Capacitors and Electric Fields Flashcards
Define the electric field strength
The force per unit charge on a positive test charge placed at that point
Define capacitance
charge per unit potential difference
What is the Farad?
coulomb per volt
Define the Electric Potential
Work done per unit charge to bring a positive charge from infinity to that point in the field
Define the Electric Potential Energy
Work done to bring a small positive charge from infinity to that point in the field
What is the time constant of a circuit containing a capacitorand a resistor?
the time taken for current (or charge or voltage) to fall to 1/e of its initial value
How do you calculate the total capacitance for capacitors in parallel?
CT = C1 + C2 + C3 + ….
How do you calculate the total capacitance for capacitors in series?
1/CT = 1/C1 + 1/C2 + 1/C3….
If capcitors are connected in series what quantitiy is the same for all capacitors?
Charge stored
If capcitors are connected in parallel what quantitiy is the same for all capacitors?
p.d.
What is representd by the area under a p.d. - charge graphs?
Electrical energy transferred OR work done.
What is represented by the gradient of a charge-p.d. graph for a capacitor?
Capacitance
Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when discharging a capacitor?
Q, V and I
Out of Q, V and I which variable(s) follow the relationship x = x0 e-(t/RC) when charging a capacitor?
I
Out of Q, V and I which variable(s) follow the relationship x = x0 (1 - e-(t/RC) ) when charging a capacitor?
Q and V
What is meant by the constant-ratio nature of exponential decay?
In equal time intervals the quantity decreases by equal factors.
How much charge remains after 1 time constant when discharging a capacitor?
e⁻¹ OR 0.37 OR 37%
How much charge remains after* n* time constants when discharging a capacitor?
e^(-n)
What is represented by the gradient of charge-time graph when charging a capacitor?
Charging current
State a two uses of capacitors.
Energy storage e.g. for camera flashes, spot welding…; timing circuits; voltage smoothing; audio filters.
What does the direction of an electric field line show?
The direction of the force exerted on a positive test charge.
What does the density of electric field lines show?
Field strength.
When is the equation E = V/d used?
To find the uniform field strength between two parallel plates.
State Coulomb’s law in words.
The force between two point charges is proportional to the product of the two charges and inversely proportional to the square of their separation distance.
What is the relationship between the electrostatic force between two point charges and their seperation distance?
It is inversely proportional to the square of separation OR inverse square law
What is the relationship between potential and distance from a point charge?
Potential α 1/distance
What is a uniform electric field?
A region of space in which the electric field strength is the same at all points
What is a radial field?
An electric field generated by a point charge OR a field that obeys the inverse square law.
Is electric field strength a vector or a scalar quantity?
Vector
Is electrical potential a vector or scalar quantity?
Scalar
State a similarity between electric and gravitational fields.
Both obey inverse square law for field strength/an inverse proportionaility between potential (around a point mass/charge). Both have infinite range.
What are the differences between electric and gravitational fields (at least 2)?
Gravitational fields act on masses, electric fields on charges: electric fields are much stronger that gravitational fields: electric fields can be attractive or repulsive, gravitational fields are always attractive: potential is always negative in a gravitational field.
When is the equation V = Q / (4πε₀r) used?
To find the potential around a point charge/in a radial field.
When is the equation C = 4πε₀R used?
To calculate the capacitance on an isolated sphere.
What is represented by R in the equation C = 4πε₀R used?
he radius of an isolated sphere.
What is the relationship between electric potential energy and electric potential?
EPE = V x q where q is the charge in the field.
