Unit 4: Capacitors

Question 1

What is a capacitor?

Answer 1

A device used to store charge

Question 2

What is a capacitor made up of?

Answer 2

Two parallel metal plates placed near each other that are insulated from each other

Question 3

How does each plate of the capacitor gain an equal and opposite charge?

Answer 3

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

Question 4

What happens to the conductors when a capacitor is connected to a battery?

Answer 4

One of the two conductors gain electrons from the battery and the other conductor loses electrons to the battery

Question 5

What is meant by a capacitor storing a charge of Q?

Answer 5

One conductor stores charge +Q and the other conductor stores charge -Q

Question 6

Describe the set-up of the experiment that is used to charge a capacitor at constant current

Answer 6

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

Question 7

How can you investigate how the capacitor pd changes with time for a constant current?

Answer 7

You can use a stopwatch to measure the voltmeter reading at measured times OR use a data logger instead of a voltmeter

Question 8

What is the formula for Q in terms of I?

Answer 8

Q = It

Question 9

Describe the graph of charge against pd

Answer 9

A linear graph is produced that passes through the origin with charge on the y axis and pd on the x axis

Question 10

What is the capacitance, C, of a capacitor defined as?

Answer 10

The charge stored per unit pd

Question 11

What is the unit of capacitance and what is it equivalent to?

Answer 11

The unit is the Farad and it is equal to 1 coulomb per volt

Question 12

What is the formula for C in terms of V?

Answer 12

C = Q/V

Question 13

What type of circuits are capacitors used in?

Answer 13

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

Question 14

What type of power supplies are capacitors used in?

Answer 14

Back-up power supplies which are circuits that take over when the mains supply is interrupted

Question 15

Why is energy stored in a capacitor when it is charged?

Answer 15

Electrons are forced onto one of its plates by the battery and taken off the other plate

Question 16

What form of energy is the energy in a capacitor stored as?

Answer 16

Electric potential energy

Question 17

Describe what happens when a capacitor is discharged across a torch bulb

Answer 17

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

Question 18

What is the formula for the energy stored in a capacitor in terms of Q and V?

Answer 18

E = 0.5QV

Question 19

What does the pd across the plates increase in proportion to?

Answer 19

The charge stored

Question 20

What happens to the energy supplied by the battery in the charging process?

Answer 20

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

Question 21

Describe how you would measure the energy stored in a charged capacitor

Answer 21

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

Question 22

What does a thundercloud and the Earth below act like?

Answer 22

A pair of charged parallel plates

Question 23

What is the result of the thundercloud being charged?

Answer 23

A strong electric field exists between the thundercloud and the ground

Question 24

What is the formula for the potential difference between the thundercloud and the ground, V?

Answer 24

V = Ed where E is the electric field strength and d is the height of the thundercloud above the ground

Question 25

What is the formula for the energy stored by the thundercloud, E?

Answer 25

E = 0.5QEd

Question 26

What happens to the electric field strength when the distance between the ground and the thundercloud is increased?

Answer 26

It remains unchanged since it depends on the charge per unit area if the distance is increased

Question 27

What is the effect of the distance between the ground and the thundercloud increasing on the energy stored?

Answer 27

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

Question 28

When does the insulating property of air break down?

Answer 28

If it is subjected to an electric field strength of more than about 300kVm^-1

Question 29

What happens when a capacitor discharges through a fixed resistor?

Answer 29

The discharge current gradually decreases to zero, the resistor current decreases as the pd decreases

Question 30

Why does the current decrease gradually when a capacitor discharges through a fixed resistor?

Answer 30

The pd across the capacitor decreases as it loses charge

Question 31

What does exponential decay mean?

Answer 31

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

Question 32

What is the relationship between the current, the pd and the charge?

Answer 32

They are all proportional to each other , all three quantities decrease exponentially in capacitor discharge

Question 33

What is the relationship between I and RC?

Answer 33

I = Q/RC

Question 34

Does the charge on the plates ever reach zero?

Answer 34

No

Question 35

What is the capacitor discharge equation?

Answer 35

Q = Q0e^-t/RC

Question 36

What is the time constant?

Answer 36

RC

Question 37

What is the unit of the time constant?

Answer 37

Second

Question 38

What does the time constant represent?

Answer 38

The time taken for the charge to fall to 0.37 of its initial value

Question 39

What is the effect of R or C increasing on the discharge of a capacitor?

Answer 39

This would make the discharge of C through R slower so increasing the time for the capacitor voltage to decrease

Question 40

Describe the setup of investigating capacitor discharge

Answer 40

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

Question 41

What happens to the current as a capacitor is charged?

Answer 41

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

Question 42

What happens when the capacitor is fully charged?

Answer 42

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)

Question 43

What is the time constant of a circuit?

Answer 43

The time taken for the charge to reach 63% of the final charge

Question 44

What is the source pd equal to?

Answer 44

Resistor pd + capacitor pd: V0 = IR + Q/C and I0 = V0/R