Electricity

Question 1

Define capacitance

Answer 1

The charge stored on one plate per unit of potential difference between the plates. (C = Q/V)

Question 2

Describe the process by which energy is stored on a capacitor

Answer 2

Work is done to push additional negative electrons towards the negative plate of the capacitor, against the repulsion of electrons already present, to increase the charge on the negative plate.

Question 3

Energy stored in a capacitor (E) =

Answer 3

Area under a voltage-charge graph
E = 1/2 VQ
E = 1/2 V^2 C
E = 1/2 (Q^2 / C)

Question 4

Total capacitance for capacitors in parallel =

Answer 4

Capacitance in parallel is added (C total = C1 + C2).

Total increases.

Question 5

Total capacitance for capacitors in series =

Answer 5

1/C total = 1/C1 + 1/C2

Decreases total.

Question 6

Define an electric field

Answer 6

A field of force created by an electric charge.

Question 7

State Coulomb’s law

Answer 7

Any 2 point forces exert a force on each other which is directly proportional to the product of their charges, and inversely proportional to the square of their separation. (F = (Q1Q2)/(4piE0 r^2) )

Question 8

Electrical field strength of a point charge in a radial field (E) =

Answer 8

E = F/Q = V/d
E = Q / (4piE0 r^2)

Question 9

State and explain the relationship between potential energy and distance moved in an electric field

Answer 9

Potential energy and distance moved against the field lines are directly proportional.
Work is done to move the charge against the field lines.
Therefore when a charged particle moves against the field lines it gains electrostatic potential energy.

Question 10

Define the electric potential at a point

Answer 10

The work done in bringing unit positive charge from infinity to that point.

Question 11

Potential gradient (m) =

Answer 11

Gradient on an electric potential - distance graph
m = - V/r
Potential gradient = -field strength (m = - E)

Question 12

Define root-mean-square current (Irms = )

Answer 12

The steady current which delivers the same average power as the a.c. to the resistive load. (rms = I0/rt2)

Question 13

Explain the advantage of using alternating current

Answer 13

The voltage can be changed (e.g. Stepped up or down)

Question 14

Explain the advantage of using higher voltages

Answer 14

The flow of current is reduced

So less power is lost as heat

Question 15

State the turns-ratio equation

Answer 15

Np/Ns = Vp/Vs

Question 16

Explain the source of energy loss in a transformer

Answer 16

1) Resistance of windings (small amount of energy lost as heat)
2) flux leakage (but only if designed badly)
3) Eddy currents (induced in the iron core by the magnetic field)
- cause energy to be lost as heat
- can be reduced by having core laminated

Question 17

Define rectification

Answer 17

The process of changing AC to DC using a diode.

Question 18

Explain the disadvantage of half-wave vs full-wave rectification

Answer 18

In half-wave rectification, the input power is reduced, because for half of the time voltage = 0.

Question 19

Explain the process of ‘smoothing’ (alternating current)

Answer 19

A capacitor is incorporated in parallel to the resistor R
The capacitor charges while voltage is present, and discharges when voltage = 0.
Produces an output voltage with a ripple.

Question 20

Define a sensing device

Answer 20

An electronic component with a property which changes when a physical quantity is changed.

Question 21

Define the piezo electric effect

Answer 21

When a force is applied to piezo-electric crystals, causing the crystals to change shape, an electric field is produced.

Question 22

Explain the source of the piezo electric effect

Answer 22

Piezo electric crystals consist pf a regular arrangement of positive and negative ions.
When the crystal is stressed, the movement of ions will produce a small voltage.

Question 23

Describe the function of a piezo electric transducer in a microphone

Answer 23

The sound wave hits the side of the sheet
Compressions and rarefactions cause the pressure to increase and decrease.
The crystal changes shape and a small voltage is produced.

Question 24

Describe the way in which LDRs operate

Answer 24

Made of a high resistance semi-conductor
If the light falling on the LDR is of a high enough frequency, photons will be absorbed.
These will cause electrons to be released from atoms in the LDR.
These free electrons can conduct electricity, so resistance is decreased.

Question 25

Describe the operation of an NTC

Answer 25

(Negative temperature coefficient)
Made from a semi-conductor material
As temperature rises, number of free electrons increases, so resistance decreases.

Question 26

Describe a metal-wire strain gauge (🔺R = )

Answer 26

Measures changes in resistance due to changes in length and cross-sectional area. (🔺R = pl/A)

Question 27

Describe how LDRs and thermistors can be incorporated in an electric circuit, to function as sensing devices

Answer 27

The LDR/thermistor is placed in series with a fixed resistor (forming a potential divider).
The voltage is shared in proportion to the resistance.
Vout = Vin (R2 /(R1+R2))

Question 28

Define the gain of an op-amp

Answer 28

The magnitude by which the input voltage is amplified (G0 = output voltage / input voltage).

Question 29

State the properties of an ideal op-amp (6)

Answer 29

1. Infinite open-loop voltage gain
2. Infinite input resistance / impedance
3. Zero output resistance / impedance
4. Infinite bandwidth
5. Infinite slew rate
6. Zero noise contribution

Question 30

State an equation for the op-amp as a comparator

Answer 30

Vout = G0 (V+ ➖ V-)

Question 31

Define negative feedback

Answer 31

When the output of an op-amp is connected to the inverting (-) input
So as to reach a point of equilibrium.

Question 32

Closed loop gain (G) =

Answer 32

G = Vout/Vin = G0/1+G0

Question 33

State the benefits of negative feedback

Answer 33

1) Less distortion
2) increased bandwidth
3) A more stable gain,
4) A low output impedance and high input impedance

Question 34

Describe an inverting amplifier

Answer 34

The non-inverting input is connected to the 0V line.
Vin is connected to the inverting input.
Part of the output voltage is also connected to the inverting input.

Question 35

Explain the virtual earth approximation

Answer 35

Point connecting the output voltage with Vin is referred to as a virtual earth.
Because the gain is very large (so difference between V(-) and V(+) is around 0)
V(+) is earthed, so both points must have almost 0 voltage.

Question 36

State an equation for the gain of an inverting amplifier

Answer 36

G = - Rf/Rin

Question 37

State an equation for the gain of a non-inverting amplifier

Answer 37

G = 1 + (R1/R2)

Question 38

Define a relay

Answer 38

An electromagnetic switch used to switch on a large current circuit using a small current.

Question 39

Explain the purpose of the reverse-biased diode (D1) in an electric circuit

Answer 39

Switching off a relay can damage an op-amp, because there will be a rapid fall in the magnetic flux within the coil, inducing a large e.m.f.
D1 can pass current around the coil without damaging the op-amp

Question 40

Explain the purpose of resistors in a circuit containing LEDs

Answer 40

Resistors added in series to limit current reaching diode,

Because large currents can damage LEDs