AH 2.2.1 Capacitors Flashcards

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1
Q

In the circuit below the capacitor is initially uncharged.

The switch is moved to position 1.

a) what happens to the capacitor?
b) sketch the current v time graph for this situation.
c) sketch the voltage v time graph for this situation

A

a) The capacitor charges
b) & c) See the graphs below.

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2
Q

In the circuit below the capacitor is initially charged.

The switch is moved to position 2.

a) what happens to the capacitor?
b) sketch the current v time graph for this situation.
c) sketch the voltage v time graph for this situation.
d) Explain the sign of the current in the current discharge graph.

A

a) The capacitor discharges.
b) & c) See the graphs below which show both the charging and DISCHARGING of the capacitor.

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3
Q

a) Define the time constant τ in an RC circuit (a circuit containing a resistor R in series with a capacitror C).
b) State the equation for the time constant in an RC circuit and sate the unit of τ.

A

a)

τ = the time for the charge (or voltage) to increase to 63% of its initial value.

= the time for the charge (or voltage) to decay to 37% of its initial value.

b) τ = RC

τ has the unit of seconds

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4
Q

Insert the missing words below.

In an RC circuit, an uncharged capacitor can be considered to be fully charged after a time approximately equal to ………..

In an RC circuit, a fully charged capacitor can be considered to be fully discharged after a time approximately equal to ………….

A

In an RC circuit, an uncharged capacitor can be considered to be fully charged after a time approximately equal to 5τ.

In an RC circuit, a fully charged capacitor can be considered to be fully discharged after a time approximately equal to 5τ

Where τ = the time constant for the RC circuit

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5
Q

The graphs below show the graphs of current in an RC circuit.

a) In the left-hand graph is the capacitor charging or discharging?
b) In the right -hand graph is the capacitor charging or discharging?

The graphs show the effect of changing the values of resistance R or capacitance C.

c) Copy the graphs and replace the letters D, E, F & G with the either ‘resistor’ or ‘capacitor’

A

a) Charging
b) Charging
c) See the graphs below.

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6
Q

The circuit below may be used het investigate the way current behaves in a A.C. capacitive circuit.

a) AC current I is directly proportional to three different variables

What are they?

b) State the equation which relates I to these variables

A

a) I is directly prportional to

  • the supply voltage Vs
  • the AC supply frequency f
  • the capacitance itself C

b) I = 2πfVC

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7
Q

​Look at the circuit below which shows a capacitor with an AC supply.

Assuming the capacitance C is contant and the supply Voltage V is constant, sketch a graph to show the variation of current I with frequency f for a capacitive circuit.

A
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8
Q

a) State the equation for capacitive reactance in terms of voltage and current in an AC circuit.
b) Develop the equation to include frequency

A

a) Xc = V/I
b) Xc = V/I

= V / 2πfCV

Xc = 1 / 2πfC V cancels

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9
Q

State as many uses of capacitors as you can.

A
  1. Timing circuits (e.g. in a pelican crossing)
  2. High-pass electronic filter (e.g. cross-over network in loudspeaker)
  3. Flashing neon light circuit.
  4. Smoothing circuits (smoothing rectified AC)
  5. Radio receiver circuit
  6. Camera flash units
  7. Capitive touch screens
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10
Q

Look at the circuit below. The switch is now closed.

a) Sketch the charging graph of current v time
b) Calculate the current at switch-on and indicate this on your graph
c) Sketch the graph of voltage v time for the capacitor
d) Indicate the maximum voltage across the capacitor on your V-t graph
e) Add to your voltage graph a line indicating the voltage across the resistor
f) Calculate the charge stored on the capacitor.
g) If it takes 0.6 s for the capacitor to fully charge, find the average charging current

A
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