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## Basic Quantities

• Force : Force can be defined by Newton's second law:

The force of 1 newton will cause a mass of 1 kilogram to accelerate at 1 meter per second per second.

Comparison:

• The gravitational force between masses and is

In particular, on surface of Earth, the weight'' of a mass is

where is the gravitational acceleration that measures the intensity of Earth's gravitational field.
• The electric force is

where is the intensity of the electric field caused by charge , and is the force this field asserts on charge .

• Energy : (a conservative quantity, can be neither created nor destroyed)

Potential Energy: When an object with weight of 1 newton is raised by 1 meter, it receives 1 Joule of potential energy.

In gravitational field, the potential energy is , while in electric field, it is , where the field intensity is .

Kinetic Energy: A mass of 1 kilogram moving with a velocity of 1 meter per second possesses 1/2 Joule of kinetic energy.

Another unit for energy is calorie:

• Power : The rate of energy transformation. The transformation of 1 Joule of energy in 1 second represents a power of 1 watt. The instantaneous and average power are defined by:

Another unit for power is horse power:

Energy can also be measured by kilowatt-hours (kWh) Joules.

• Charge: The charge of electrons is a coulomb, i.e., the charge each electron carries is coulomb. Charge is conservative.

• Current: The rate of flow of positive charges:

which is a through variable.

• Voltage: The energy required to move a unit charge from one electric potential to another is the difference between the two potentials or voltage . When a unit charge of 1 coulomb is moved through a voltage of 1 volt, it receives or delivers an energy of 1 Joule

Comparison:

• The energy needed to move a charge from point A with potential to point B with potential is , and the voltage is the potential difference between the two points, and is therefore an across variable.
• The energy needed to move a mass from height to height is , and is the potential difference between the two heights.

• Electric Field: a force field defined as:

Comparison:

• Electric field is the electric potential difference per unit distance.
• Gravitational field is the gravitational potential difference per unit distance.

But as

we get the alternative definition of electric field

Comparison:

• Electric field is electric force per unit charge.
• Gravitational field is gravitational force per unit mass.

• Electrical power and energy:

• Magnetic Flux: The intensity of magnetic effect (lines per unit area in magnetic field or flux) is measured by magnetic flux density in tesla, and the magnetic flux within an area is

This is the dot product of magnetic intensity vector and area vector (in the normal direction of the area), and is the angle between the two directions.

When and are in the same direction (), and if is 1 tesla and is 1 square meter, the flux is 1 weber.

• Electric Magnetic Interaction:

In a magnetic field , a force is exerted on a charge moving with velocity :

where the force vector is the cross product of velocity vector and magnetic flux vector (right-hand rule).

A force of 1 newton is experienced by a charge of 1 coulomb moving with a velocity of 1 meter per second normal to a magnetic flux density of 1 tesla.

Next: Capacitor and Inductor Up: Chapter 1: Basic Quantities Previous: Chapter 1: Basic Quantities
Ruye Wang 2012-07-02