It comes from Newton's law of universal gravitation. It states that every matter which has a mass attracts other matters with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between the centers of gravity of the two matters.

where:

We already studied about gravitational force of an object on earth, which is F = m*g, where "m" is mass of the object and "g" is the gravity of the earth. Then, we can say that . Therefore, gravity (g) of the earth is , where "m_{e}" is the mass of the earth and "d" is its radius (we are talking about gravitational force on the surface of the earth.).

However, electric field E is a little bit different from gravitational field g. Gravitational force depends on mass, whereas electric force does not depend on mass. Instead, electric force depends on charges on both objects.

By rearranging the formula, we get:

What are the magnitude and direction of the electric field 1.5 m away from a positive charge of 2.1*10^{-9} C?

(e.g. 1.0 N/C)

(e.g. "outward" or "inward")

PE_{G} = mgh

PE = qEd (see right)

Voltage is also related to force.

V = Ed = (F/q)*d = Fd/q= W/q

(W = Fd -- force times displacement in the direction of force is work (J))

A high voltage means that each individual charge is experiencing a large force. A low voltage means that each individual charge is experiencing a small force.

From the diagram of A and B above, r_{A} is 0.0005 m and r_{B} is 0.0003 m. What is the force acting on q at A and B, if the charge of q is 1.2 * 10^{-11} C and the charge of Q is 1.5 * 10^{-11} C?

(e.g. 1.00*10^-10 N) A =

(e.g. 1.00*10^-10 N) B =

B has high potential energy because these particles want to come together. It is the same priciple as the gravity.

D has lower potential energy compare to B for the same reason.

Try Chapter 12 Quiz and see how much you learned.

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