ELECTRICITY & MAGNETISM

ATOM REVIEW

Neutron Charge

q = 0 C Mass

m = 1.67 x 10-27 kg

Electron Charge

q = - 1.6 x 10-19 C (-e) Mass

m = 9.11 x 10-31 kg

Parts of an atom:Protons, Neutrons, Electrons

ProtonCharge

+1 +1 what?

+1 elementary charge e = 1.6 x 10-19 C

q = + 1.6 x 10-19 CMass

m = 1.67 x 10-27 kg

CHARGE

Objects are charged because… They have excess electrons. They have an electron deficit.

Therefore, the charge of an object must be a multiple of the elementary charge. 1e = 1 * (1.6 X 10^-19 C) = 1.6 X 10^-19 C 2e = 2 * (1.6 X 10^-19 C) = 3.2 X 10^-19 C 3e = 3 * (1.6 X 10^-19 C) = 4.8 X 10^-19 C And so on…

How many electrons make up a coulomb?

Conservation of Charge

The total charge of a system is conserved. Like energy Like momentum

The particles of a system will exchange electrons in order to achieve equal charge and to become closer to neutral.

PROTONS DO NOT MOVE!!!!!

CONSERVATION OF CHARGE

-4+6

+1

+1-4

+6-3+5-2+4-1+3

0+2

+1

+1

Conservation of Charge

+3 -3

+3 -1

+3 +3

+3 +1

+4 * 10^-6 C +1 * 10^-6 C

+6 * 10^-6 C -7 * 10^-6 C

ELECTROSTATIC FORCE

ELECTROSTATIC FORCE

Like charges repel.

Opposite charges attract.

STATIC ELECTRICITY – PITH BALL

- - - - - - - - - -+

+++

--

-- +

+ ++

----

--+

+

++

--- - --

Electroscope Metal Knob

Metal Rod

Metal Leaves

ELECTROSCOPE - - - - - - - - -

-

ELECTROSCOPE+ + + +

- - - -

- - - -

- - - - - - - - -

-

ELECTROSCOPE

- -

- -

- - - -

- - - - - - - - -

-+ + + +

ELECTROSCOPE

ELECTROSCOPE

What if I want to make the leaves stay separated after I take the rod away?

Two ways:

Electrostatic Conduction Electrostatic Induction

ELECTROSTATICCONDUCTION

- -

- -

- - - -

- - - - - - - - -

-+ + + +- - - - - - - - -

-

ELECTROSTATICCONDUCTION

- - - - - -

- -

- -

- - - -

ELECTROSTATICCONDUCTION

- -

- -

- - - -

ELECTROSTATICCONDUCTION

- -

-

- - -

- -

ELECTROSTATIC CONDUCTION

Charging object touches electroscope.

Resulting charge on the electroscope is the same as the charge of the charging object.

ELECTROSTATICINDUCTION

- -

- -

- - - -

- - - - - - - - -

-+ + + +

ELECTROSTATICINDUCTION - - - - -

- - - -

-+ + + +

ELECTROSTATICINDUCTION - - - - -

- - - -

-+ + + +

+ -

+ -

ELECTROSTATICINDUCTION

+ - + -

ELECTROSTATICINDUCTION

+ - + -

+ +

ELECTROSTATIC INDUCTION

Charging object DOES NOT touch electroscope.

Electroscope is grounded.

Resulting charge on the electroscope is the opposite of the charge of the charging object.

ELECTROSTATIC FORCE

HOW MUCH FORCE?

More force if charges are big or small?BIGFE q (direct relationship)

More force if charges are close or far apart?CloseFE r (inverse relationship)

FE = k q1 q2

Coulomb’s Lawr2

PRACTICE PROBLEMS

What is the electrostatic force between two +4 μC charges that are 1 cm apart? FE = 1438.4 N Attractive or Repulsive?

What is the electrostatic force between a charge of +3 μC and -2 μC that are 1 mm apart? FE = -53940 N Attractive or Repulsive

PRACTICE PROBLEMS The proton and electron in a hydrogen atom

are 0.1 nm apart.

What is the electrostatic force between the proton and electron? FE = 2.3 x 10-8 N

What is the gravitational force between the proton and electron? Fg = 1 x 10-47 N

Which force is stronger? Electrostatic!

