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
What is the formula for the energy stored in a capacitor in terms of Q and V?
E = 0.5QV
What does the pd across the plates increase in proportion to?
The charge stored
What happens to the energy supplied by the battery in the charging process?
In the charging process, the battery supplies charge Q at pd V to the circuit and therefore transfers energy QV to the circuit - 50% of the energy supplied by the battery is stored in the capacitor and the other 50% is wasted due to resistance in the circuit as it is transferred to the surroundings when the charge flows in the circuit
Describe how you would measure the energy stored in a charged capacitor
A joulemeter is used to measure the energy transfer from a charged capacitor to a light bulb when the capacitor discharges; the capacitor pd V is measured and the joulemeter reading recorded before the discharge starts; when the capacitor has discharged, the joulemeter reading is recorded again and the difference of the two joulemeter readings is the energy transferred from the capacitor during the discharge process; this is the total energy stored in the capacitor before it discharged and this can be compared with the calculation of the energy stored using E = 0.5CV^2
What does a thundercloud and the Earth below act like?
A pair of charged parallel plates
What is the result of the thundercloud being charged?
A strong electric field exists between the thundercloud and the ground
What is the formula for the potential difference between the thundercloud and the ground, V?
V = Ed where E is the electric field strength and d is the height of the thundercloud above the ground
What is the formula for the energy stored by the thundercloud, E?
E = 0.5QEd
What happens to the electric field strength when the distance between the ground and the thundercloud is increased?
It remains unchanged since it depends on the charge per unit area if the distance is increased
What is the effect of the distance between the ground and the thundercloud increasing on the energy stored?
If the distance is increased then there is an increase in the energy stored as work is done by the force of the wind to overcome the electrical attraction between the thundercloud and the ground and make the charged thundercloud move away from the ground
When does the insulating property of air break down?
If it is subjected to an electric field strength of more than about 300kVm^-1
What happens when a capacitor discharges through a fixed resistor?
The discharge current gradually decreases to zero, the resistor current decreases as the pd decreases
Why does the current decrease gradually when a capacitor discharges through a fixed resistor?
The pd across the capacitor decreases as it loses charge
What does exponential decay mean?
Quantities decrease by the same factor in equal intervals of time, it occurs whenever the rate of change of the quantity is proportional to the quantity itself
What is the relationship between the current, the pd and the charge?
They are all proportional to each other , all three quantities decrease exponentially in capacitor discharge
What is the relationship between I and RC?
I = Q/RC
Does the charge on the plates ever reach zero?
No
What is the capacitor discharge equation?
Q = Q0e^-t/RC
What is the time constant?
RC
What is the unit of the time constant?
Second
What does the time constant represent?
The time taken for the charge to fall to 0.37 of its initial value
What is the effect of R or C increasing on the discharge of a capacitor?
This would make the discharge of C through R slower so increasing the time for the capacitor voltage to decrease
Describe the setup of investigating capacitor discharge
There is a capacitor connected to a switch, a battery and a resistor which leads off to an oscilloscope; the resistor has a very high resistance so the discharge current from the capacitor passes only through the fixed resistor; the oscilloscope measures the capacitor pd at regular intervals (could also use a data logger or digital voltmeter instead), the measurements can then be used to plot a graph of voltage against time and you can also determine the time constant
What happens to the current as a capacitor is charged?
When a capacitor is charged by connecting it to a source of constant pd, the charging current decreases as the capacitor charge and pd increases
What happens when the capacitor is fully charged?
Its pd is equal to the source pd and the current is zero as no more charge flows in the circuit, the capacitor charge builds up until the capacitor pd V is equal to the source pf V0, Q0 = CV0 (this is where the curve would flatten out)
What is the time constant of a circuit?
The time taken for the charge to reach 63% of the final charge
What is the source pd equal to?
Resistor pd + capacitor pd: V0 = IR + Q/C and I0 = V0/R
