Aqa Physics A-Level: Capacitors

Front 1

Tests for measuring how voltage changes as a capacitor discharges:

Back 1

Use a high-resistance voltmeter connected in parallel with the capacitor. Keep the current constant by constantly adjusting a variable resistor. Use a stopwatch to measure the voltmeter reading at regular intervals or use a data logger. The graph produced will have a straight line whose gradient is equal to the capacitance of the capacitor.

Front 2

Capactiance

Back 2

The charge stored per unit pd.

Front 3

Energy stored in a capacitor

Back 3

1/2QV = 1/2CV^2 = 1/2 Q^2/C

1/2 of the energy supplied by the battery is stored in the capacitor, the other half is wasted due to resistance in the circuit.

Front 4

Measuring the energy stored in a charged capacitor.

Back 4

Using a joulemeter, measure the reading when the capacitor is fully charged. Then when the capacitor has fully discharged record the joulmeter's reading again. The energy stored in the capacitor/the energy transferred from the capacitor is the difference in these two readings.

Front 5

Dielectric

Back 5

An electrically insulating material that increases the ability of a parallel-plate capacitor to store charge when placed between the plates.

Front 6

Functionality of a dieletric

Back 6

When placed between the oppositely charged plates, each molecule of the dielectric becomes polarised. Its electrons are pulled slightley towards the positive plate so the surface of the dielectric ear the positive plate gains a negative charge at the expense of the other side which gains a positive charge.If the molecules are already polarised (polar molecules) then they align themselves such that the positive side faces the negative plate and the negative side faces the positive plate. More charge is stored on the plates because:

The positive side of the dielectric attracts more electrons from the battery onto the negative plate.

The negative side of the dielectric pushes electrons back to the battery from the positive plate.

Front 7

Relative Permittivity (εr) =

Back 7

Q/Q0 Where Q is the charge stored when the space is completely filled by the dielectric and Q0 is the charge stored when the space is completely empty.

Front 8

Capacitance =

Back 8

Aε0εr/d

Front 9

Orientation polarisation

Back 9

Occurs in substances which contain molecules where covalent bonds are formed between atoms of different elements. The two atoms form a permanent dipole in which one atom is positively charged and the other is negatively charged due to the electrons being shared unequally. When an electric field is applied the two atoms in each bond are displaced in opposite directions so the dipole tends to align with the field by turning slightly. When the field is absent they lie in random directions.

Front 10

Ionic polarisation

Back 10

Occurs in substances where ions are held together by ionic bonds. The oppositely charged ions of each ionic bonds are displaced in opposite directions when an electric field is applied. As a result the ions of each bond form a dipole that tends to align with the field.

Front 11

Electric polarisation

Back 11

Occurs where the electrons of each atom are displaces relative to the nucleus of the atom when an electric field is applied. The centre of the election distribution no longer coincides with the nucleus. As a result the election distribution and the nucleus from a dipole that tends to align with the field.

Front 12

Capacitor in an AC field

Back 12

The polar dipoles rotate and the non polar dipoles oscillate one way then the opposite way as the field strength increases and decreases.At low frequencies, the three polarisation mechanisms alternate in phase with the field. As the frequency increases, each mechanism ceases to work due to the inertia of particles involved and the resistive forces that oppose the motion of the dipoles.

The mass of the particles being moved determines which mechanism ceases first.

Orientation polarisation ceases first, then ionic followed by electric.