Term 2

Question 1

Define capacitance and farad

Answer 1

The charge stored on one plate per unit of potential difference between the plates.

The unit of capacitance is the farad. Equivalent to Charge per Volt.

Question 2

State the formula for capacitance

Answer 2

C = Q/V

Question 3

Describe how the formulas for capacitance is derived for capacitors in parallel and series

Answer 3

Parallel:

• Total charge is the sum of charge going through each capacitor
• As voltage is constant the capacitance becomes C = C1+ C2…

Series:

• Total voltage is the sum of the potential differences of the capacitors.
• As charge is constant capacitance becomes C = 1/C1 + 1/C2

Question 4

State the formulas required to calculate the work done in charging up a capacitor

Answer 4

• area under graph of p.d against charge is equal to work done
  W = 1/2 QV
  W = 1/2 CV^2
  W = 1/2 (Q^2/C)

Question 5

Describe what happens if a capacitor is charged and then connected to a second capacitor

Answer 5

• Charge stored is equivalent to charge when the initial capacitor is charged.
• This means the charge is shared between the capacitors in proportion to their capacitances.

Question 6

Describe how light dependent resistors work

Answer 6

• made of high resistance semiconductor
• as light intensity increases resistance decreases, non-linear relationship
• if light falling on LDR is of high enough frequency then electrons are released from the semiconductor.
• these free electrons conduct electricity and resistance is decreased.

Question 7

Describe how thermistors work

Answer 7

• negative temperature coefficient thermistors mean that as temp rises, resistance falls
• made of semiconductor material
• relationship is not linear

Question 8

Describe how piezo electric transducers work and explain their use in microphones

Answer 8

• contains crystals that when force is applied the shape changes and an electric field is produced
• consists of positive and negative ions in regular arrangement so when stressed a small voltage is produced.
• microphones contain the crystal which is made into a thin sheet with metal connections on opposite ends.
• Sound wave causes compressions and rarefactions causing pressure to increase and decrease and small voltage is created across connections.

Question 9

Describe how a metal wire strain gauge works

Answer 9

• As wire is stretched metal wire becomes longer and narrower increasing electrical resistance
• opposite occurs if wire is compressed
• change in length of wire is directly proportional to change in resistance.

Question 10

Describe the properties of an ideal op-amp

Answer 10

Infinite open loop voltage gain: With no feedback loop then a small input signal will be amplified to an ‘infinite’ output signal resulting in output being saturated.

Infinite input resistance: no current is drawn from the supply meaning there are no lost volts and input voltage is max.

Zero output resistance: no lost volts in current supplied by op amp

Infinite bandwidth: Range of frequencies that are amplified by the same amount. Ideally all signals of all frequencies are amplified the same.

Infinite slew rate: Means output is produced instantaneously as input is changed. No time delay.

Zero noise contribution: Does not produce any noise itself

Question 11

State the formula for open loop gain

Answer 11

G = Vout/ (V+ - V-)

Question 12

Describe the use of op amps as a comparator

Answer 12

• compares V+ and V-
• if V+ is slightly greater than V- then Vout will be equal to positive power supply voltage and vice versa.
• Tells us which input is larger
• Can also be used to compare light levels or temperatures if LDR or thermistor is used in place of fixed resistor.

Question 13

State the formula for closed loop gain

Answer 13

G = Go/ (1+Go)

Question 14

State the benefits of using negative feedback

Answer 14

• less distortion
• increased bandwidth
• gain is more stable
• output impedance is low and input impedance is high.

Question 15

Describe the virtual earth approximation

Answer 15

• Vin must be very close to 0 so that the op-amp is not saturated
• this is required in inverting amplfiers

Question 16

State the formula for gain in an inverting amplifier

Answer 16

G = -Rf/Rin

Question 17

State the formula for gain in a non-inverting amplifier

Answer 17

G = 1 + (Rf/Rg)

Question 18

Describe the use of relays as output devices and explain how the op amp is protected from it

Answer 18

• electromagnetic switch operated by small current
• magnetic field attracts magnetic field causing contacts to be connected completing a second circuit that contain larger voltages or currents
• when current is turned off a large change in magnetic flux causes a large emf to be produced by relay causing damage to op-amp
• reverse biased diode is placed across relay coil allowing current to flow without damaging op-amp.
• diode is also used to ensure current only flows through one end.

