Electric Fields Flashcards
(26 cards)
Coulomb’s law
Coulomb’s law states that the magnitude of the force between two point charges in a vacuum is directly proportional to the product of their charges, and inversely proportional to the square of the distance between the charges
Define the electric field strength at a point in an electric field
(The electric field strength at a point) is the force per unit charge
On a (small) positive charge (at that point)
Coulomb’s law equation
Constant total = 8.99×10
^9
Air can be treated as a vacuum when using the above formula, and for a charged sphere, charge may be assumed to act at the centre of the sphere.
Compare the magnitude of electrostatic forces between subatomic particles
The magnitude of electrostatic forces between subatomic particles is magnitudes greater than the magnitude of gravitational forces, this is because the masses of subatomic particles are incredibly small whereas their charges are much larger.
Electric field strength (E)
Electric field strength (E) is the force per unit charge experienced by an object in an electric field. This value is constant in a uniform field, but varies in a radial field. There are three formulas you can use to calculate this value; the first is general, the second is used to find the magnitude of E in a uniform field formed by two parallel plates, while the third is used only for radial fields:
Electric uniform field
The field lines show the direction of the force acting on a positive charge. A uniform field exerts that same electric force everywhere in the field, as shown by the parallel and equally spaced field lines
Electric radial field
in a radial field the magnitude of electric force depends on the distance between the two charges.
Calculate the work done by moving a charged particle between the parallel plates of a uniform field
W=QΔV
W = work done (Joules)
Q = charge moved (Coulombs)
ΔV = potential difference between the plates (Volts)
If you want to express it in terms of field strength (E) and distance moved (d):
𝑊=𝑄𝐸𝑑
E = electric field strength (V/m)
d = distance moved in the direction of the field (m)
Uniform electric fields made by two parallel plates
Uniform electric fields made by two parallel plates can sometimes be used to find out whether a particle is charged, and whether its charge is negative or positive. This is done by firing the particle at right angles to the field and observing its path: a charged particle will experience a constant electric force either in or opposite to the direction of the field (depending on its charge), this causes the particle to accelerate and so it follows a parabolic shape. If the charge on the particle is positive it will follow the direction of the field, if the charge is negative it will move opposite to the direction of the field.
Absolute electric potential (V)
Absolute electric potential (V) at a point is the potential energy per unit charge of a positive point charge at that point in the field. The absolute magnitude of electric potential is greatest at the surface of a charge, and as the distance from the charge increases, the potential decreases so electric potential at infinity is zero. To find the value of potential in a radial field you can use the formula:
What does the value of potential is negative or positive depends on?
the sign of the charge (Q), when the charge is positive, potential is positive and the charge is repulsive, when the charge is negative, potential is negative and the force is attractive (similarly to gravitational potential which is always attractive).
The gradient of a tangent to a potential (V) against distance (r) graph gives?
the value of electric field strength (E) at that point: E = ΔV / Δr
Electric potential difference V Δ
is the energy needed to move a unit charge between two points. Therefore, the work done ( ) in moving a charge across a potential difference is equal W Δ to the product of potential difference and charge.
Electric fields equipotential surfaces
The potential on an equipotential surface is the same everywhere, therefore when a charge moves along an equipotential surface, no work is done. Between two parallel plates the equipotential surfaces are planes which are equally spaced and parallel to the plates, whereas equipotential surfaces around a point charge form concentric circles.
If you plot a graph of electric field strength (E) against distance (r), you can find?
the electric potential difference by finding the area under the graph.
Capacitance (C)
Capacitance (C) is the charge stored (Q) by a capacitor per unit potential difference (V).
C = capacitance (farads, F)
Q = charge stored on the plates (coulombs, C)
V = potential difference across the plates (volts, V)
A capacitor
is an electrical component which stores charge. It is made up of two conducting parallel plates with a gap between them which may be separated by an insulating material known as a dielectric. When the capacitor is connected to a source of power, opposite charges build up on the two parallel plates causing a uniform electric field to be formed.
permittivity (ε)
which is a measure of the ability to store an electric field in the material. Using this value, you can find the relative permittivity (εr ), also known as the dielectric constant of a dielectric, which is used to calculate the capacitance of a capacitor. You can calculate relative permittivity by finding the ratio of the permittivity of the dielectric to the permittivity of free space:
The capacitance of a capacitor (C) can be calculated by…
measuring the area of the plates (A), the distance between the plates (d), and the relative permittivity (εr ) of the dielectric in the gap of the capacitor
How is a dielectric formed?
A dielectric is formed of polar molecules, which are molecules with one end which is positive and one which is negative (these are demonstrated by the orange ovals on the diagram below). Usually, when there is no electric field, these molecules are arranged in random directions, however when an electric field is present these polar molecules will move and align themselves with the field, as shown in the diagram. The negative ends will be rotated towards the positive plate of the capacitor and the positive ends to the negative plate; each molecule has its own electric field, the strength of which depends on the dielectric’s permittivity, which will now oppose the field formed by the capacitor, reducing this field. Due to this, the potential difference required to charge the capacitor decreases (as electric field strength is decreased), causing capacitance to increase because . C = Q / V
The electrical energy stored by a capacitor (E) is given by…
the area under a graph of charge (Q) against potential difference (V). As potential difference is directly proportional to charge, this
In order to charge a capacitor you must…
connect it in a circuit with a power supply and resistor, as shown in the circuit diagram below:
Once the capacitor is connected to a power supply, current starts to flow and negative charge builds up on the plate connected to the negative terminal. On the opposite plate, electrons are repelled by the negative charge building up on the initial plate, therefore these electrons move to the positive terminal and an equal but opposite charge is formed on each plate, creating a potential difference. As the charge across the plates increases, the potential difference increases but the electron flow decreases due to the force of electrostatic repulsion also increasing, therefore current decreases and eventually reaches zero.
You could use a data logger to measure the values of potential difference and current in order to plot a graph of voltage and current against time.
As , you can draw a graph of Q = I × t charge against time by measuring the area under the current-time graph.
To discharge a capacitor through a resistor, you must…
connect it to a closed circuit with just a resistor. Again you could use a data logger to measure voltage and current to plot graphs of voltage, current and charge against time.
When the capacitor is discharging the current flows in the opposite direction, and the current, charge and potential difference across the capacitor will all fall exponentially, meaning it will take the same amount of time for the values to halve.
