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A Level Physics - LVI B > Capacitors > Flashcards

Flashcards in Capacitors Deck (19)
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What is a capacitor?

A device designed to store charge.


How does a capacitor charge?

Two parallel metal plates placed near each other. When connected to a battery, electrons move through the battery and are forced onto one of the plates. this makes the plate become negative. On the other plate, an equal number of electrons are repelled making that plate positive.


What is the symbol for a capacitor in a circuit?

Two equal length parallel lines.


What would a charge against pd graph for a capacitor look like? what can be found from this graph?

A straight proportional line passing through the origin.
Area = work done = energy stored


Define capacitance

The charge stored per unit pd.


How is the energy stored in a capacitor obtained from a graph?

Charge against pd graph
Area under is a triangle of QV/2


In what proportionality does current, charge and pd of a capacitor decrease when discharging?



What is the time constant of a circuit?

the rate of discharge of a capacitor. the time take for the voltage/charge/current to fall to 1/e (37%) of its original value
time constant = RC


When t=RC in a capacitor's discharge, what percentage charge remains?



When t=2RC in a capacitor's discharge, what percentage charge remains?



At what point, is a capacitor considered discharged?

When t=5RC


Define a farad

One coulomb of charge stored per volt


What factors affect the capacitance of a capacitor?

surface area of plates
Separation of plates


Define permittivity

Permittivity is a measure of how difficult it is to generate an electric field in that medium. The higher the permittivity of the material the more charge needed to generate an electric field.


Define relative permittivity

Relative permittivity is the ratio of the permittivity of a material to the permittivity of free space, it is sometimes known as the dielectric constant.


Explain permittivity

When no charge is stored by a capacitor, no electric field is generated, the polar molecules are randomly aligned.
When a charge is applied to the plates of a capacitor an electric field is generated between them. The negative ends of the molecule are attracted to the positive plate and vice versa. This causes the molecules to rotate and align them selves anti-parallel to the field.
The molecules each have their own electric field, which now opposes the applied electric field of the capacitor. The larger the permittivity, the larger the opposing field. This reduces the overall electric field between the plates and therefore reduced the p.d. needed to transfer a given charge to the capacitor. This increases the capacitance (Q = CV).


If you measure voltage and time for a capacitor discharging, what graph would you plot to obtain a straight line? What could you find from this graph?

ln V against t
Gradient = - 1/RC
Intercept = ln Vo


What are the units of time constant



How can you prove a graph is exponential?

An exponential graph always shows a constant ratio property. This means in equal amount of time the Y axis will always decrease by the same ratio.