capacitors unit 4

Question 1

What is capacitance?

Answer 1

Charge stored per unit potential difference

Question 2

what are the units for capacitance?

Answer 2

farradas (F) which is C/V

Question 3

what is micro?

Answer 3

10^-6 μ

Question 4

what is nano?

Answer 4

10^-9

Question 5

what is pico?

Answer 5

10^-12

Question 6

what is the test circuit that you would set up to test capacitance?

Answer 6

have a battery connected to a variable resisitor an ampmeter the capacitor a switch and back to the battery. Have a voltmeter around the capacitor.

Question 7

what is the symbol for a capacitor?

Answer 7

Like the one for a battery but the lines have the same length.

Question 8

when a test circuit has been set up how can the stored charge get investigated?

Answer 8

Reduce the variable resistance to try and keep the current constant. (this is impossible when it is nearly fully charged)
record the voltage regularly until it is equal to the pd of the battery.

Question 9

when you have done the practical investigation of capacitance, what do you do with your data?

Answer 9

You can draw a graph of current against time. Current should be fairly constant. The area under the graph is the charge stored in the capacitor.
You can then draw a charge against p.d graph. The gradient of this is capacitance.

Question 10

How can the electrical energy stored in a capacitor be found out?

Answer 10

The area under a graph of pd by current.

E=0.5QV

Question 11

When a capacitor is connected to a battery, what happens to current?

Answer 11

It will flow around the circuit until the capacitor is fully charged then it stops.
It will start high but as charge builds up on the plates electrostatic forces of repulsion will make it harder for more electrons to be deposited. When the pd around the capacitor is the same as that of the battery the current will flow to 0.

Question 12

How does a capacitor build up charge?

Answer 12

The plate connected to the negative end of the battery will build up with electrons while the positive plate will lose electrons to the plate. An equal and oposite charge will build up in the plates causing a pd between them, though no charge can flow between them because they are separate.

Question 13

How can you discharge a capacitor?

Answer 13

Take out the battery and reconnect the circuit.

Question 14

what happens when a charged capacitor is connected around a resistor?

Answer 14

Charge will flow in the opposite direction to the charging current.

Question 15

When has a capacitor has been fully discharged?

Answer 15

The pd around it and the current in the circuit will be 0.

Question 16

What might a discharging graph for a capacitor look like?

Answer 16

V or Q against T. A smooth curve that starts at infinite gradient and ends at 0 gradient.
You could also have -V or -Q to show that the charge is flowing in the opposite direction to the charging current.

Question 17

What can the time taken to charge or discharge a capacitor depend on?

Answer 17

The resistance of the circuit. This will affect the current flowing.
The capacitance.

Question 18

When a capacitor is discharging, what is the relationship of the rate of fall of the amount of current?

Answer 18

The amount of charge left falls exponentially. It will take the same amount of time to halve.

Question 19

Why can V and Q both be used on the y axis on the discharging graph of a capacitor?

Answer 19

They are proportional to each other.

Question 20

What does Q0 mean in the decay of charge equation?

Answer 20

It is the charge when it is fully charged.

Question 21

What is the time constant?

Answer 21

Given by the symbol tau and it equals RC. If put it in the equation of charge decay you get the Q0 divided by e which will give around 37% of Q0.
It is the time for it to fall to 37% and also the time for a charging capacitor to get to 63% of Q0.

Question 22

In practise what is the time needed for a capacitor to fully charge or discharge?

Answer 22

Around 5RC

Question 23

Outline the essential features of a step-down transformer when in operation. (2)

Answer 23

primary coil with more turns than secondary coil (wound around) a core or input is ac

Question 24

Describe the energy changes that take place as the bob of a simple pendulum makes one complete oscillation, starting at its maximum displacement. (2)

Answer 24

grav) potential energy → kinetic energy → (grav) potential energy → kinetic energy → gravitational potential energy (1)
energy lost to surroundings in overcoming air resistance (1)

Question 25

If a capacitor is in a circuit adjusted for constant current, what are the only equations you need to use?

Answer 25

Q= I*T
Q= C*V

Question 26

The energy stored in a capacitor = 0.5QV. What is the work done by the battery and give reasons for any discrepancies

Answer 26

Work by cell = V*Q

Half the energy is lost in overcoming resistance of circuit.

Question 27

A capacitor is marked “0.22 F 15v” What do these two numbers mean?

Answer 27

0.22 of charge produced a p.d. of 1 v between the two terminals
15V is the maximum safe volatge or it is the point where the insulator starts to conduct.

Question 28

How would you find the maximum current in a circuit due to the discharge of a capacitor?

Answer 28

Voltage / resistance

Question 29

What does the graph of current look like when a capacitor is being charged?

Answer 29

An inversely proportional relationship, never touching the x axis