Module 6 Flashcards
What are capacitors?
Electrical components in which charge is separated. They consist of two metallic plates separated from each other by an insulator.
What is capacitance?
The charge stored per unit p.d. across a capacitor.
What is the unit for capacitance?
Farads (F).
How do you calculate the capacitance of a capacitor?
C = Q/V
If two or more capacitors are connected in parallel, is the p.d. the same across them or different?
The p.d. across each capacitor is the same.
How do you calculate the total charge stored by capacitors in parallel?
The total charge stored is equal to the sum of the individual charges stored by the capacitors/
Qt=Q1+Q2+Q3+…
How do you calculate the total capacitance of capacitors connected in parallel?
The total capacitance is equal to the sum of the individual capacitances of the capacitors.
Ct=C1+C2+C3+…
When capacitors are connected in series, how do you calculate the p.d. across the combination?
The sum of the individual p.d.s across the capacitors.
V=V1+V2+V3+…
When capacitors are connected in series, is the charge stored by each capacitor the same or different?
Same.
How do you find the total capacitance of capacitors connected in series?
1/Ct=1/C1+1/C2+1/C3+…
How can you use a p.d.-charge graph to determine the energy stored in a capacitor?
Area under graph = work done.
W=1/2VQ
W=1/2V^2C
W=1/2Q^2/C
For a discharging capacitor, what do V-t. I-t, and Q-t graphs show?
Exponential decay.
What is the time constant (tau) of a capacitor-resistor circuit?
The product of the capacitance and resistance. It is equal to the time taken for the p.d./current/charge to decrease to e^-1 (about 37%) of its initial value.
What is the electrical field strength of an electrical field at a point in space?
The force experienced per unit positive charge at that point.
E=F/Q
What are the uses of capacitors?
Camera flash, provide back-up power for computers and emergency lighting for when the mains supply cuts out briefly.
Smoothing capacitors help to convert AC to DC.
How do you use electrical field lines to map electrical field patterns?
- the arrow on an electric field line shows the direction of the field
- electric field lines are always at right angles to the surface of a conductor
- equally spaces, parallel electric field lines represent a uniform field (one in which the field strength is the same everywhere)
- closer electric field lines represent greater electric field strength
What is the electric field strength of an electric field?
The force exerted per unit positive charge at that point.
E=F/Q
What is Coulomb’s law?
Any two point charges exert an electrostatic force on each other that is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
What are some differences between electric and gravitational fields?
- Gravitational fields are always attractive whereas Electric fields can be attractive or repulsive
- the property that creates a gravitational field is mass whereas the property that creates an electric field is charge
What are some similarities between electric and gravitational fields?
- Both force per unit (property creating charge)
- Force is inversely proportional to the separation squared
- Point masses/charges produce a radial field
How do you find the electric field strength between two parallel plates?
W=Fd
VQ=EQd
V=Ed
E=V/d
What does the capacitance of a capacitor depend on?
- separation between the plates
- the area of overlap between the plates
- the dielectric used between the plates
What is the equation for capacitance when a dielectric other than a vacuum is used?
C= epsilon(r)epsilon(0) A/d
How do you describe the motion of an electron travelling at right angles to an electric field?
Horizontally:
- there is no acceleration (ignore gravitational effects as mass of electron is so small)
- the time spent in the field is L/v
Vertically:
- vertical acceleration given by a= F/m = EQ/m
- initial vertical velocity is 0
- final vertical component of the velocity v is given by v=u+at. v=0+ EQ/m x L/v = EQL/mv