Electricity & Magnetism

Universal ForcesThroughout all galaxies far, far away………

Remember, FORCES exert a push or pull on objects and can do WORK

They can act over a distance without touching• Gravitational force – a weak force due to

mass• Electromagnetic force – a stronger force

due to charged particles

• ELECTRICITY• MAGNETISM

What is a charge?All matter is made up of atoms

All atoms are made up of :Protons (+)

Neutron ( 0)

Electrons (-)

Electrons extremely SMALL mass

travels extremely FAST

has A LOT of KINETIC energy !

What is a charge?Atoms are normally NEUTRAL because:

# of PROTONS = # of ELECTRONS

Electrons can move easily between objects

When an object LOSES electrons positively charged

# of PROTONS > # of ELECTRONS

When an object GAINS electrons negatively charged

# of PROTONS < # of ELECTRONS

This movement of electrons can transfer energy and do WORK.

Check for Understanding

What would occur to make a positive or negative particle?

POSITIVE NEGATIVE

-

--

-

--

++

+

+++-

Law of ChargesLike charges REPEL or PUSH (+ and +) OR ( - and -)

Unlike charges ATTRACT or PULL (+ and - )

FORCE is exerted by electric charges!

Check for Understanding:Would the following repel or attract

each other?

REPEL

ATTRACT

REPEL

- -

+

+ +

-

and

and

and

Electrons (-) push each other away

Electrons (-) are pulled toward protons (+) or other positively charged objects

Induction Some objects that really have an equal amount of charges, can temporarily shift their electrons so that an area may become more positive or negatively charged.

Static Electricity

Charges that build up is called static electricity.

Pollution control systems to capture smoke particles

Photocopiers, speakers, microphones

Lightning

Muscle cells

However, in general, static electricity is a nuisance!

Van der Graaf and Rockin’ Bill Nye

Static dischargeDischarge is a sudden movement of electrons• Unpredictable• Unreliable• Its just not really useful.

Static discharge occurs when a new path forms between two areas of opposite charge.

The electrons move suddenly to try to restore the balance between charged areas.

Pair Share:How would you explain getting “shocked” by the doorknob?

Electric CurrentStatic discharge is brief and limited in the number of electrons it moves.

To do USEFUL WORK, we need a continuous flow of charge.

This is an electric current.

Electric Current, Cont.

For an Electric Current, we need…

A steady supply of moving electrons

A path for the electrons to follow and return to be reused.

Something to “push” the electrons along the path.

Water AnalogyWater in a high reservoir (PE) causes a downward force through pipes to a lower reservoir.

The higher the reservoir, the higher the PE.

Water flows through pipes in a continuous path. Higher reservoir creates flow with more KE.

Water is pumped back up to the higher reservoir to be re-used.

The electric circuit…

Electrons “flow” along a closed path with no breaks.

Battery chemicals provide the “push” of electrons, acting as a “pump”. (ELECTRICAL FORCE )

The “strength” of the battery provides the PE of the circuit.

Work is done as electrons flow through a circuit .

Circuit Construction

pHet Circuit Construction interactive

Let’s talk numbers!A battery is a device that converts

chemical energy to electrical energy.

In a battery , one terminal (end) is Positive and the other terminal (end) is Negative.

The PE of the electrical flow of electrons from one end to the other is the voltage of the battery.

Higher voltage = Higher “push” of electrons through the circuit (Joules/Coulomb)

Let’s talk numbers!

Coulomb (C) Is a bunch of electrons (6.25X 1018

electrons)

6,250,000,000,000,000,000 electrons

1 Volt is the PE that can move 1 Coulomb of charge using 1 Joule of energy

Ampere (amps) is 1 Coulomb in one second ( C / s)

What if we add…a water wheel ?

A wheel would hinder the flow – like friction.

Water uses some of it’s energy to make the wheel turn (work)

Water flows on through the wheel to continue the flow.

More than one wheel could be put into the flow of water.

ResistorsResistor = any object or substance that slows down the flow of electrons.

Electrons transfer some of their Kinetic energy when going through a resistor, so this is a place of energy conversion:

Light bulb light energy

Heating element thermal energy

Speaker sound energy

Resistance measured in ohms. (Ω)

A short circuit:Resistors also protect a circuit from becoming a SHORT CIRCUIT!

Too much current!!

Wires get HOT!

Plastic covering can melt.

Fire may start.

Drains battery fast – or flips protective circuit breaker in electrical panel.

Stop Here

MagnetismEarly discovery of natural occurring lodestones

they always oriented themselves north when floated on a piece of wood on water and

they concentrated iron shavings

There are Naturally occurring magnets and Man-made magnets.

Current science understands magnets to ALWAYS have a NORTH and a SOUTH pole, no matter how many times you divide a magnet (Di-Pole)

Effects of MagnetismMagnetic fields show the strength and direction of magnetic forces.

Closer lines = more strength

Force lines point from north pole to south pole

Same poles REPEL

Opposite poles ATTRACT

Effects of magnetism is very similar to the effects of static electrical charges.

Magnetism from ElectricityMagnets attract only a small group of materials.

Iron, nickel, cobalt, steel

We can INDUCE magnetism in some materials, especially iron.

A wire carrying a current that is coiled around a metal core strengthens the magnetic field.

The more coils, the STRONGER the magnetic field (lab!)

Electromagnets = magnets created by use of electric current.

Li nk

Electricity from MagnetismDoes this work the other way around?

YES! If a magnet is moved through a coil of wire, a current is produced in the wire.

pHet Faraday interactive

What did you notice about the current flow?

The current only flows when the magnet is moving! When the magnet stops moving, so does the current.

The magnet PULLS the electrons along through the wire.The more quickly the magnet moves, the faster the electrons move, SO….more current. Remember… current is more electrons per second

More work is being done!