Unit 4.1 Capacitance Flashcards by Shirou Aizen

What is a capacitor?
(3-way)

An electric device designed to
store electrical charge
and hence electrical energy

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What does a capacitor consist of?
(4-way)

2 conducting metal plates
parallel to each other
separated by an insulating material:
“dielectric”

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List 5 different uses of capacitors in a circuit?
(Pick and match type sh)

Local energy storage
Voltage spike suppression
Timing circuits
Signal filtering
Nuclear fusion research (high energy laser)

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How do u draw how a capacitor works?
(Get whiteboard)
(Booklet sorry)

Once again….

2 cells
the capacitor
electrons
Positive and negative sides

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Explain how a capacitor becomes charged when a battery is placed across it? PT. 1
(4-way)

Electrons from capacitor attracted to positive terminal of the battery
Leaves a positively charged plate on capacitor,
attracts electrons from negative terminal of the battery
Electrons stored on capacitor plate

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Explain how a capacitor becomes charged when a battery is placed across it? PT. 2
(3-way)

One plate = positively charged, other = negatively charged
Charge = equal
Once voltage across capacitor = battery, charging stops

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What is charge (Q) measured in?

Coulombs
(C)

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Do u know the circuit symbol for a capacitor?

Imagine paying to add pictures.

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Define capacitance
(2-way)

The charge per unit potential difference
across the 2 plates

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What is the formula for a capacitor?
(In data booklet)

C = Q/V

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Formula for a capacitor expressed in words?

Capacitance = charge on either plate/pd between plates

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Define C
(Capacitor formula)

Capacitance
(F aka Farad)

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Define Q
(Capacitor formula)

Charge
(C)

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Define V
(Capacitor formula)

Voltage
(V)

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1 Farad = ?

1 coulomb per volt

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1F = ?

1CV^-1

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How would u calculate t on a charged plate?

Q = It

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How would u find the charge stored on each conducting plate of the capacitor
(3-way)

Q = CV
They’re only positive & negative sides
therefore same answers but…

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Which capacitor will have greater capacitance in terms of distance between plates?
How come?
(3 things)

Smaller distance
Stronger field between plates
Stronger interaction between the charges

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Distance and Capacitance relationship?

They are inversely proportional
(C ∝ 1/d)

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Which capacitor will have greater capacitance in terms of area of plates?
How come?
(1 + 2-way)

Bigger area
More capacity
… to store electric charge

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Area and Capacitance relationship?

They are directly proportional
(C ∝ A)

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What’s the equation for capacitance?
(In data booklet)

C = ε₀A/d

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Define C
(Capacitance equation)

Capacitance
(F)

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Define A (Capacitance equation)

Area of plates (m²)

Define d (Capacitance equation)

Distance between plates (m)

Define ε₀ (Capacitance equation) (A constant)

Permittivity of free space (8.85 x 10^-12 **Fm^-1**)

What is a dielectric? (2-(1thing)-way)

- A material used in-between the 2 plates - within capacitor - That has effects on the value capacitance

How does a dielectric effect the value capacitance? (4 things)

- Has dipolar effects inside - If polarised, increases capacitance - Unpolarised = no effect - Polarises due to pd across it

How is a uniform electric field obtained with a capacitor in a circuit? (3 things)

- Between the 2 parallel metal plates w/ - distance apart (d) - & has a pd (v)

Describe field lines (3 things + () <- literally)

- Parallel to each other - At right angles to the plate - Directed from positive to negative plate - (Direction a positive test charge would move)

How do u increase the electric field strength (E)? (2 ways)

- Increase the pd (V) - Decrease plate separation (d)

Electric field strength equation? (In data booklet)

E = v/d

Define E (Electric field strength equation)

Electric field strength.... (Nc^-1)

Define v (Electric field strength equation)

Voltage, potential difference (V or Jc^-1)

Define d (Electric field strength equation)

Plate separation (m)

Tip for finding out the electric field strength? (2-way)

- As separation always very small… - E will always be the bigger answer

How is energy being stored in a capacitor? (3 things)

- When charging the capacitor, - work is being done - assembling the charge on each plate

How is the energy stored in a capacitor equation formed? (4 steps)

1. Voltage against charge 2. Straight line 3. Area under = energy stored 4. J = ½ QV

Therefore, energy stored in capacitor equation? (in data booklet)

J = ½ QV

What are the 2 alternate formulae to use to find energy stored in capacitor?

