Electricity Review

Electricity Review

Unit 6

Part I: Charge and Materials

Charge

Charge is a property of matter

positive -- protons

negative -- electrons

neutral -- neutrons

Like charges repel.

Unlike charges attract.

All charged objects are attracted to neutral objects.

Electricity and materials

​Electricity is the motion of electrons

Static electricity is the motion of electrons all at once

Current electricity is the continuous motion of electrons.

Materials

​Conductors -- allow electrons to flow easily

usually metals

Best: silver

Others: copper, gold, aluminum

Materials

​Insulators

Insulators do not allow charge to flow. Any charge added stays on the surface.

Examples: rubber, styrofoam, plastics

Charging an object

​Conduction

To charge by conduction, use friction to physically remove electrons from one object and move to the other object.

Examples: rubbing a balloon on your head, rubbing the rod with rabbit fur.

Charging an object

Polarization

Charged objects are attracted to neutral conductors as the unlike charges attract and the ones on the surface of the neutral object will be compelled toward the charged object; thus making the neutral object polarized with the like charges away from the charged object and the unlike charges near the charged object.

Charging an object

Induction

Only works with conductors.

Charge one conductor with polarization.

Bring another conductor to the first conductor and touch them together.

The "like charges" will move to the second conductor, as the "unlike" charges are still attracted to the charged insulator.

Now separate the two conductors. Each now has a net charge.

Multiple Choice

What causes the shock you receive when you touch a doorknob?

• Electrons being pulled from one surface to another

• Protons being pulled from one surface to another

• Neutrons being pulled from one surface to another

Multiple Choice

What are electrons?

• Negatively charged subatomic particles

• Positively charged subatomic particles

• neutrally charged subatomic particles 

• charged atoms

Multiple Choice

How does rubber differ from most metals?

• Rubber conducts electrons; most metals conduct protons

• Rubber is a good conductor; most metals are good insulators

• Rubber is a good power source; most metals are good conductors

• Rubber is a good insulator; most metals are good conductors

Multiple Choice

True/False: Opposite charges repel one another.

• False

• True

Multiple Choice

the charging of an object by the contact of a charged object to a neutral object (like touching a door knob)

• induction

• conduction

• polarization

Part II:

Static Electricity

energy and voltage

​voltage is the energy per unit charge

Voltage is the amount of energy in each charge carrier. The more carriers you have, the more total energy you have.

One can have the same amount of total energy with a different number of charge carriers by having each carrier hold a different amount of stored energy.

capacitors

capacitors store energy

A capacitor is a device that will hold charge on it until it cannot hold any more. A good example is a cloud in a thunderstorm. The water vapor becomes charged by friction as the wind blows the vapor around. When the cloud can hold no more, the energy is released as a lightning strike. Cloud-to-cloud lightning is also common.

​where the charge lies

In any object being charged, since the charges are all "like" and they will repel, they want to be as far away as possible. Thus, they all stay on the surface of the object. You are safe in your car during a lightning storm!

batteries

​batteries separate charge chemically

Unlike capacitors, batteries use chemical means to separate charges in an electrolyte (like lemon juice) and create different charges on dissimilar metals (like zinc and copper). This difference of charge is called the electric potential. Also Volts.

Multiple Choice

Charge is mostly easily transferred in 

• nonconductors

• conductors

• semiconductors

• insulators

Multiple Choice

What is voltage?

• The amount of electric potential energy per one coulomb of charge

• The amount of 1 coulomb of charge per unit of potential energy

• The number of volts per coulomb

• The amount of electric potential energy per volt

Multiple Choice

If you have a copper electrode (positive) and a zinc electrode (negative), how do the electrons flow between them?

• Electrons leave the copper electrode and transfer them to the zinc electrode

• Electrons leave the zinc electrode and transfer them to the copper electrode

• The electrons don't flow once the terminals become positive or negative

• The electrons flow back and forth but more electrons are attached to the zinc electrode which is why it is negative

Multiple Choice

What is an electrolyte?

• A gas that allows electrons to flow freely

• A substance that allows electricity to flow

• The two ends of a battery (positive and negative)

• What separates the charges inside a battery

Part III: Current and Circuits

Current

​Current is the flow of electrons

Current

​Direction of current flow

Even though it is electrons that carry the energy, we define current to be from positive to negative.

So, the opposite direction of the actual electrons.

Resistance and resistors

Resistance inhibits current flow

Resistance slows the current down.

A resistor is a device that is designed to convert the energy from the charge and turn it into other forms -- light, motion, heat, etc. These are the everyday devices that we use.

Circuits

​Circuits direct current flow

A circuit is a complete loop of wires and devices that allow current to flow and energy to go through devices. Circuits begin and end with a battery.

A short circuit has very little resistance and the current is uncontrolled.

An open circuit does not make a complete loop and current will not flow.

As the voltage increases, the current increases.

As the resistance increases, the current decreases.

V = voltage; energy! Measured in Volts

I = current; measured in Amps

R = resistance; measured in Ohms, Ω

V = IR

Ohm's Law

Multiple Choice

In a closed circuit, how does battery voltage affect current?

• As voltage increases, current increases.

• As voltage increases, current decreases.

• Voltage has no affect on current.

• If voltage exceeds current, a short circuit circuit will develop.

Multiple Choice

Which of the following correctly states relationship between voltage, resistance, and current in a circuit?

• Current is directly related to voltage but inversely related to resistance.

• Current is directly related to both voltage and resistance.

