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.
The following 5 quantities are some common terms.
Quantity
Symbol Unit
Definition
Unit Abbreviation
Current
I
amp
A
coulomb/second
Voltage (Potential V
Difference)
volt
V
joule/coulomb
Resistance
Power
R
ohm
Ω
volt/amp
P
watt
W
joule/second
Energy
E
kilowatt KWH
hour
power x time
1
Electrical Potential Energy When 2 charges interact there is an electric force between them.
The potential energy associated with this force is like the potential energy associated with the position of an object position relative to Earth.
When an object is moved away from the Earth is has gained gravitational potential energy.
2
Electrical Potential Energy Remember from last chapter that the electric field direction is directed away from the positive charge and directed towards negative charges.
Two positive charges __________ each other. To move a positive charge from
location A to location B will require work and will INCREASE the electrical potential energy
of the positive charge.
A positive charge will naturally move from
location B to location A. Work is not required and the electrical potential energy
of the positive charge will DECREASE.
Opposite charges __________ each other. A positive charge will naturally move from
location A to location B. Work is not required and the electrical potential energy
of the negative charge will ___________.
To move a negative charge from
location B to location A will require work and will __________ the electrical potential energy
of the negative charge.
3
Electrical potential energy is the energy of a charged object due to its location in an electric field. The unit is JOULE.
In the picture below, it took energy to push both positively charged particles close to the large green charged object.
The larger particle has a _________________ electrical potential energy, because it has a ______________ charge, but the electric potential of both particles are the same because they are at the same location from the large green positively charged object.
Electric potential is a electrical potential energy per unit charge. The unit is JOULE
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Electrical potential energy = _____ J
Electric potential = _____ J
Electrical potential energy = _____ J
Electric potential = _____ J
Electrical potential = Joule (unit of energy)
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.
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.
Symbol is V and the SI unit is volts. It is one joule per coulomb.
In a simple circuit with a 12 volt battery every coulomb
of charge gains 12 joules of potential energy as
it moves from the low to high potential terminals.
This is the push to move the charges.
Every coulomb of charge loses 12 joules of electrical
potential energy as it passes through the circuit.
The lose of this electrical potential energy results in a
gain in light, thermal energy and other forms of non­
electrical energy. Battery
The potential difference between 2 terminals of a small battery is 1.5 V
For your car battery it is about 13.2 V.
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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: ___ Ω has been overcome
_____________
10 V
10 A
0 V
resistance means
________ current
___ Ω has been overcome
can flow.
10 V
1 A
0 V
The voltage drops due to the resistance of the part of the electrical circuit.
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
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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. Current will flow in a closed circuit.
V = 20 v
R = 10 Ω
I = ?
A
10
Electrical Potential Energy When 2 charges interact there is an electric force between them.
The potential energy associated with this force is like the potential energy associated with the position of an object position relative to Earth.
When an object is moved away from the Earth is has gained gravitational potential energy.
When a charged object is moved away from an object with an opposite charge is has gained electrical potential energy. When a charged object is moved toward an object with the same charge is has gained 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
11
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 electrical potential energy.
The increased potential energy is the result of work required to reach the closer location. The unit for electrical potential energy is JOULES.
This energy due to the location of the small particle with respect to the big particle 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.
12
If there is no difference in electric potential
If there is a 10 Volt different in electric potential
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