Electricity and Magnetism

Electric ChargeCoulomb’s LawCapacitors

Electric Charges and Forces

Objectives

1. Describe and calculate the forces between like and unlike electric charges.

2. Identify the parts of the atom that carry electric charge.

3. Apply the concept of an electric field to describe how charges exert force on other charges.

4. Sketch the electric field around a positive or negative point charge.

5. Describe how a conductor shields electric fields from its interior.

6. Describe the voltage and current in a circuit with a battery, switch, resistor, and capacitor.

7. Calculate the charge stored in a capacitor.

Vocabulary Terms: homework

charge electrically

neutral static

electricity positive charge negative

charge electric forces charge by

friction electroscope protons neutrons

electrons gravitational

field charged induction Coulomb’s law capacitor parallel plate

capacitor microfarad coulomb electric field

capacitance charge polarization shielding test charge farad field inverse square law discharged

field lines

ElectrostaticsIn Physics

Electric Charge

Key Questions:1.Define electrically

neutral2.Define net charge or

excess charge3.What causes static

electricity

Electric Charge

All ordinary matter contains both positive and negative charge.

You do not usually notice the charge because most matter contains the exact same number of positive and negative charges.

An object is electrically neutral when it has equal amounts of both types of charge.

Electric Charge

Objects can lose or gain electric charges.

The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges.

A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

Electric charge

Answer the following:4. What is the unit of electrical charge?5. What is the mass of an electron, proton,

and neutron?6. What are the charges in coulombs of a

protron, neutron, and electron?7. Describe the attracting and repelling

characteristics of protons and electrons

Electric Charge

Electric charge is a property of tiny particles in atoms.

The unit of electric charge is the coulomb (C).

A quantity of charge should always be identified with a positive or a negative sign.

Electric forces

Electric forces are created between all electric charges.

Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

8. Draw a picture of an electroscope with a charge (positive or negative)

9. Describe how the electroscope becomes positively charged through conduction

10. Describe how the electroscope becomes negatively charged through conduction

Electric forces

The forces between the two kinds of charge can be observed with an electroscope.

Electric forces

Charge can be transferred by conduction.

11. What moving particles create a current in metals?

12. Define current 13. Write the formula for current14. What is the unit for current?

In conductive liquids (salt water) both positive and negative charges carry current.

In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons.

The direction of current was historically defined as the direction that positive charges move.

Both positive and negative charges can carry current.

Electric current

Current is the movement of electric charge through a substance.

Current (amps)

Charge that flows

(coulombs)

Time (sec)

I = q t

Calculate current

15. Two coulombs of charge pass through a wire in five seconds.

Calculate the current in the wire.

16. Compare the movement of an electron in a conductor and insulator

17. Describe a semi-conductor18. Describe charging by friction19. What causes a negatively charged

balloon to stick to a wall?

Conductors and insulators

All materials contain electrons.

The electrons are what carry the current in a conductor.

The electrons in insulators are not free to move—they are tightly bound inside atoms.

Conductors and insulators A semiconductor has a few free electrons

and atoms with bound electrons that act as insulators.

Conductors and insulatorsWhen two neutral objects

are rubbed together, charge is transferred from one to the other and the objects become oppositely charged.

This is called charging by friction.

Objects charged by this method will attract each other.

20. Define coulomb’s law and write the formula

21. What is coulomb’s constant?22. Describe the force between two charges if

the distance between them is tripled23. Describe the force between two charges if

both charges are increased by a factor of two.

Coulomb's Law

Coulomb’s law relates the force between two single charges separated by a distance.

Force (N)

Constant9 x109 N.m2/C2

Distance (m)

F = K q1 q2

r2

Charges (C)

Coulomb's Law

The force between two charges gets stronger as the charges move closer together.

The force also gets stronger if the amount of charge becomes larger.

Coulomb's Law

The force between two charges is directed along the line connecting their centers.

Electric forces always occur in pairs according to Newton’s third law, like all forces.

Coulomb's LawThe force between

charges is directly proportional to the magnitude, or amount, of each charge.

Doubling one charge doubles the force.

Doubling both charges quadruples the force.

Coulomb's Law

The force between charges is inversely proportional to the square of the distance between them.

Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4).

This relationship is called an inverse square law because force and distance follow an inverse square relationship.

Now answer questions #22, 23

Calculating force

24. Two balls are each given a static electric charge of one ten-thousandth (0.0001) of a coulomb.

Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter.

25. Describe an electric field26. What type of field does mass create?27. Compare the gravitational force and

electrical force between two charges.28. Draw an electrical field between: a. A proton and electron b. A proton and proton c. An electron and electron29. Draw the electric field coming from a

single proton and a single electron

Fields and forcesThe concept of a field is used to describe any

quantity that has a value for all points in space.You can think of the field as the way forces are

transmitted between objects.Charge creates an electric field that creates forces

on other charges.

Fields and forcesMass creates a gravitational field that exerts

forces on other masses.

