Electrical Fields

Question 1

What happens when an object or material is polarised?

Answer 1

There is a slight redistribution of charge within the atoms and they effectively have a positive and negative end

Question 2

State Coulomb’s law

Answer 2

The force between two point charges separated by distance r in a vacuum is directly proportional to the product of the two charges and inversely proportional to the square of their separation

Question 3

Why can the equation for Coulomb’s law be used when two charges are in air, as it is only truly valid for situations where they are in ‘free space’ (a vacuum)?

Answer 3

The effect of the air is negligible

Question 4

When dealing with interactions between charged subatomic particles, why can the force of gravity be considered as negligible?

Answer 4

The electrostatic force between a proton and an orbiting electron is over 10^39 times bigger than the gravitational force

Question 5

How do you determine the direction of field lines?

Answer 5

Imagine placing a tiny positive test charge at a particular point in the field and work out the direction of the force on that test charge/the direction it will travel

Question 6

Define electric field strength E at a point in a field

Answer 6

The force per unit charge on a positive test charge placed at that point. It is a vector quantity and it’s unit is N/C

Question 7

What is the equation for electric field strength in a uniform field? Radial field?

Answer 7

1) E=F/Q

2) E=Q/4πεor²

Question 8

What is electric potential difference ΔV?

Answer 8

The work done per unit charge in moving the charge between two points

Question 9

In a uniform field, the electric field strength…

Answer 9

… constant, meaning a charged particle in the field experiences the same force no matter where it is in the field

Question 10

What is an electron gun?

Answer 10

An arrangement that uses an electric field to accelerate electrons to form a beam. It consists of an anode and a cathode. The cathode is heated causing it to emit electrons when they have sufficient thermal energy to overcome the work function. A high p.d. between the two metal electrodes creates an electric field, which accelerates the electrons towards a hole in the cylindrical anode, to form a beam.

Question 11

What is the equation of a parabola?

Answer 11

y = kx² where k = EQ/2mv²

Question 12

What is the electric potential at a point defined as?

Answer 12

The work done per unit charge in bringing a small positive test charge from infinity to that point

Question 13

Is electric potential scalar or vector?

Answer 13

Electric potential is a scalar quantity. Δ electric potential is a vector quantity

Question 14

In the isolated field of a positive charge, electric potential…

Answer 14

… is always positive because external work must be done to move a small positive test charge towards that isolated charge

Question 15

In the isolated field of a negative charge, electric potential…

Answer 15

… is always negative. This is because no external work is required to move small positive test charge towards that isolated charge. The electric field itself does work accelerating the test charge towards the isolated negative charge. This results in a loss of electric potential energy

Question 16

When considering the electric field strength in a uniform, E=V/d, the symbol V represents…

Answer 16

… the potential difference between the plates

Question 17

When considering electric potentials, the symbol V represents… …and the symbol ΔV represents…

Answer 17

1) … electric potential (radial electric field)

2) … electric potential difference

Question 18

How much work is required to move a charge along an equipotential surface?

Answer 18

None, the potential is the same everywhere

Question 19

The potential difference between adjacent equipotentials is…

Answer 19

…constant

Question 20

Equipotentials meet field lines at…

Answer 20

… 90°

Question 21

Describe equipotential surfaces in a uniform field?

Answer 21

Equally spaced planes parallel to the plates

Question 22

What is the electric field strength inside a spherical charge? Why?

Answer 22

1) Zero
2) This is because the charge on the surface will equally distribute so that the potential is the same over the whole surface so there is no difference in potential from one side of the sphere to the other

Question 23

Why can there not be an electric field within an isolated charge? (Why must the electric field strength be zero?)

Answer 23

If the field were not zero, the charge would move (to the centre of the sphere where it is zero) to equalise the potential

Question 24

What are the similarities between gravitational fields and electric fields?

Answer 24

• The gravitational force between two masses and the electrostatic force between two charges both have laws that vary inversely with the square of separation
• The gravitational/electrostatic force between two masses/charges is directly proportional to the product of the two masses/charges
• Gravitational fields and electric fields can both be represented by field lines, which indicate the direction of the force on a point mass/charge in the field, who’s density indicates the strength of the field
• The expressions for gravitational/electric field strength near a point mass/charge both vary inversely with the square of the distance from the mass/charge
• Gravitational/electric potential is the work done per unit mass/charge in bringing a mass/charge from infinity to that point
• Both gravitational potential and electric potential are inversely proportional to the distance from the centre of the source of the field
• Gravitational fields and electric fields can be represented by equipotential surfaces, which are at right angles to field lines

Question 25

What are the differences between gravitational fields and electric fields?

Answer 25

• There are positive and negative charges, but only positive masses
• The gravitational force is always attractive, whereas the electrostatic force may be attractive or repulsive, depending on the polarity of the charges present
• The gravitational force is independent of the medium between the masses, whereas the size of the force between two charges can be affected by the material between them