Electric fields

Question 1

Name some basic rules of charges and electric fields

Answer 1

• Electric charge,Q, is measured in coulombs(C) and can be either positive or negative.- Opposite charged particles attract each other.Like charges repel.- If a charged objet is placed in an electric field, then it will experience a force. - Any object with charge has an electric field around it.

Question 2

What is the equation for Coulomb’s law?

Answer 2

F = kQ₁Q₂/r²

Where K = 1/4πE

E = permittivty of material between charges.

r = is the distance between the charges.

• The force on Q₁ is equal and opposite to the force on Q₂.

Question 3

What is the definition of electric field strength, E?

Answer 3

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

Question 4

What is the equation for electric field strength?

Answer 4

E=F/q

• F is the force on a charge q.
• E is a vector pointing in the direction that a positive charge would move.

Question 5

Draw the field lines around a positive charge and the field lines around a negative charge.

Question 6

What is the equation for electric field strength at diffirent points along a radial field?

Answer 6

E = kQ/r²

• r is the distance from a point charge.

Question 7

Draw the graph of E against r. (Where r is the distance from a point charge Q.)

• State what the relationship is between the two variables.
• What the area under the graph is.

Answer 7

• The relationship between E and r is an inverse square one.
• The area under the graph is the electric potential.

Question 8

What is electric potential energy?

Answer 8

The work that would need to be done to move a small charge,q, from infinity to a distance r away from a point charge, Q.

Question 9

What is the equation for electric potential energy.

Answer 9

E = kQq/r

• Q is a point charge and q is another charge.
• In a repulsive force field(Q and q are both positive) you have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases.
• In an attractive field the charge q gains potential energy as r increases.

Question 10

Draw the graph of the electric potential energy of a charge q when in a repulsive field and when in an attractive field at distances r from the source.

Question 11

What is electric potential, V.

Answer 11

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

Question 12

What is the equation for electric potential.

Answer 12

V = kQ/r

• V is measured in volts
• As with E, V is positive when the force is repulsive, and negative when the force is attractive.

Question 13

Draw the graph of electric potential , V, against distance for a repulsive force.

• What does the gradient of the graph tell you?

Answer 13

The gradient of a tanget gives the field strength.

Question 14

How can a uniform field be made?

Answer 14

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

Question 15

How can the field strength, E, be calculated from a uniform field between two plates?

Answer 15

Field strength E is the same at all points between the two plates and is given by:

E= V/d

V is the potential diffirence between the plates and d is the distance between them.