# A2 Advancing Physics OCR B - Chapter 16

\A2 Advancing Physics OCR B - Chapter 16

- Created by: Hamed
- Created on: 14-06-11 13:34

First 429 words of the document:

Chapter 16. Charge and Field:

Electric Fields

All charged objects have a field around them. Any charged object within this field will be

either attracted or repelled depending on the charges of the 2 objects.

kQq

You can work out the force acting upon the objects using Coulomb's Law , F = 2 . F is

r

the force on each object acting away from the other, so if F is positive, they will be

repelled, if it is negative they will be attracted. Q and q are the charges on the objects and

r is the distance in between them. Finally k is the electric force constant which is given on

the first page of the data booklet for free space. Difference materials will have a different

value of , however, you can calculate k for other values of using the formula given in the

1

data book, k - .

4 0

GMm

You should notice similarities between this force and gravity, F= 2 . Both force are

r

inverse square forces which means, as the objects become closer, the force becomes

stronger at an exponential rate. The difference is the gravitational force is always negative,

so objects are always attracted, however, the electric force can be both positive and

negative, since Q and q can be either, this means electric charges can both attract and

repel.

Electric Field Strength (E)

The electric field strength is the force for each unit of charge, Newtons per Coulomb, NC -1.

F

E= . This is also like gravity, where the gravitational field strength (g) is the force per

q

unit mass. F=mg, F=qE.

Radial Fields

Objects which have their charge at a single point, or can be modelled like that, for example

a charged sphere, have a radial field. Like the gravitational field about a planet. At any

kQ

given point the field strength can be worked out as, E = 2 . Which is another inverse

r

1

square law. E 2 .

r

Electrical Potential Energy

The electrical potential energy is the amount of energy required to move an object from

infinity to a distance r from the particle. This means, if the objects are attracted to each

other, then this will be a negative value. Electric Potential Energy can be calculated using

kQq

the formula E elec = . So at an infinite distance, Eelec = 0.

r

Anthony Stewart

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