Capacitors

Question 1

What is the Capacitor Symbol?

Answer 1

Question 2

What happens when the switch in this is closed, ignore the resistor?

Answer 2

1 - Electrons flow from negative terminal of the cell to the right hand plate

2 - Plates become negatively charged

3 - At the same time electrons flow from the left hand plate towards the positive terminal of the cell

4 - This flow of charge is an electric current

5 - There is an exponential change in charge build up

6 - Discharges when the P.D of the capacitor = the P.D of supply

Question 3

What is the Time Constant of a Capacitor?

Answer 3

• 37% - time taken for the charge to drop to this value
• Time Constant = CR

Question 4

Why do we use LN with Capacitors?

Answer 4

We use ln to turn the exponential in to a linear graph

Question 5

What is a Capacitor?

Answer 5

Is an electrical Component that can store electrical charge.

Question 6

What are Capacitors made up of?

Answer 6

Made of…

• Two conducting plates separated by an air gap or an insulating material

Question 7

What happens when a Capacitor is connected to a Power Source?

Answer 7

• Positive and Negative Charge build up on opposite plates
• Insulating material stops charge moving between the two plates.
• So a P.D is created
• This creates a uniform electric field

Question 8

What kind of Electric Field does a Capacitor create?

Answer 8

Uniform Electric Field between the plates

Question 9

Define Capacitance.

Answer 9

Is the amount of charge per unit voltage stored by a Capacitor

Question 10

What is the Capacitance equation?

Answer 10

C = Q / V

C - Capacitance in Farads (1 F)

Q - Charge (C)

V - Potential Difference (V)

Question 11

What is Capacitance measured in?

Answer 11

Farads (F)

Question 12

What units do we usually have to express Capacitance in?

Answer 12

μF - Microfarads ( x 10*-6)

nF - nanofarads ( x 10*-9)

pF - picofarads ( x 10*-12)

Question 13

Capacitors store…

Answer 13

Energy

Question 14

What happens in this circuit when the switch is flicked to the LEFT?

Answer 14

• Charge builds up on the plates of the Capacitor
• Electrical Energy, provided by the battery, is stored by the Capacitor.

Question 15

What happens in this circuit when the switch is flicked to the RIGHT?

Answer 15

• The charge stored on the plates will discharge through the bulb, converting electrical energy into light and heat.

Question 16

How do you calculate the electrical energy from the battery needed to charge the Capacitor?

Answer 16

Charge X Average P.D

Question 17

What is done when charging the capacitor?

Answer 17

WORK is done removing negative charge from one plate and depositing it into the other plates

Question 18

What is the energy stored by a Capacitor equal to?

Answer 18

The energy stored by a Capacitor is equal to the work done by the battery

Question 19

How do you find out the energy stored in a Capacitor graphically?

Answer 19

It’s the area underneath the graph of P.D against charge stored.

Question 21

Why is the P.D over Charge graph linear?

Answer 21

P.D across the Capacitor is PROPORTIONAL to the charge stored on it, so the graph is a straight line through the origin.

Question 22

Give the Equation for the energy stored by a Capacitor.

Answer 22

W = 0.5 x Q x V

W can also stand for E

Question 23

Where does the Equation W = 0.5 x Q x V come from?

Answer 23

W = 0.5 x Q x V comes from this graph

Question 24

What is the Capacitance Equation?

Answer 24

C = Q / V

Question 25

Give the Equation derived by using these equations for Work Done, W = 0.5 x Q x V & Q = CV ? Capacitor equations

Answer 25

W = 0.5 x C x V\*2

Question 26

Give the Equation derived by using these equations for Work Done, W = 0.5 x Q x V & V = Q / C ? Capacitor equations

Answer 26

W = 0.5 x Q\*2 / C

Question 27

Investigate what happens when you CHARGE a Capacitor.

Answer 27

- Set up the circuit - Close the switch to connect the uncharged Capacitor to the power supply - Let the Capacitor charge whilst the data logger Records both the potential difference (from the voltmeter) and the current (from the ammeter) over time. - You can use a computer to plot a graph of Charge, P.D or Current against Time. Like these...

Question 28

What does Q0 & V0 & I0 stand for?

Answer 28

Q0 - Charge on the Capacitor when it is fully charged V0 - Potential Difference across the capacitor when it is fully charged I0 - Maximum Current flowing through the circuit

Question 29

How do you generate a Q-t graph? CHARGING a Capacitor Experiment.

