe connected at the bottom leading to a faucet is determined by the size of the pipe and the size of the outlet of the faucet. This difference of pressure between the two can be thought of as potential Voltage.

What is current? An analogy would be the amount of flow determined by the pressure (voltage) of the water thru the pipes leading to a faucet. The term current refers to the quantity, volume or intensity of electrical flow, as opposed to voltage, which refers to the force or "pressure" causing the current flow.

What is resistance? An analogy would be the size of the water pipes and the size of the faucet. The larger the pipe and the faucet (less resistance), the more water that comes out! The smaller the pipe and faucet, (more resistance), the less water that comes out! This can be thought of as resistance to the flow of the water current.

All three of these: voltage, current and resistance directly interact in Ohm's law.Change any two of them and you effect the third.

Current, Resistance & VoltageRaising resistance reduces current.Raising voltage increases current.

We can combine these relationships in what we call "Ohm's Law".

Another way to write this is that:

OR V = IR

V RI

=

IV. CircuitsA. Definition

• In electronics, a circuit is a path between two or more points along which an electrical current can be carried.

IV. CircuitsB. Parts

Drawing realistic pictures of circuits can be very difficult. For this reason, we have common symbols to represent each piece.

Resistor Battery

Wire

*Note: Circuit diagrams do not show where each part is physically located.

Bulb

Switch

IV. CircuitsC. Types of circuits

1. Series• In series circuits, current can only take one path.• The amount of current is the same at all points

in a series circuit.

• Each resistance in a series circuit adds to the total resistance of the circuit.

Rtotal = R1 + R2 + R3...Total

resistance(ohms)

Individual resistances (W)

Resistance in a series circuit

• Light bulbs, resistors, motors, and heaters usually have much greater resistance than wires and batteries.

Voltage in a series circuit• Each separate resistance creates a

voltage drop as the current passes through.

• As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy

• Ohm’s law is used to calculate the voltage drop across each resistor.

• In parallel circuits the current can take more than one path.• Because there are multiple branches, the current is not the

same at all points in a parallel circuit.

IV. CircuitsC. Types of circuits

2. Parallel

• Sometimes these paths are called branches.

• The current through a branch is also called the branch current.

• When analyzing a parallel circuit, remember that the current always has to go somewhere.

• The total current in the circuit is the sum of the currents in all the branches.

• At every branch point the current flowing out must equal the current flowing in.

• This rule is known as Kirchhoff’s current law.

Voltage and current in a parallel circuit

• In a parallel circuit the voltage is the same across each branch because each branch has a low resistance path back to the battery.

• The amount of current in each branch in a parallel circuit is not necessarily the same.

• The resistance in each branch determines the current in that branch.

Advantages of parallel circuits

Parallel circuits have two big advantages over series circuits:1. Each device in the circuit sees the full battery

voltage.2. Each device in the circuit may be turned off

independently without stopping the current flowing to other devices in the circuit.

Calculate current

• Two bulbs with different resistances are connected in parallel to batteries with a total voltage of 3 volts.

• Calculate the total current supplied by the battery.

Adding resistance in parallel circuits

• A circuit contains a 2 ohm resistor and a 4 ohm resistor in parallel.

• Calculate the total resistance of the circuit.

D. Power

P = VI Current (amps)Voltage (volts)

Power (watts)

Calculate power

• A light bulb with a resistance of 1.5Ω is connected to a 1.5-volt battery in the circuit shown at right.

• Calculate the power used by the light bulb.

Paying for electricity• Electric companies charge for the

number of kilowatt-hours used during a set period of time, often a month.

• One kilowatt-hour (kWh) means that a kilowatt of power has been used for one hour.

• Since power multiplied by time is energy, a kilowatt-hour is a unit of energy.

• One kilowatt-hour is 3.6 x 106 joules.

Calculate power

• Your electric company charges 14 cents per kilowatt-hour. Your coffee maker has a power rating of 1,050 watts.

• How much does it cost to use the coffee maker one hour per day for a month?

E. Safety Features

E. Safety Features1. Insulators

• Electrical wires, primarily made of copper, are surrounding by plastic and rubber….why?

2. Grounding Many electrical appliances have metal cases, including

cookers, washing machines and refrigerators – the grounding wire creates a safe route for the current to flow through if the live wire touches the casing

You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing

The grounding terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock

A strong current surges through the earth grounding because it has a very low resistance – this breaks the fuse and disconnects the appliance

3. 4.) Fuses & Circuit Breakers• Fuses and circuit breakers protect electrical circuits and appliances

3. Fuses• The features of a plug are: -

– The case is made from tough plastic or rubber, because these materials are good electrical insulators

– The three pins are made from brass, which is a good conductor of electricity

– There is a fuse bbetween the live terminal and the live pin

• There is a fuse between the live terminal and the live pin

• The fuse breaks the circuit if too much current flows• The cable is secured in the plug by a cable grip – this

should grip the cable itself, and not the individual wires inside it

3. Fuses• The fuse breaks the circuit if a fault in an appliance causes too much current

flow, protecting the wiring and the appliance

• The fuse contains a piece of wire that melts easily – if the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit

• Fuses in plugs are made in standard ratings (3A, 5A, 13A etc…)

• The fuse should be rated at a slightly higher current than the device needs: -– If the device works at 3A, use a 5A fuse– If the device works at 10A, use a 13A fuse etc…

4. Circuit Breakers• The circuit breaker does the same job as the fuse, but works slightly

differently – a spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt

• An electromagnet is arranged so that it can pull the bolt away from the switch

• If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position

Circuit working Circuit broken