Electricity

Current

Current electricity is the continuous flow of electric charge

• Electric current is measured in amps or amperes (A)

Conductors and Insulators

Why is metal wire usually coated with plastic or rubber?

• Because metal wire is a good conductor
• The plastic and rubber are good insulators
• The coating around the wire helps to isolate the current and keep it where it’s needed

Conductors and Insulators

• An electrical conductor is a material through which charge can flow easily
• An electrical insulator is a material through which charge cannot flow easily

Conductors and Insulators

• Good electrical conductors are copper and silver
• Good electrical insulators are glass, plastic, and rubber

Resistance

• As electrons move through a conducting wire, they collide with electrons and ions
• These collisions convert some kinetic energy into thermal energy

Resistance

Because less energy is available to move electrons through the wire, the current is reduced.

more collisions = more resistance = more energy lost

Resistance

Resistance is the opposition to the flow of charges in a material.

• SI unit of resistance is the ohm (Ω)

Think

Is it easier to drink a milkshake with a thick straw or a thin straw?

Milkshake Analogy

When drinking a milkshake, you would rather use a thicker straw.

In the same way, resistance is lowered if you make a wire thicker because more electrons flow through it.

Resistance

A material’s thickness, length, and temperature affect is resistance

• resistance is greater in a longer wire because the charges travel farther
• As temperature increases, a metal’s resistance increases because electrons collide more often

Voltage

Because there is resistance in the wires and the bulb, charges do not flow on their own without a source of energy, �or voltage.

Voltage

In order for a charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.

battery

solar cell

generator

Battery

A battery is a device that converts chemical energy to electrical energy.

Ohm’s Law

German scientist Georg Ohm determined how resistance and current affect voltage, the difference in electrical potential energy between any two points.

Ohm’s Law

Ohm’s Law tells us the relationship between voltage (V), current (I), and resistance (R).

V = I x R

V = voltage measured in volts

I = current measured in amperes (amps)

R = resistance measured in ohms (Ω)

Electric Circuit

• An electric circuit is a complete path through which _______ can flow

charge

• An electrician uses circuit diagrams to keep track of how different parts of a circuit are connected

Electric Circuit

• An electric circuit is a complete path through which _______ can flow

charge

• Switches show places where the circuit can be ________ or _______

• If a switch is open, the circuit is not a complete loop, and the current ______ – this called an open circuit

• When the switch is closed, the circuit is complete and charge _____ flow – this is called a closed circuit

opened

closed

stops

can

Electric Circuit

• In a closed circuit, charge ______ flow
• In an open circuit, charge ________ flow

can

cannot

Closed circuit diagram

Open circuit diagram

switch

switch

load

voltage

lightbulb

battery

Series Circuits

• In a series circuit, charge has only one path which it can flow
• If one light bulb on the strand of lights blows, they all go out

Series Circuits

The bulbs in a circuit are a source of resistance.

• Adding bulbs to a series circuit increases the resistance.

Parallel Circuits

• A parallel circuit is an electric circuit with two or more paths through which charges can flow

• If one light bulb on the strand goes out, the rest will keep shining

Circuit: Review

• In a series circuit, charge has only _____ path which it can flow; if one light bulb goes out, they ALL go out

• A parallel circuit is an electric circuit with ____________ paths through which charges can flow
• If one goes out, the rest KEEP working

one

2 or more

Schematic Circuit Symbols

• Some circuit symbols used in schematic diagrams are shown below

Single cell

Battery

Connecting wire

Resistor

Switch (open)

Switch (closed)

Light bulb