AQA A Level Physics复习笔记7.6.1 Capacitance

• Capacitors are electrical devices used to store energy in electronic circuits, commonly for a backup release of energy if the power fails
• They are in the form of two conductive metal plates connected to a voltage supply (parallel plate capacitor)
  • There is commonly a dielectric in between the plates, this is to ensure charge does not freely flow between the plates

   

• The capacitor circuit symbol is:

The capacitor circuit symbol is two parallel lines

• Capacitors are marked with a value of their capacitance. This is defined as:

The charge stored per unit potential difference (between the plates)

• The greater the capacitance, the greater the energy stored in the capacitor

• The capacitance of a capacitor is defined by the equation:

• Where:
  • C = capacitance (F)
  • Q = charge (C)
  • V = potential difference (V)

   

A capacitor used in small circuits

• Capacitance is measured in the unit Farad (F)
  • In practice, 1 F is a very large unit
  • Often it will be quoted in the order of micro Farads (μF), nanofarads (nF) or picofarads (pF)

   

• If the capacitor is made of parallel plates, Q is the charge on the plates and V is the potential difference across the capacitor
  • The charge Q is not the charge of the capacitor itself, it is the charge stored on the plates

   

• This capacitance equation shows that an object’s capacitance is the ratio of the charge stored by the capacitor to the potential difference between the plates

Step 1: Write down the known quantities

• • Capacitance, C = 1 nF = 1 × 10-9 F
  • Potential difference, V = 0.3 kV = 0.3 × 103 V

   

Step 2: Write out the equation for capacitance

Step 3: Rearrange for charge Q

Q = CV

Step 4: Substitute in values

Q = (1 × 10-9) × (0.3 × 103) = 3 × 10-7 C = 300 nC

• A dielectric is made up of many polar molecules
  • These are molecules that have a 'positive' and 'negative' end (poles)

   

• When no charge is applied to the capacitor:
  • There is no electric field between the parallel plates and the molecules are aligned in random directions

   

• When there is a charge applied:
  • One of the parallel plates becomes positively charged and the other negatively charged hence an electric field is generated between the plates (from positive to negative)
  • The negative ends of the polar molecules are attracted to the positive plate and vice versa
  • This means all the molecules rotate and align themselves parallel to the electric field

   

Polar molecules align themselves when an electric field is between two parallel plates