Chapter 18. Electrical fields Flashcards
(25 cards)
Explain charging by induction
When a charged object is brought near a neutral object, the electrons in the neutral object are repelled or attracted by the charged object. This causes a separation of charges in the neutral object, with electrons accumulating on one side and protons on the other.
Define an electric field
A region of space where a stationary electric charge experiences a force.
Define the direction of an electric field
The direction of the electric field is the direction of the force on a positive test charge.
Explain why there is no resultant electric field in a conductor in electrostatic equilibrium
In a conductor, the free electrons are not bound to any particular atom and can move freely. When an electric field is applied, the electrons move in such a way as to cancel out the field inside the conductor. They move until the field is equal and opposite to the applied field.
How can a conductor be charged by induction
A conductor can be charged by induction by bringing a charged object near it without touching it. The charges in the conductor will be repelled or attracted by the charged object, causing a separation of charges. The conductor can then be grounded, allowing the charges to flow away, leaving the conductor with a net charge.
Define electric field strength at a point
The force per unit charge acting on a small stationary positive test charge placed at that point. E = F/q (NC-1)
Define a uniform electric field
An electric field in which the lectric field strength is the same at all points in the field.
Derive the formula for electric field strength of a uniform electric field
There is a force F acting on a positive charge in the direction of the field. So work must be done to bring the charge to the positive plate. The work is given by Fd. From the definition of a volt, V = W/q so W = Vq = Fd, but F/q = E = V/d hence electric field strength = V/d. N.B It is assumed that voltage changes at a constant rate with d, so E = △V/△d
Derive the formula for the velocity of a positive charge in a uniform electric field parralel to the field
Work done by the particle = gain in kinetic energy. so Vq = (1/2)mv2 so v = √(2Vq/m) OR F = Eq = ma so a = Eq/m using v2 = u2 + 2as, we get v =2 = 0 + 2(Eq/m)d but E = V/d so = 2(Vq/md)d = 2Vq/m so v = √(2Vq/m)
How would you deal with a charged particle with initial velocity going perpendicular to a field?
You would apply parabolic motion. N.B The particle continues in a straight line after passing through the field.
What is the direction of field lines on a positively charged sphere?
Outwards
State Coulomb’s law
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them. F = Q1Q2/(4πε0r2)
Show understanding of the behaviours of charges on a conductor and on an insulator
On a sconductor the charges spread evenly but on an insulator they remain static.
What is the ε0?
The permittivity of free space is 8.85 x 10-12 Fm-1 in a vacuum. Can also be used in air but for air its 1.0005ε0
What are the effects of a charged particle on the electric field of a small positive charge?
The field is distorted by the presence of the charge. The field is stronger closer to the charge and weaker further away. Instead of being radial, the field lines are curved. It would be radial if it was an isolated charge.
What is the formula for electric field strength at a point due to a point charge and derive it?
- From Coulomb’s law, F = Qq/(4πε0r2) so E = F/q from the definition of E. 2. E = Q/(4πε0r2)
Why do we need a reference point to measure potential?
We can only measure potential differences. We need a reference point to define the potential as 0.
What is electric potential at a point?
The work done per unit positive charge in bringing a small test charge from infinity to the point.
What is the formula for electric potential?
V = Q/(4πε0r)
Define potential gradient
The potential difference per unit distance. The rate of change of potential with distance.
Formula connection between potential gradient and electric field strength
E = -potential gradient
Show understanding of why potential gradient is negative when electric field strength is positive
They act in different directions. When potential decreases a positive charge tends to move in the direction of the field, thus potential gradient is negative but electric field strength is positive.
Describe the potential gradient in a uniform electric field
The potential decreases uniformly with distance. Hence potential gradient is constant and so is electric field strength. But they are still in opposite directions.
Describe the graph of potential against distance of a point charge
A decreasing curve that has a decreasing gradient.
