Electricity

 14.1 Electric Charge and Current
 14.2 Electric Current, Resistance, and
Voltage
 14.3 Capacitors

14.1 Investigation: Electric Charge
Key Question:
What is static electricity?
Objectives:
 Explain that static electricity is caused by an imbalance of charge.
 Build an electroscope and describe its function.
 Infer that a relationship exists between charge and the strength of electrical force.

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

 Electric charge , like mass, is also fundamental property of matter.
 Inside atoms found in matter, attraction between positive and negative charges holds the atoms together.

 The unit of charge is the coulomb (C).
 The name was chosen in honor of Charles Augustin de Coulomb (1736-1806), the French physicist who performed the first accurate measurements of the force between charges.

 Virtually all the matter around you has electric charge because atoms are made of electrons and protons (and neutrons).
 Because ordinary matter has zero net (total) charge, most matter acts as if there is no electric charge at all.

 An object is charged when its net charge is not zero.
 If you have ever felt a shock when you have touched a doorknob or removed clothes from a dryer, you have contacted a charged object.

 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 forces are incredibly strong.
 A millimeter cube of carbon the size of a pencil point contains about 77 coulombs of positive and negative charge.

 The force between two charges depends on the charge and the distance.
 Coulomb’s law explains the relationship between the amount of each charge ( q
1 and q
2
), the distance between their centers ( r ), and the electrical force
( F
E
).

 The 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.

 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 2 2 = (4), decreasing the force to onefourth its original value (1/4).
 This relationship is called an inverse square law because force and distance follow an inverse square relationship.

 Coulomb’s law is similar in form to Newton’s law of universal gravitation.
Force
(N)
Constant
9 x10 9 N .
m 2 /C 2
F = K q
1 q
2 r 2
Charges (C)
Distance (m)

 Electrostatics is the part of physics that studies the forces created by unmoving charges.
 A photocopier uses electrostatic forces.

Using Coulomb’s law
Two steel marbles are each given a static electric charge of 0.001 coulombs. Calculate the size of the force on the marbles, in newtons, if they are held 2 m apart.
1.
2.
3.
Looking for: … force
Given: … charges (.001 C), distance (2 m)
Relationships: Use Coulomb’s law:
F = -Kq
1 q
2
÷ r 2
4.
Solution:
F = -(9 x 10 9 Nm 2 /C 2 ) (0.001 C)(0.001 C) ÷ (2 m) 2
= 2,250 N

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

 Once an electroscope is charged it can be used to test other charged objects.

 Under dry conditions, a balloon rubbed on hair will transfer electrons from hair to the balloon.
 This is called charging by friction .
 Objects charged by this method will attract each other.
Once the balloon is charged, it can also stick to a neutral wall through
“polarization”

 Charging by induction is a method of using one object to charge another without changing the net charge on the first object.

 To charge by induction, the electroscope is first connected by a wire to a large neutral object.
 When the balloon comes near, the charge on the balloon induces an opposite charge to flow through the wire onto the electroscope.
 The wire is then removed quickly so the charge on the electroscope cannot flow back where it came from.
 The electroscope stays charged after the balloon is removed.
 The leaves spread apart because the added charges repels each other and spread out onto the electroscope leaves .

 14.1 Electric Charge and Current
 14.2 Electric Current, Resistance, and
Voltage
 14.3 Capacitors

14.2 Investigation: The Flow of Electric
Charge
Key Question:
How much charge moves when current flows?
Objectives:
 Discuss how the voltage across a capacitor varies with time as it discharges.
 Use Ohm’s law to calculate the current in a circuit when voltage and resistance are known.
 Create a current versus time graph and use it to calculate the electrical charge flowing in a circuit.

 The direction of current was historically defined as the direction that positive charges move.
 Benjamin Franklin first used the terms “positive” and
“negative” to describe charge.
 According to Franklin’s theory, a positive object’s extra fluid naturally flowed toward a negative object.

 Because of Franklin’s work, the direction of electric current is defined as going from positive to negative.
 Long after Franklin’s work, scientists discovered that current in wires is the flow of electrons which move in a circuit from negative to positive.

 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.

 In a conducting metal like as copper, the atoms of copper bond together by sharing electrons.
 Some of the electrons can move freely anywhere within the copper.

 If a copper wire is not connected to a battery, the free electrons move around at high speeds.
 They have no net motion because as many move one way as the other way.

 If voltage is applied to a copper wire, electrons slowly drift while randomly colliding with atoms in the wire.
 This drift velocity is what creates electrical current.

 A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

 Now that you know that current is really moving charge, how do we understand voltage in terms of charges?
 Voltage measures electrical potential energy per unit of charge .
 One volt is 1 joule per coulomb.

 14.1 Electric Charge and Current
 14.2 Electric Current, Resistance, and
Voltage
 14.3 Capacitors

14.3 Investigation: Making an Electrophorus
Key Question:
How do electric charges interact?
Objectives:
 Triboelectrically charge different materials.
 Use a triboelectric series to make predictions about charged objects.
 Make an electrophorus and explain how it works.

 A capacitor is a storage device for electric charge .
 Capacitors can be connected in series or parallel in circuits, just like resistors.

 The 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.

 A 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.

The current flowing into or out of a particular capacitor depends on four things:
1.
The amount of charge already in the capacitor.
2.
3.
4.
The voltage applied to the capacitor by the circuit.
Any circuit resistance that limits the current flowing in the circuit.
The capacitance of the capacitor.

 As the capacitor charges, the current in the circuit decreases.
 As the voltage on the capacitor increases, the circuit’s voltage difference decreases and so does its current flow.

 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.

The amount of charge a capacitor can store depends on three factors:
1.
The insulating ability of the material between the positive and negative plates.
2.
3.
The area of the two plates
(larger areas can hold more charge).
The separation distance between the plates.

 If connected in a circuit with a low resistance, a capacitor can discharge very quickly, creating a large amount of current.
 This can be very useful in devices that require a brief burst of a large amount of current for their operation, but can also be dangerous.
How is a defibrillator both useful and dangerous?

 Lightning originates in towering, dark storm clouds. Inside these clouds, charges begin to separate.
 Scientists still don’t completely understand how this happens.
Lightning