Fields and forces

Gravitational forces are far weaker than electric forces.

Drawing the electric field

30. Describe the units and equations for both a gravity field and electric field

31. Describe how electric fields accelerate particles causing changes in voltage

32. Why is shielding necessary around electrical wires inside of sensitive electronic equipment?

Electric fields and electric forceOn the Earth’s surface, the gravitational field

creates 9.8 N of force on each kilogram of mass.With gravity, the strength of the field is in newtons

per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

Electric fields and electric forceWith the electric field, the strength is in newtons

per coulomb (N/C).The electric field describes the amount of force

per coulomb of charge.

Accelerators

An electric field can be produced by maintaining a voltage difference across any insulating space, such as air or a vacuum.

Electric fields are used to create beams of high-speed electrons by accelerating them.

Electron beams are used in x-ray machines, televisions, computer displays, and many other technologies.

Electric shielding

Electric fields are created all around us by electric appliances, lightning, and even static electricity.

These stray electric fields can interfere with the operation of computers and other sensitive electronics.

Many electrical devices and wires that connect them are enclosed in conducting metal shells to take advantage of the shielding effect.

Practice problems: 33. A particle in a microwave has a charge

of -9 x 10-15 C and is deflected into popcorn with a force of 4.5 x 10-2 N. What is the strength of the electric field. USE F = Eq

q = -9 x10-15 CF = 4.5 x 10-2 NE = F/q = 4.5 x 10-2 N/- 9 x10-15 C = 5 x 1012

N/C

34. A positive and negative particle, each with a 5 x 10-8 charge, are separated by a distance of 6 mm. What is the Force acting on them? Use coulomb’s law

F = k q1 q2/r2

F = (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)2 = -6.25x10-1N

35. What would the force be in number 34 if the distance were tripled

F = k q1 q2/r2

If r is tripled the r2 = 32 = 9 so the force would be 1/9 as strong = -6.25x10-1N/9

36. Find the size of the electrical field at the location of one charge due to the other charge? USE F = Eq, Use information from question #35 E = F/q = -6.25x10-1N/ (5 x 10-8 )

37. Find the potential energy of the charged pair.

Use UE = ke q1q2/r

Use UE = ke q1q2/r

= (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)

38. How many excess electrons does the microwave particle in #33 have?

Q = Ne where Q = the total charge N = the number of electrons e = electronic charge 1.6 x 10-19 CN = Q/e = 9 x10-15 C/ 1.6 x 10-19 C = 5.625 x

104

Capacitors

39. What is a capacitor?40. What is the symbol for a capacitor in a

circuit diagram?41. How is a capacitor charged?42. How is a capacitor discharged?43. What determines flow in and out of a

capacitor?44. Describe a parallel plate capacitor45. Describe how the parallel plate capacitor

works

Capacitors

A capacitor is a storage device for electric charge.

Capacitors can be connected in series or parallel in circuits, just like resistors.

CapacitorsA capacitor can be charged by connecting it to a

battery or any other source of current.A capacitor can be discharged by connecting it to

any closed circuit that allows current to flow.

CapacitorsThe current flowing into or out

of a particular capacitor depends on four things:

1. The amount of charge already in the capacitor.

2. The voltage applied to the capacitor by the circuit.

3. Any circuit resistance that limits the current flowing in the circuit.

4. The capacitance of the capacitor.

How a capacitor works insideThe simplest type of

capacitor is called a parallel plate capacitor.

It is made of two conductive metal plates that are close together, with an insulating plate in between to keep the charges from coming together.

Wires conduct charges coming in and out of the capacitor.

46. What determines how much charge a capacitor can hold?

47. Define capacitance48. What is the formula for capacitance49. What is the unit for capacitance50. What is a microfarad?

How a capacitor works inside

The amount of charge a capacitor can store depends on several factors:

1. The voltage applied to the capacitor.2. The insulating ability of the material

between the positive and negative plates.

3. The area of the two plates (larger areas can hold more charge).

4. The separation distance between the plates.

Capacitance

The ability of a capacitor to store charge is called capacitance (C).

Charge

(C)

Capacitance(coulombs/volt)

q = C V

Voltage (volts)

Cameras use capacitors to supply quick bursts of energy to flash bulbs.

Capacitance

Capacitance is measured in farads (F). A one-farad capacitor can store one coulomb of

charge when the voltage across its plates is one volt.

One farad is a large amount of capacitance, so the microfarad (μF) is frequently used in place of the farad.

Calculate capacitance51. A capacitor holds

0.02 coulombs of charge when fully charged by a 12-volt battery.

Calculate its capacitance and the voltage that would be required for it to hold one coulomb of charge.

C = q/V

Capacitors: electric field = V/d52. 2000 volts moves across a parallel plate

capacitor with a separation of 5 mm. What is the electric field across the capacitor?

E = V/d = 2000V/5x10-3m = 4 x 10 N/C

Application: How a Television Works