Electrical Energy and Circuits Date ________
(Parts of Chapter 17 and 18)
Last chapter we studied charges at rest and this chapter is about moving charges.
3 Characteristics ­ these are interrelated as current flows through a material
Potential Difference: Symbol:V (unit is volts, v)
Current: Symbol:I (unit is amps, A)
Resistance: Symbol:R (unit is ohms, Ω)
1
Electrical Potential Energy When 2 charges interact there is an electric force between them.
The potential energy associated with this force, like the potential energy associated with the position of an object position relative to Earth, is called the electrical potential energy.
An object has gravitational potential energy due to its location in a gravitational field.
An charged object has electrical potential energy due to its location in an electric field. Unlike charges attract
When a charged particle is moved away from an oppositely charged particle, work has been done on the first particle and it has more electric potential energy. 2
The big charged sphere and the small charged particle have the SAME charge.
If the small charged particle is pushed closer to the large charged sphere it has MORE potential energy.
The increased potential energy is the result of work required to reach the closer location. This energy due to the location is electric potential energy.
If the particle is released, it accelerates away from the sphere.
Its electrical potential energy decreases and its energy changes into kinetic energy.
If the charge doubles, twice as much work is done pushing it closer and the particle has twice as much electrical potential energy.
A more convenient quantity is the electrical potential energy per unit charge or electric potential.
3
The blue particle has a larger electrical potential energy, because it has a greater charge, but the electric potential of both particles are the same because they are at the same location.
Electric potential is a property of space not charge.
Electrical potential energy is a quantity of energy
with units in Joules.
Electrical potential is a measure of energy per
unit charge.
Electrical potential = Joule (unit of energy)
(Voltage) Coulomb (electric charge) One coulomb is the charge of 6.25 x 1018 electrons.
4
In electric circuits we care about the difference in electric potential.
If there is no difference in electric potential
If there is a 10 Volt different in electric potential
Potential Difference (V) is the measure of the difference in the electrical potential energy between 2 points in space divided by the charge. This is the pressure to move charges.
The SI unit is volt and it is one joule per coulomb. One coulomb of charge moves through a potential difference of one volt and gains one joule of energy.
The potential difference between 2 terminals of a small battery is 1.5 V
For your car battery it is about 13.2 V.
5
What is moving when there is a potential difference?
In your house there are electrons in the wires. When a light is turned on, an electric field is established in the wire. The wire guides the electric field lines.
(Remember the electric field is the force on a charged particle per unit charge.)
The force due to the electric field sets the electrons in motion, creating current.
A current exists whenever there is a net movement of electric charge through a medium.
The electric company sells
energy, not electrons. The energy
is carried by the electric field.
The SI unit for current is
ampere (amps).
1 amp = 1 coulomb = 6.25 x 1018 electrons Current is the rate of flow of charge.
1 sec 1 sec
6
The amount of current depends on the ease with which the charge can move through a conductor.
Resistance is the opposition to the motion of charge through a material.
Resistance, R, has the units of ohms, Ω.
For many materials, like most metals, the resistance is constant over a wide range of potential differences. This is know as Ohm's Law.
If 1 volt causes 1 amp of current to flow then 1 ohm of resistance has been overcome.
Ohm's Law: 7
Greater distance means higher resistance
Harder for electrons to flow through a smaller cross sectional area
Different structures of atoms gives materials different resistances
Atoms at a higher temp vibrate faster making it harder for electrons to flow
8
Summary
Symbol Units Quantity Definition
V
volts (v) Potential difference 1 V = 1 joule
coulomb R ohms (Ω) Resistance 1 Ω = 1 volt
amp
I amps (A) Current 1 A = 1 coulomb
sec 9
These 3 quantities are interrelated in Ohm's Law.
Ohm's Law can be used to solve circuit problems. A circuit is a conductive pathway.
V = 20 v
R = 10 Ω
I = ?
A
10