Answer 29

Because Q = I x t the charge is equal to the area under the graph at that point.

Question 30

Explain the shape of this Capacitor Graph. CHARGING a Capacitor Experiment. (LOOK AT THE AXIS)

Answer 30

- When the switches close, Current starts to flow. Electrons flow onto the plate connected to the negative terminal of the power supply, there is a build up of negative charge. - The negative charge on the plate repels electrons off the plate connected to the positive terminal of the power supply, making that plate positive. These electrons are attracted to the positive plate.

Question 31

Explain the shape of this Capacitor Graph. CHARGING a Capacitor Experiment. (LOOK AT THE AXIS)

Answer 31

An EQUAL but OPPOSITE charge builds up on each plate, causing Potential Difference between the plates. Remember that no charge can flow between the plates because they are separated by an insulator.

Question 32

Explain the shape of this Capacitor Graph. CHARGING a Capacitor Experiment.

Answer 32

- At first the Current flowing through the circuit is high. - But as charge builds up the electrostatic repulsion (of the electrons) makes it harder for electrons to be deposited. - When the P.D across the capacitor is equal to the P.D across the power supply, the current falls to zero - The capacitor is fully charged

Question 33

How do you Discharge a Capacitor?

Answer 33

Remove the power supply and close the switch

Question 34

Investigate what happens when you DISCHARGE a Capacitor.

Answer 34

- Let the Capacitor Discharge whilst the data logged Records Potential Difference and current over time. - When the current through the ammeter and the Potential Difference across the plates are zero the Capacitor is fully discharged - Then plot these graphs

Question 35

Explain the shape of these graphs. DISCHARGING a Capacitor Experiment.

Answer 35

When a Capacitor is discharging, the amount of charge and P.D between the plates falls exponentially with time. This is also true for current.

Question 36

What does the time it takes for a Capacitor to charge or discharge depend on?

Answer 36

- The Capacitance of the Capacitor (C), this affects the amount of charge that can be transferred at a given P.D - The resistance of the circuit, which affects the current

Question 37

What is the Capacitance equation?

Answer 37

C = Q / V

Question 38

What is meant by a Resistor-Capacitor Circuit?

Answer 38

A circuit that includes both a Capacitor and a Resistor

Question 39

What is the relationship between charge and time when a Capacitor is discharging?

Answer 39

The charge falls exponentially with time

Question 40

What is the equation for the charge left on the plates at a given time after a Capacitor begins discharging from being fully charged?

Answer 40

Q0 - Charge of the Capacitor when it’s fully charged (C) t - time since charging began (s) R - Resistance (Ohms) C - Capacitance (F)

Question 41

Give the equations for Potential Difference and Current as a Capacitor discharges.

Answer 41

V0 - P.D of the Capacitor when it’s fully charged (V) I0 - Current of the Capacitor when it’s fully charged (A) t - time since charging began (s) R - Resistance (Ohms) C - Capacitance (F)

Question 42

What is the time constant equation?

Answer 42

t = RC

Question 43

What do you get if you put the time constant into the Q = Q0e^ (-t/RC) discharging Equation?

Answer 43

Q = Q0e^-1 The same applies to V and I

Question 44

Define what is meant by the Capacitor time constant.

Answer 44

Is the time taken for the charge, P.D or current of a discharging Capacitor to fall to 37% of its value when fully charged.

Question 45

What is the time constant in terms of CHARGING a Capacitor?

Answer 45

Time taken for the Charge, P.D or Current to rise to 63% of its value when fully charged.

Question 46

What are the log rules for this, Log (A x B)?

Answer 46

Log(A) + Log(B)

Question 47

What are the log rules for this, Log (A^B x C)

Answer 47

B x Log(A) + Log (C)

Question 48

Show the equation Q = Q0e^ (-t/RC) in terms of natural logs and y = mx + c.

Answer 48

Ln(Q) = Ln(Q0) - t / RC

Question 49

Because you have the equation Ln(Q) = Ln(Q0) - t / RC in terms of natural logs and y = mx + c how do you plot a graph to get the TIME CONSTANT?

Answer 49

- Plot ln(Q) over time and you get a straight line - Gradient of this line is - 1 / RC - Y-Intercept is ln(Q0) - To get the time constant divide the gradient by -1

Question 50

Can this equation, Ln(Q) = Ln(Q0) - t / RC be expressed in terms of V and I?

Answer 50

YES!