• Current is inversely related to both voltage and resistance.

• Current is directly related to voltage but is not affected by resistance.

Multiple Choice

An electrical circuit is powered by a 12-volt battery and contains a light bulb. The current flowing in the circuit is 20 amps. What is the resistance of the light bulb? HINT: V = IR

• 0.6 Ohms

• 240 Ohms

• 1.67 Ohms

• 12 Ohms

Multiple Choice

An electrical circuit is powered by a battery and contains a light bulb producing 1.5 Ohms of resistance. The circuit is flowing at 4 amps of current. What is the voltage of the battery?

HINT: V = IR

• 6.0 volts

• 1.5 volts

• 9.0 volts

• 12.0 volts

Multiple Choice

This circuit won't work. What essential part is missing from this circuit?

• resistance

• source of electric current

• conductor

• ammeter

Multiple Choice

The flow of electricity through a conductor is called...

• resistance

• voltage

• current

• source of electric current

Multiple Choice

What slows down the flow of electricity in a circuit?

• resistance

• voltage

• current

• source of electric current

Multiple Choice

Which of the following can work as an energy source?

• Battery

• Light bulb

• Wire

• All of them

Multiple Choice

In order for electricity to flow through a circuit, the circuit must be a closed, complete:

• Path, so that the electricity can make a complete circle.

• Donut, because circuits get hungry.

• Wire, made out of rubber.

• Solar panel, because circuits only work with the sun.

A circuit diagram is a sketch that depicts the elements in a circuit. Symbols are used to depict devices.

Circuit Diagrams

Devices are connected in parallel when they are connected head-to-head and tail-to-tail.

The electrons can choose which device to go through.

​Parallel

Devices are connected in series when they are connected head-to-tail.

There is only ONE PATH for the electrons to follow.

​Series

Types of connections

Voltmeters measure the energy "dropped" across a device.

It measures the energy difference between two points.

Voltmeters are connected in parallel across a device.

​

​Voltmeters

Ammeters measure current.

They "count" the electrons going through a device.

Therefore, an ammeter must be connected in SERIES with the device.

​

Ammeters

Series Circuits

A series curcuit is one in which the devices are connected in series.

• As more resistors are added, the total resistance increases and the current decreases.

• The current is the same throughout.

• The battery voltage is distributed across each resistor.

• The voltages will add to the battery voltage.

​

​Series circuits

Solving Series Circuits

Multiple Choice

Solve for the total resistance of the circuit shown

• 50 V

• 1970 ohms

• 705 K ohms

• 20 Amps

Multiple Choice

Solve for the voltage drop at resistor 1

• 38 volts

• 60 Kvolts

• 50 volts

• 25 volts

Multiple Choice

With three 10 resistors connected in series with a 30 volt power supply, the total circuit current will equal ____.

• 300 amps

• 1 amp

• 150 amps

• 1 millamp

Multiple Choice

Total series circuit wattage is equal to source voltage ____.

• multiplied by the voltage value going through any resistor

• multiplied by the current value going through any resistor

• divided by the current value going through any resistor

• divided by the voltage applied to the circuit and the total resistance of the circuit

Multiple Choice

When a resistor in a series circuit is replaced by another resistor with an increased resistance value, the total circuit current will __________.

• stay the same

• decrease

• increase

• double in value

Multiple Choice

__________ remains the same in a series circuit.

• Resistance

• Voltage

• Current

• Power

Multiple Choice

The potential difference across the 8.0-Ω resistor is ---.

• 1.5 V

• 12 V

• 0.4 V

• 3.2 V

Multiple Choice

The voltage drop across R1 is ______________________________ V.

• 20

• 30

• 40

• 50

Parallel Circuits

In a parallel circuit, the elements are connected in parallel. Each "loop" can be thought of as its own circuit.

• As more loops/resistors are added, the resistance decreases and the current increases.

• The voltage across each resistor equals the battery voltage.

• The currents in each loop are different. They will add to the total current.

​

​Parallel Circuits

Solving Parallel Circuits

Multiple Choice

This is an example of a ______ circuit

• Series

• Parallel

• Open

• Dihexihedral

Multiple Choice

The current going through the 45 ohm resistor is 

• 0.05 amps

• 0.2 amps

• 0.1 amps

• 5 amps

Multiple Choice

The total resistance of this circuit is 

• 25.7 ohms

• 315 ohms

• 324 ohms

• 90 ohms

Multiple Choice

The total current going through the battery of this circuit is

• 0.35 amps
• 0.0286 amps
• 0 amps
• 9 amps

Multiple Choice

In a parallel circuit which of the following is the same value throughout the circuit?

• Voltage

• Current

• Resistance

• Charge

Multiple Choice

As the resistance of a circuit increases, the current will...

• increase

• decrease

• stay the same

Alternating Current

​Direction of the current changes

Using AC instead of DC means that there is less overall resistance since the electrons only move back and forth and aren't pushed through the entire circuit.

Transformers

Coils create magnetic fields.

Alternating currents create magnetic fields that change direction.

This will induce a current in any nearby coils.

Transformers use conservation of energy. The energy in each coil will be the same.

More current ==> Less voltage

Less current ==> More voltage

Decreases voltage, increases current.

Used to create safe voltages inside homes.

Used to power devices that only need a small voltage (cell phones, etc).

​Step Down

Increases the voltage, decreases current.

Used to go from the power station to transport electricity across a long distance (BIG LINES)

Used to power devices that need more voltage.

​Step Up

Using Transformers