1. J = ½ CV² 2. J = ½ Q²/C

(Simple) How do u find total capacitance in series?

- Same to finding resistance in parallel - 1/ C_T = 1/C₁ + 1/C₂ + 1/C₃

(Simple) How do u find total capacitance in parallel?

- Same to finding resistance in series - C_T = C₁ + C₂ + C₃

What's another way of saying the total capacitance?

The effective capacitance

How to find voltage in capacitor in series? (Use data booklet)

Charge/Capacitance = Voltage

How to find voltage in capacitor in parallel? (Use data booklet)

Charge/Total capacitance = Voltage

How to find charge in capacitors in series? (3 steps)

1. Find total capacitance 2. Q = VC 3. Charge = same

How to find charge in capacitors in parallel? (We partly don't know yet but...)

1. Probably use the formula

What does it mean for a capacitor to discharge? (2 things)

- Provide electrons for the circuit - Like a battery

What 3 factors determine how long it takes a capacitor to fully discharge?

- Capacitance (C) - Voltage (V) - Resistance (R)

How is C and V a factor of how long a capacitor takes to discharge? (2-way)

- Determines how much charge, - is stored Q_o

How is R a factor of how long a capacitor takes to discharge? (2-way)

- Determines the size of current - that flows to discharge the capacitor

What type of decrease would the graph show on how the charge on the plates changes with time?

Exponential decay

How would u define exponential decay for the graph showing "charge on the plates changes with time"? (2-time)

- The percentage decrease, - is constant per unit time

What's the general formula for exponential decay? (In data booklet)

A = A_oe^-λx

What effects the rate of decrease of the exponential decay formula? (2-wayway)

- Decay constant - aka λ

What are the 3 measurements that can be subbed in exponential decreases for capacitor discharge?

- Charge - Current - Voltage

What's the exponential decrease formula for Q of capacitor discharge? (In data booklet)

Q = Q_oe^-t/CR

How do u find the exponential decrease formula for I and V? (2 things)

- Just swap the Q - Don't forget -t/CR

Formula for time constant (CR)? (In data booklet)

Q = Q_oe^-t/CR

Define Q (Time constant formula)

Charge at any time t (C aka coulombs)

Define Q_o (Time constant formula)

Initial charge (C aka coulombs)

Define t (Time constant formula)

Any time after discharging has begun (s aka seconds)

Define C (Time constant formula)

Capacitance of capacitor (F aka farads)

Define R (Time constant formula)

Resistance of load (Ohms)

What's the difference if CR is small? (2 features)

- Q quickly decays - Steeper curve on graph

What's the difference if CR is large? (2 features)

- Q slowly decays - Less steep curve on graph

How to show CR (time constant) must have units of seconds? (4 by 4 steps)

1. FxR 2. C = Q/V & R = V/I 3. It/v x V/I 4. = t

Definition of time constant? (3-way although cut 1 way…)

- Time constant, for a discharging capacitor, - is time taken for charge/current/voltage - to decrease to 37% of og val.

How do u find the "time constant" from the CR formula? (Wow... <- clue right there)

1. Take logs. 2. Use the whiteboard 3. That one chemistry lesson

What are the 2 ways to obtain a value for the capacitance of the capacitor with graphs?

- t = RC for normal graph @ 37% (y axis) - In v graph and do y = mx + c

The clarification is lost D: ^^

Lesson learned?

What's an opposite for a capacitor discharging?

.... the capacitor charging

What's the difference for capacitor charging? (3 things)

- **Only** charge & voltage increases - No current + no decrease - Different formula

Equation for capacitor charging? (**Not** in data booklet)

Q = Q_o(1 - e^-t/CR))

How can u tell on the graph that it's capacitor charging?

The opposite of capacitor discharging: - Goes up insteada down

Probably not the best..... once again.... however.

I'd probably spam ppq, surely it can make things more clarifying?