“WHAT-IF” PRACTICE What happens to the electrostatic force if the

charge of both particles is doubled? 4Fe

What happens to the electrostatic force if the distance between the particles is halved? 4Fe

What happens to the electrostatic force if the mass of one particle doubles and the charge of the other particle is halved? ½ Fe

What happens to the electrostatic force if the charge of both particles is doubled and the distance between them is doubled? Fe

ELECTRIC FIELDS

Electric field: the region around a charged particle through which a force is exerted on another charged particle.

We represent electric fields by drawing electric field lines.

Electric field line: the imaginary line along which a positive test charge would move in an electric field. Electric field lines DO NOT cross.

ELECTRIC FIELDS

ELECTRIC FIELDS

ELECTRIC FIELDS

ELECTRIC FIELDS

ELECTRIC FIELDS

Electric Fields

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Electric Fields

Field lines go away from positive.

Field lines go toward negative.

Field lines do NOT cross.

ELECTRIC FIELD STRENGTH

Not all electric fields are the same strength.

The more force a charged particle feels, the stronger the electric field.

+1 C

+1 C

+1 C

- 1 mC +1

C

- 5 mC

ELECTRIC FIELD STRENGTH

E = FE / q

E – electric field strength Units – N / C

PRACTICE PROBLEMS

What is the strength of an electric field in which a 5 C charge feels a force of 100 N? E = 20 N/C

What is the strength of an electric field in which an electron feels a force of 1 N? E = 6.3 x 1018 N/C

What force will a proton feel if it is placed in an electric field with a strength of 500 N/C? F = 8 x 10-17 N

Potential Difference We have a positive charge

at point A. In which direction does it

feel force? We move the same charge

to point B. In which direction does it

feel force? Does it feel more or less

force than when it was at point A?

There is a difference in the potential energy from A to B.

A

B

Potential Difference

What must we do to move a positive charge from point A to point B? Exert force? Do work!

We must do work to move a charge from point A to point B.

V = W/q V – potential difference

A

B

Practice Problems

If 6 joules of work are done to move 2.0 C of charge from point A to point B, what is the potential difference between points A and B? V = 3V

How much energy is needed to move one electron through a potential difference of 100V? W = 1.6 * 10-17 J

An alpha particle (helium nucleus) with a charge of +2e is accelerated by a potential difference of 5000V. What is the kinetic energy gained by the ion? KE = 1.6 * 10-15 J How fast is it moving?

CIRCUITS

Basic Circuit

batteryswitch

bulb

Circuits

What happens when the switch is closed? Electricity flows!

What is the flow of electricity called? Current.

Current What is another use for the word current?

Current of a river.

What does current mean in this sense? How much water flows in a certain amount of time.

Current for circuits is similar, but it is how much charge flows in a certain amount of time.

Current: the rate at which charge flows in a circuit.

I = Δq/t I – current

Units for Current

I = Δq/t q – coulomb t – seconds

I – coulombs / second

I – ampere (A)

Practice Problems

A total of 20.0 C of charge pass a given point in a conductor in 4.0 seconds. What is the current in the wire? I = 5 A

A wire carries 2.0 A. How much charge passes a given point in the wire in 1.0 minute? Δ q = 120.0 C

RESISTANCE

The water mill wheel resists the flow of current. Like resistors in a circuit.

A water mill uses energy from the water to do work.

The resistors in a wire (light bulb) use electrical energy to do work. Light Heat

RESISTANCE

Anything you include in a circuit (ie. a light bulb) restricts the flow of current.

Resistor: a circuit component used solely to regulate the current in a circuit.

Resistance: a measure of how much a substance impedes the flow of electric current.

Units – ohms (Ω)

POTENTIAL DIFFERENCE (VOLTAGE)

How does the energy of the water at A compare to the energy of the water at B? More at A – energy taken by wheel.

How could you determine how much energy was taken by the wheel? Measure energy at A and measure energy at B

and subtract.

A B

MEASURING POTENTIAL DIFFERENCE

If we wanted to know the Potential Difference across R1, where would we have to measure potentials?

Each component (ie. light bulb) in a circuit uses some (or all) of the potential.

Voltmeter: instrument used to measure potential difference (voltage) across a component of a circuit. The voltmeter is attached on either side of the component (in parallel).

R1 R2 R3

a b c d

V

MEASURING CURRENT

If we want to measure how much current is flowing through R2, where should we look? b or c However much current goes in must also come out.

