Electric Fields

Electric Fields

 

Any object with a charge has an electric field around it- the region where it can attract or repel other charges.

 

1. Electric charge, Q, is measured in Coulomb's (C) and can be either positive or negative
2. Oppositely charged objects attract each other and like charges repel.
3. If a charged object is placed in an electric field then it will experience a force.

Coulomb's Law

You'll need Coulomb's law to work out F- the force of attraction or repulsion between two point charges.

 

Coulomb's law:

 

 

Q1= a charge

Q2= a charge

 

So the top one shows an attractive force and the force is towards the centre and F will be negative since -q-(+q) =-F

 

The bottom one shows a repulsive force where the force is directed outwards and F will be positive since -q-(-q)=+F

 

1. The force on Q1 is always equal and opposite to the force on Q2
2. It's an inverse square law. The further apart the charges are, the weaker the force between them
3. The size of the force, F, also depends of the permittivity of the material between the two charges.

Electric Field Strength

Electric field strength, E, is defined as the force per unit positive charge- the force that +1C would experience if it was placed in an electric field.

1. E is a vector pointing in the direction that a positive charge would move
2. The units of E are newtons per coulomb (N/C)
3. Field strength depends on where you are in the field
4. A point charge- or any body that behaves as if all is charge is concentrated at the centre has a radial field.

Radial Fields

1. E is the force per unit charge that a small, positive test charge, q, would feel at different points in the field. In a radial field, E, depends of the distance, r, from the point charge Q...
2. It's another inverse square law, E is proportional to r^2

 

This is what you use for finding the electric field strength of a point charge.

 

1. Field strength decreases as you go further away from Q

On the Y axis you have E, and on the X axis you have r. As r increases E decreases- inverse square law!

 

Electric Potential Energy

A charge in an Electric field has electric potential energy. Electric potential energy is the work that would need to be done in order to move a small charge, q, from infinity to a distance r away from a point charge Q...

U is another notation for E. This equation tells us the electric potential energy to move q from infinity to a distance r away from Q.

 

1. Infinity is used as if q were an infinite distance away from Q, the charged particle q would have zero potential energy.
2. In a repulsive force field (Q and q are both positive) you would have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases.
3. In an attractive field (Q is negative and q is positive) the charge q gains potential energy as r increases.

Electric Potential

Electric potential, V, is electric potential energy per unit positive charge...

This is the equation for electric potential. The difference between electric potential energy and electric potential is that one equation involves two q's the other only has one since it with electric potential, the q's cancel as it is per unit positive charge.

 

1. It is measure in Volts (V)
2. As with E, V is positive when the force is repulsive and negative when the force is attractive.

Uniform fields

A uniform field can be produced by connecting two parallel plates to the opposite poles of a battery.

1.  Field strength, E, is the same at all points between two plates and is...

1. V is the potential difference between the plates d is the distance between them
2. E can be measured in volts per metre