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Electric Charge Coulomb’s Law Capacitors. Electric Charges and Forces. Electricity and Magnetism. Objectives. Describe and calculate the forces between like and unlike electric charges. Identify the parts of the atom that carry electric charge.
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Electric Charge Coulomb’s Law Capacitors Electric Charges and Forces Electricity and Magnetism
Objectives • Describe and calculate the forces between like and unlike electric charges. • Identify the parts of the atom that carry electric charge. • Apply the concept of an electric field to describe how charges exert force on other charges. • Sketch the electric field around a positive or negative point charge. • Describe how a conductor shields electric fields from its interior. • Describe the voltage and current in a circuit with a battery, switch, resistor, and capacitor. • Calculate the charge stored in a capacitor.
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 Vocabulary Terms: homework
Electrostatics In Physics
Take notes on key points during the film • E:\Physics 2010\Unit 8 Electrostatic Forces\Physics__A_World_in_Motion__Electrostatics.asf
Key Question: How do electric charges interact? 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. 21.1 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
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 Charge
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. Electric forces
The forces between the two kinds of charge can be observed with an electroscope. Electric forces
Charge can be transferred by conduction. Electric forces
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 positivecharges move. • Both positive and negative charges can carry current. Electric current
I = q t • Current is the movement of electric charge through a substance. Charge that flows (coulombs) Current (amps) Time (sec) Electric current
Two coulombs of charge pass through a wire in five seconds. • Calculate the current in the wire. Calculate current
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 insulators
When 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. Conductors and insulators
F = K q1 q2 r2 • Coulomb’s law relates the force between two single charges separated by a distance. Constant 9 x109 N.m2/C2 Force (N) Charges (C) Coulomb's Law Distance (m)
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 Law
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. 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. Coulomb's Law
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. • Compare the force with the weight of an average 70 kg person. Calculating force
The 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 forces
Mass creates a gravitational field that exerts forces on other masses. Fields and forces
Gravitational forces are far weaker than electric forces. Fields and forces
On 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 force
With the electric field, the strength is in newtons per coulomb (N/C). • The electric field describes the amount of force per coulomb of charge. Electric fields and electric force
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. Accelerators
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. Electric shielding
Key Question: How strong are electrical forces? Coulomb’s Law
A capacitor is a storage device for electric charge. Capacitors • Capacitors can be connected in series or parallel in circuits, just like resistors.
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. Capacitors
The current flowing into or out of a particular capacitor depends on four things: • The amount of charge already in the capacitor. • 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. Capacitors
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. How a capacitor works inside
The amount of charge a capacitor can store depends on several factors: • The voltage applied to the capacitor. • The insulating ability of the material between the positive and negative plates. • The area of the two plates (larger areas can hold more charge). • The separation distance between the plates. How a capacitor works inside
The ability of a capacitor to store charge is called capacitance (C). Capacitance (coulombs/volt) Charge (C) q = C V Voltage (volts) Capacitance Cameras use capacitors to supply quick bursts of energy to flash bulbs.
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. Capacitance
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. Calculate capacitance
Key Question: homework How does a capacitor work? Capacitors