Each component will have some (or all) of the current flowing through it.

Ammeter: instrument used to measure the current in a circuit. The ammeter is placed in the circuit before or after the component it is measuring (in series).

R1 R2 R3

a b c dA

BATTERY

This symbol represents a battery. Batteries are rated in volts. What is the voltage of a AA battery? What about a

AAA battery? On the big side (the left) the potential is the

maximum (the rating of the battery). On the small side (the right) the potential is

the minimum (zero).

All of the potential of the battery is used up as electricity flows through the circuit.

Ohm’s Law

The resistance, current, and potential difference are related by Ohm’s Law:

R = V/I

A

V

R

Practice Problems

The current through a light bulb that has a potential difference of 110 volts applied across it is measured to be 0.55 amps. What is the resistance of the light bulb?

A potential difference of 12 volts is applied across a circuit having a 4.0 ohm resistor. What is the current in the circuit?

A 20 ohm resistor has 40 coulombs of charge passing through it in 5.0 seconds. What is the potential difference across the resistor?

Resistivity Water analogy!

There is a section of pipe that narrows. Is the resistance of this section greater if it narrows a lot or a little? A lot!

Is the resistance of this section greater if the section is short or long? Long!

What else might the resistance of a component depend on ? The material it’s made of.

CROSS SECTIONAL AREA

Cross sectional area: the area of a slice of a wire.

A

Resistivity

SO,

A↓ R↑

L ↑ R↑

Depends on material.

R = ρL/A ρ: resistivity of the material L: length A: cross-sectional area

Practice Problems What is the resistance of a 10 meter long

copper wire having a cross-sectional area of 1.5 X 10-6 m2 at 20°C?

A 5.0 meter long tin wire has a cross-sectional area of 2.0 X 10-6 m2 and a resistance of 0.35 ohm. What is the resistivity of tin?

Electrical Power The power of an electrical component is

equal to the product of the potential difference across the component and the current through the component.

P = VI V = IR (from Ohm’s Law) I = V/R (from Ohm’s Law)

P = (IR)I = I2R P = V(V/R) = V2/R

P = VI = I2R = V2/R

Power Practice Problems

The current through a toaster connected to a 120 V source is 8 A. What is the power dissipated by the toaster? 960 W

A light bulb has a resistance of 100 ohms and a potential difference of 120 V across it. What is the power dissipated by the light bulb? 144 W

Work (Electrical Energy)

How are work and power related? P = W / t W = Pt

Reminder: P = VI = I2R = V2/R

W = ?

W = Pt = VIt = I2Rt = (V2/R)t

Work Practice Problems

How much energy does a 60 W light bulb use in 5 minutes? 18000 J

A current of 1.2 A flows through a 50 ohm resistor for 2 minutes. How much heat was generated by the resistor? 8640 J

SERIES CIRCUIT

Series circuit – a circuit in which there is only one path for electrons to travel; each electron travels through ALL the resistors.

What is the total resistance of this circuit? R = 24 Ω

RT = R1 + R2 + R3 + …

R1 = 10 Ω

R2 = 8 Ω

R3 = 6 Ω

V = 12 V

SERIES CIRCUIT

What is the total potential difference of the circuit? V = 12 V

What is the total current of the circuit?

V = IR Ohm’s Law! I = 0.5 A

R1 = 10 Ω

R2 = 8 Ω

R3 = 6 Ω

V = 12 V

LET’S MAKE A TABLE

R (Ω) V (V) I (A)

R1 10

R2 8

R3 6

RT 24 12 0.5

R1 = 10 Ω

R2 = 8 Ω

R3 = 6 Ω

V = 12 V

POTENTIAL DIFFERENCE & CURRENT IN SERIES

How did the total current compare to the current through each resistor?

IT = I1 = I2 = I3 = …

How did the total potential difference compare to the current through each resistor?

VT = V1 + V2 + V3 + …

SERIES CIRCUIT PRACTICE

What is the total current in the circuit? IT = 0.33 A

What is the potential difference across each resistor? V1 = 4 V

V2 = 6 V

V3 = 2 V

R1 = 12 Ω

R2 = 18 Ω

R3 = 6 Ω

V = 12 V

Hint: MAKE A TABLE!!!

There is a current of 1.5 A through resistor 1. Find the total potential difference of the circuit.

