Unit 4: Capacitors Flashcards
What is a capacitor?
A device used to store charge
What is a capacitor made up of?
Two parallel metal plates placed near each other that are insulated from each other
How does each plate of the capacitor gain an equal and opposite charge?
When the plates are connected to a battery, electrons from the negative terminal of the battery flow onto one of the plates, an equal number of electrons leave the other plate and return to the battery via its positive terminal so each plate gains an equal and opposite charge
What happens to the conductors when a capacitor is connected to a battery?
One of the two conductors gain electrons from the battery and the other conductor loses electrons to the battery
What is meant by a capacitor storing a charge of Q?
One conductor stores charge +Q and the other conductor stores charge -Q
Describe the set-up of the experiment that is used to charge a capacitor at constant current
There is a switch, the capacitor is connected in series with a microammeter and a variable resistor which is continuously adjusted to keep the microammeter reading constant, there is also a high resistance voltmeter in parallel to the capacitor which enable the capacitor pd to be measured
How can you investigate how the capacitor pd changes with time for a constant current?
You can use a stopwatch to measure the voltmeter reading at measured times OR use a data logger instead of a voltmeter
What is the formula for Q in terms of I?
Q = It
Describe the graph of charge against pd
A linear graph is produced that passes through the origin with charge on the y axis and pd on the x axis
What is the capacitance, C, of a capacitor defined as?
The charge stored per unit pd
What is the unit of capacitance and what is it equivalent to?
The unit is the Farad and it is equal to 1 coulomb per volt
What is the formula for C in terms of V?
C = Q/V
What type of circuits are capacitors used in?
Smoothing circuits which are circuits that smooth out unwanted variations in voltage, timing circuits which switch on or off automatically after a preset delay, pulse producing circuits which switch on and off repeatedly, tuning circuits which are circuits that are used to select radios and TV channels and filter circuits which are circuits that remove unwanted frequencies
What type of power supplies are capacitors used in?
Back-up power supplies which are circuits that take over when the mains supply is interrupted
Why is energy stored in a capacitor when it is charged?
Electrons are forced onto one of its plates by the battery and taken off the other plate
What form of energy is the energy in a capacitor stored as?
Electric potential energy
Describe what happens when a capacitor is discharged across a torch bulb
It will release its energy in a brief flash of light from the bulb as long as the capacitor has been charged initially to the operating pd of the bulb