What happens when an object or material is polarised?
There is a slight redistribution of charge within the atoms and they effectively have a positive and negative end
State Coulomb’s law
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
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)?
The effect of the air is negligible
When dealing with interactions between charged subatomic particles, why can the force of gravity be considered as negligible?
The electrostatic force between a proton and an orbiting electron is over 10^39 times bigger than the gravitational force
How do you determine the direction of field lines?
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
Define electric field strength E at a point in a field
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
What is the equation for electric field strength in a uniform field? Radial field?
1) E=F/Q
2) E=Q/4πεor²
What is electric potential difference ΔV?
The work done per unit charge in moving the charge between two points
In a uniform field, the electric field strength…
… constant, meaning a charged particle in the field experiences the same force no matter where it is in the field
What is an electron gun?
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.
What is the equation of a parabola?
y = kx² where k = EQ/2mv²
What is the electric potential at a point defined as?
The work done per unit charge in bringing a small positive test charge from infinity to that point
Is electric potential scalar or vector?
Electric potential is a scalar quantity. Δ electric potential is a vector quantity
In the isolated field of a positive charge, electric potential…
… is always positive because external work must be done to move a small positive test charge towards that isolated charge
In the isolated field of a negative charge, electric potential…
… 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
When considering the electric field strength in a uniform, E=V/d, the symbol V represents…
… the potential difference between the plates
When considering electric potentials, the symbol V represents… …and the symbol ΔV represents…
1) … electric potential (radial electric field)
2) … electric potential difference
How much work is required to move a charge along an equipotential surface?
None, the potential is the same everywhere
The potential difference between adjacent equipotentials is…
…constant
Equipotentials meet field lines at…
… 90°
Describe equipotential surfaces in a uniform field?
Equally spaced planes parallel to the plates
What is the electric field strength inside a spherical charge? Why?
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
Why can there not be an electric field within an isolated charge? (Why must the electric field strength be zero?)
If the field were not zero, the charge would move (to the centre of the sphere where it is zero) to equalise the potential
What are the similarities between gravitational fields and electric fields?
- 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