VT = 27 V

Find the potential difference across each resistor. V1 = 3 V

V2 = 10.5 V

V3 = 13.5 V

SERIES CIRCUIT PRACTICE

R1 = 2 Ω

R2 = 7 Ω

R3 = 9 Ω

PARALLEL CIRCUITS

Parallel circuit – a circuit in which there are multiple paths for electrons to travel; each electron only flows through one of the resistors.

What is the total resistance of this circuit? Been to a concert recently? Easier for the crowd to exit through 1 door or 3

doors? 3! Total resistance in parallel is LESS THAN any

individual resistor!

R1 = 3 Ω R2 = 4 Ω R3 = 6 ΩV = 12 V

RESISTANCE IN PARALLEL

Calculate the total resistance of this circuit WITHOUT a calculator.

Hint: common denominator?

RT = 1.33 Ω

R1 = 3 Ω R2 = 4 Ω R3 = 6 ΩV = 12 V

1 1 1 1RT R1 R2 R3

…

RESISTANCE IN PARALLEL

What is the total resistance of this circuit? (use a calculator)

Is your answer less than 5 Ω? It should be!

RT = 2.4 Ω

R1 = 5 Ω R2 = 8 Ω R3 = 11 ΩV = 12 V

POTENTIAL DIFFERENCE IN PARALLEL CIRCUITS

What is the electric potential at a? 12 V

What is the electric potential at b? 0 V

If an electron flows across R1, what is the potential difference? V = 12 V

What if the electron flowed across R2 or R3? The same!

VT = V1 = V2 = V3 = …

R1 = 5 Ω R2 = 8 Ω R3 = 11 ΩV = 12 V

a

b

PARALLEL CIRCUITS

R1 = 5 Ω R2 = 8 Ω R3 = 11 ΩV = 12 V

R (Ω) V (V) I (A)

R1 5

R2 8

R3 11

RT 2.4 12

CURRENT IN PARALLEL

Whatever current passes through R1, does not pass through R2 or R3.

The less resistance a path has, the more current will flow through it.

IT = I1 + I2 + I3 + …

R1 = 5 Ω R2 = 8 Ω R3 = 11 ΩV = 12 V

PARALLEL CIRCUIT PRACTICE

What is the total resistance of the circuit? RT = 1 Ω

What is the total current in the circuit? IT = 12 A

What is the current through each resistor? I1 = 6 A

I2 = 4 A

I3 = 2 A

R1 = 2 Ω R2 = 3 Ω R3 = 6 ΩV = 12 V

PARALLEL CIRCUIT PRACTICE

What is the total resistance of the circuit? What is the total current in the circuit? What is the current through each resistor? What is the voltage of the battery?

R1 = 3 Ω R2 = 5 Ω R3 = 15 Ω

A

I = 2A

MAGNETISM

MAGNETS Magnets have two poles:

North South

How will these magnets interact?

S N

N S

S N

S N

N S

S N

N S

N S

MAGNETS

Similar to electric charges.

Like poles repel. Opposite poles attract.

Every magnet has a north and south pole. You CANNOT have a material with only one pole.

NORTH-SOUTH POLE

N S

N S

N S

N S

N S

N S

N S

MAGNETIC FIELDS

Magnetic field lines go from North to South, and do NOT cross.

N SN

S

MAGNETIC FIELDS

N N S S

N S

Compass in Magnetic Field

A compass in a magnetic field will align itself parallel to the field line with the North pole of the compass pointing in the direction of the field line.

Complex Magnetic Fields In a topographic map (Earth Science) the

steepest part of the hill is where the lines are closest together.

In a map of a magnetic field, the closer the lines the stronger the magnetic field at that point.

ElectroMagnetism

A moving charge (electric current) creates a magnetic field.

A moving magnetic field creates an electric current in a wire.

This is why electricity and magnetism are considered the same force!!!

Conventional Current vs. Electron Current Electron Flow is what actually happens and

electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source.We don’t use this!

Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source. This was the convention chosen during the discovery of electricity. They were wrong, but we still use this. Why? I DON’T KNOW!!!Use your RIGHT hand.

1st Hand Rule

Magnetic field around a current carrying wire. Thumb points in direction of current. Fingers point in direction of magnetic field.

2nd Hand Rule

Magnetic field through a solenoid. Fingers point in the direction of the current. Thumb points in direction of North pole.