Capacitance COPY Flashcards
What is capacitance measured in?
Farads (F)
What is the most common type of capacitor?
The parallel plate capacitor
Define the capacitance of a capacitor
The charge stored per unit potential difference between it plates
What is the equation for capacitance?
C = Q/V
What does the graph of Q (on y) against V look like?
Straight line through the origin
What is the potential difference across a capacitor directly proportional to?
The charge stored on it
What does the gradient of a Q (on y) against V graph represent?
Capacitence
Describe how a capacitor is charged
- A p.d. is applyed across an uncharged capacitor
- There is a flow of e- in the circuit
- The plate connected to the negitive terminal of the battery has e- pushed onto it so becomed negatively charged
- On the other plate e- are drawn from it to the positive terminal of the battery. The plate is now positively charged.
How do you know when a capacitor is fully charged?
- The plates have opposite charges, so there is a p.d. across them
- When the p.d. across the capacitor is equal to that of the battery the charging stops
- There is no longer current flow and each plate is left with equal and opposite amounts of charge
What is a diaelectric?
An insulator
Define permittivity
Permittivity is a measure of the difficulty of forming an electric field within a particular medium
What does it mean if something has a high permittivity?
A large charge is required to produce an electric field
What is the permittivity of free space?
8.85 x 10-12 Fm-1
Why is an electric field easy to create in a vacuum?
The permittivity of free space is very low so it is relatively easy.
What is the relative permittivity of a material also known as?
The dielectric constant
What is the dielectric constant equal to? (er = e1 / e0)
The ratio of the permittivity of a material to the permittivity of free space.
What is the relative permittivity of a material equal to?
The ratio of the permittivity of a material to the permittivity of free space. ( er = e1 / e0)
What happens when a polar molecules are placed in an electric field?
- They will rotate and align with the electric field.
- The allignment is anti-parallel so that the + end of one molecule faces the - of the capacitor plate and vice versa
What happens to the value of er when polar molecules are put in an electric field?
- They have the effect of shielding the electric field
- er increases
Why does er equal 1 in a vacuum?
There are no polar molecules hence no opposition to the field and so er = 1
What does a capacitor consist of?
Two pates separated by a dielectric material
What does the capacitance of a capacitor depend on?
- The area of the plates
- The distance between the plates
- The relative permittivity of the dielectric
What is the parallel plate equation concerning capacitors?
What does the work done depend on?
The charge transferred and the potential difference
W = Q/\V
How do you calculate the energy stored on a capacitor from a V (on y) against Q graph?
The energy stored on a capacitor with a charge Q and a voltage V is given by the area underneath the graph
What does the energy of a capacitor equal in terms of Q and V?
E = 0.5QV
How much energy supplied by the power source is gained by the capacitor?
Exactly half
The rest is lost as heat
What does the energy of a capacitor equal in terms of C and V?
Derive E = 0.5CV^2
What does the energy of a capacitor equal in terms of Q and C?
E = Q^2 / 2C
Derive E = Q^2 / 2C
How does a capacitor discharge?
- The charge decreases exponentially with time as e- on the negitive plate flow around to the positive plate
What is the equation for the charge on a discharging capacitor?
What is the equation for voltage on a discharging capacitor?
What is the equation for current on a discharging capacitor?
What is the graph of voltage against time for a discharging circuit?
What is the graph of charge against time for a discharging circuit?
What is the graph of current against time for a discharging circuit?
What is I (0) equal to for a discharging circuit?
What is charge equal to for a charging capacitor?
What the the current equal to for a charging capacitor?
What the the charge equal to for a charging capacitor?
What does the graph look like for current (on y) against time for a charging capacitor?
What does the graph look like for charge (on y) against time for a charging capacitor?
What is the voltage equal to for a charging capacitor?
How does the charging capacitor behave when there is a resistor in the circuit?
The p.d across the resistor decreases as the capacitor increases until the p.d. = the V supplied.
What is RC (not the definition)?
The time constant for the discharging circuit. It is measured in seconds.
Define the time constant
The time constant is the time for the charge / current / voltage to decrease to 1/e of the original value or approximately 37% of the original value in a discharging circuit.
What happens to the value of the voltage if t = RC?
Derive a formula that tells us the time it takes for the voltage on a discharging circuit to half.
Sketch and label a graph of ln V against t.