Question 19

Describe the use of light emitting diodes as output devices

Answer 19

• Show whether state of output is positive, negative or zero
• must be placed in series with resistor if diode is between zero-volt line and output of op-amp so that voltage entering diode is decreased and diode is not damaged.

Question 20

Describe the use of calibrated meters as output devices

Answer 20

• used to measure physical quantities
• voltmeter must be calibrated in terms of physical quantity
• calibration curve is used to change the voltmeter reading into value for physical quantity

Question 21

State the formula for the force on a conductor

Answer 21

F = BILsin()

Question 22

Define magnetic flux density

Answer 22

Force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point. Unit is tesla which is equivalent to 1 N/ Am

Question 23

State the formula for the magnetic force on a moving charge

Answer 23

F = BQv sin ()

angle is angle the charged particle is moving to the magnetic field.

Question 24

Describe how a hall voltage is formed

Answer 24

• a small current flows through the probe from one end to the other meaning when a magnetic field is applied electrons are pushed sideways and so accumulate along one side of the probe
• This is the hall effect and a hall voltage is generated.
• Greater flux density means greater Hall voltage.

Question 25

State the formula for the Hall voltage

Answer 25

VH = BI/nte where t is the thickness of the slice

Question 26

State the formula for the radius of the movement of a charged particle in a uniform magnetic field

Answer 26

r = mv/Be

Question 27

State the formula for the charge to mass ratio of an electron

Answer 27

e/me = v/Br

Question 28

Describe how a velocity selector works

Answer 28

- two parallel plates are placed in an evacuated chamber providing a uniform electric field - region between plates also contains uniform magnetic field at right angles to electric field - due to F = Bev, electrons that move faster will have greater force. - if electrons have specific v then electric and magnetic field will be balanced and particle is undeflected.

Question 29

Describe how magnetic resonance imaging works

Answer 29

- nuclei of certain atoms have spin that causes them to act as tiny magnets in a magnetic field. - hydrogen is used in MRI - with strong magnetic field the protons line up in the field - magnetic axis of proton rotates around direction of external field, spinning is known as precession. - pulse of RF waves causes resonance of proton's precession. - the time taken for the protons to return to lower energy state. Rate of relaxation can be used to distinguish different tissues.

Question 30

Describe the Larmor frequency and state its formula

Answer 30

Angular frequency of precession w = γBo - γ is gyromagnetic ratio and is measure of magnetism

Question 31

Define magnetic flux and magnetic flux linkage

Answer 31

Product of magnetic flux density normal to an area and this area A Product of magnetic flux and number of turns in the coil

Question 32

State the formula for magnetic flux and magnetic flux linkage

Answer 32

Φ = BA, unit is Weber Magnetic flux linkage = NΦ or BAN cos()

Question 33

State Faraday's Law of EM induction and state its formula

Answer 33

Magnitude of induced emf is proportional to rate of change of magnetic flux linkage E = Δ(N)Φ/Δt

Question 34

State Lenz's law

Answer 34

Any induced current or emf will be established in a direction so as to produce effects which oppose the change that is producing it

Question 35

State the formulas for alternating current and voltage

Answer 35

I = Io sin wt V = Vo sin wt

Question 36

Describe the root mean square value of alternating current

Answer 36

- steady current which delivers same average power as a.c to resistive load. - I(rms) = Io/root 2

Question 37

State the formulas involved in transformers

Answer 37

ns/np = Vs/Vp Ps = Pp so VsIs = VpIp Ip/Is = Vs/Vp

Question 38

Describe how transformers work

Answer 38

- AC is supplied to primary coil and produces varying magnetic field in soft iron core - magnetic flux linkage in secondary coil is constantly changing meaning varying emf is induced

Question 39

State how transformers can be made more efficient

Answer 39

- use of low resistance wire coils to reduce heating - use of laminated iron core to reduce currents induced in core itself (eddy currents) - use of soft iron core that magnetises/demagnetises easily.

Question 40

State why AC is used for electricity supply

Answer 40

- can be transformed to high voltages so current flowing is reduced leading to less power losses - reduces costs as more power can be generated with less fuel

Question 41

Define rectification

Answer 41

The process of converting a.c to d.c

Question 42

Describe smoothing

Answer 42

- capacitor placed in parallel with load resistor - capacitor discharges gradually allowing voltage to rise again - this means a more steady d.c is formed.