6.1 Capacitors

Question 1

What is the unit for capacitance?

Answer 1

Farad (F)

Question 2

What is a capacitor and what is it made up of?

Answer 2

An electrical component that can store electrical charge.

Made up of two conducting plates separated by gap or a dielectric.

Question 3

Define capacitance

Answer 3

Amount of charge per unit voltage stored by a capacitor

Question 4

What is a dielectric?

Answer 4

An insulating material

Question 5

What is the circuit symbol for a capacitor?

Answer 5

2 parallel lines

Question 6

What happens when you connect a power source to a capacitor?

Answer 6

A positive and negative charge builds up on opposite plates. The insulating material stops charge moving between the two plates, a P.D is created.

Question 7

What is the total capacitance of capacitors in series?

Answer 7

1/C = 1/C1 + 1C2 + 1/C3 …

Question 8

What is the total capacitance of capacitors in parallel?

Answer 8

C = C1 + C2 + C3

Question 9

How do you investigate capacitors in series and parallel?

Answer 9

1. Set up test series circuit - Measure A, P.D - Add variable resistor and closed switch.
2. Constantly adjust variable resistor, to keep charging current constant.
3. Use data logger connected to voltmeter to record P.D over time-When capacitor fully charged, open switch.
4. Rearrange into a parallel circuit-make sure capacitors are discharged. Repeat step 2.
5. Plot graphs of current against time (Boxy) and charge against P.D (liney)
   * Using Q=IT*

Question 10

What is created between the plates in a capacitor?

Answer 10

Uniform electric field

Question 11

What is the area under the graph of a Current time graph?

Answer 11

Total charge stored from all capacitors

Question 12

What is the gradient of a Charge against P.D. graph?

Answer 12

Total capacitance (Q/P.D)

Question 13

What’s the equation for energy stored by a capacitor?

Answer 13

W=1/2 QV

Question 14

How is work done in a capacitor?

Answer 14

Work is done removing negative charge from one plate and depositing it onto the other plate. (Charging the capacitor)

Question 15

What are the three expressions for the energy stored by a capacitor?

Answer 15

W=1/2 V^2 C
W=1/2 Q V
W=1/2 Q^2/C

Question 16

What are the uses for capacitors?

Answer 16

Storage of energy so:

1. Flash photography - capacitor discharges quickly to give a short pulse of high current to create a flash
2. Back up power supplies - many large capacitors release charge for a short period if power supply goes off
3. Smoothing out p.d - when converting AC to DC power, capacitors charge up during the peaks and discharge during the troughs, maintain constant output.

Question 17

What is the area under V against Q graph?

Answer 17

Energy stored (W)

Question 18

Explain Techniques and procedures to investigate the charge and the discharge of a capacitor using both meters and data-loggers

Answer 18

1. Set up the test circuit in the diagram
   (Resistor capacitor-V-, A, DC power supply)
2. close switch to connect the uncharged capacitor to the power supply
3. let the capacitor charge whilst the data logger records both the p.d. (from voltmeter) and the current (from the ammeter) over time
4. when the current from the ammeter is zero, the capacitor is fully charged
5. use computer to plot a graph of charge, p.d. or current against time.

Question 19

Explain how the plates of the capacitor become charged

Answer 19

Electrons flow negative to positive

When switch closes current flows

Electrons flow on to plate connected to the negative terminal power supply, so negative charge builds up

Equal but opposite charge builds up on each plate, causing p.d. between plates

Initially current is high, but as charge builds up on plates, electrostatic repulsion makes it harder for more electrons to be deposited, when p.d. across power supply and capacitor is equal, current falls to 0.

Capacitor is fully charged.

Question 20

Define time constant

Answer 20

The time taken for the p.d / current / charge to decrease to 1/e(37%) of its (initial) value.

t=CR

Question 21

Which out of: current, voltage and charge has the same equation for charging as equation for discharging?

Answer 21

current