Notes-for-CAIE
Table of contents
- Electric Fields
- Why electric field causes the air to become conducting [3]
- The significance of the Millikan experiment [1]
- Define
electric field strength[1] - What is meant by a line of force in an electric field [2]
- What is meant by
potential energyof a body [2] - State
Coulomb's law[2] - Why charge on a spherical conductor may be considered to a point charge at centre for point outside the sphere [2]
- Electromagnetism
- What is meant by a
magnetic field[2] - State
Newton's third law of motion[1] - State
Faraday's law of electromagnetic induction[2] - State
Lenz's law[2] - Define
magnetic flux[2] - Define
magnetic flux density[3] - What is meant by the
magnetic flux linkageof a coil [3] - Why induced e.m.f. alternating [2]
- Why eddy currents are induced in the metal disc [2]
- Define
tesla[3] - What is meant by
electric potentialat a point [2] - Talking about
Hall probe - Why a Hall probe is made from a thin slice of material [2]
- Why current in Hall probe must be constant to measure magnetic flux density [1]
- Why Hall Voltage varies between a maximum positive value and a maximum negative value when rotating Hall probe [3]
- Why Hall Voltage is constant between surfaces [4]
- What is meant by a
- Capacitance
- Alternating current
- What is meant by
root-mean-square(r.m.s) value of an alternating current [3] - What is meant by
smoothing[1] - The effect of value of the capacitance of the smoothing capacitor in relation to smoothing [2]
- Why the coils are wound on a core made of iron [1]
- Why use iron core [1]
- Why the iron core is laminated [2]
- What is meant by an ideal transformer [1]
- Why thermal energy is generated in the core [2]
- Why alternating current in the primary coil gives rise to an induced e.m.f. in the secondary coil [3]
- Why alternating current in the primary coil is not in phase with the alternating e.m.f induced in the secondary coil [3]
- Why the p.d. across the load and the e.m.f. of the supply are not in phase [2]
- Why output p.d. of a transformer is alternating [3]
- What is meant by
- Direct Sensing
- Diagram of
strain gauge[3] - State properties of an
ideal operational amplifier[4] - What is meant by
zero output inpedance[2] - State what is meant by
infinite bandwidth[1] - State what is meant by
infinite slew rate[1] - What is meant by (negative)
feedback[2] - State two effects of negative feedback on the gain of an amplifier [2]
- Benefits of feedback in an amplifier circuit [2]
- What is meant by
virtual earth[3] - State two effects of negative feedback on an amplifier incorporating an op-amp [2]
- State the function of a comparator [2]
- State the function of a
non-inverting amplifier[2] - Calculating gain
- Diagram of
Electric Fields
| Quantity | Formulae |
|---|---|
Electric Field Strength |
 |
Electric Potential |
 |
Electric Potential Energy |
 |
Electric Force |
 |
ε0: permittivity of free space
Why electric field causes the air to become conducting [3]
- Field causes forces on the electrons and nucleus in opposite directions. [2]
- Electrons stripped off the atom.
The significance of the Millikan experiment [1]
- Charge is quantised.
Define electric field strength [1]
- Force per unit positive charge
What is meant by a line of force in an electric field [2]
m18 42 Q1 [1]
- Tangent to line gives direction of force on a unit positive test charge.
What is meant by potential energy of a body [2]
- Ability to do work
- … as a result of the position of an object
State Coulomb's law [2]
- Force proportional to product of two charges and inversely proportional to square of separation.
- (reference to point charges)
Why charge on a spherical conductor may be considered to a point charge at centre for point outside the sphere [2]
w17 42 Q6 [2]
- Electric field lines are radial.
- Electric field lines appear to originate from centre.
Electromagnetism
What is meant by a magnetic field [2]
- Region of space where there is a force
- … acting on moving charge.
State Newton's third law of motion [1]
- When two bodies interact, force on one body is equal but opposite in direction to force on the other body.
State Faraday's law of electromagnetic induction [2]
- Induced e.m.f. is proportional to rate of change of
- … magnetic flux linkage.
State Lenz's law [2]
- Induced e.m.f. acts in such a direction to produce effects
- to oppose the change causing it
Define magnetic flux [2]
m18 42 Q10 [2]
- Product of flux density and area
- … normal to the flux.
Define magnetic flux density [3]
w18 42 Q8
- force per unit length
- … on straight conductor carrying unit current
- … normal to the field.
What is meant by the magnetic flux linkage of a coil [3]
s18 42 Q9
- magnetic flux density × cross-sectional area
- … × number of turns of the coil
Why induced e.m.f. alternating [2]
- (Lenz) e.m.f. opposes change causing it.
- As direction of movement changes, so does e.m.f.
Why eddy currents are induced in the metal disc [2]
- Field in disc not uniform
- … so different e.m.f. in different parts of disc
- … lead to eddy currents.
Define tesla [3]
s20 43 Q8 [3]
- Flux normal to long straight wire carrying a current of 1A
- … creates force per unit length of 1 N/m.
What is meant by electric potential at a point [2]
- Work done in moving unit positive charge
- … from infinity to the point.
Talking about Hall probe
Always link the formula for VH.
Why a Hall probe is made from a thin slice of material [2]
s17 42 Q9
- Hall voltage depends on thickness of slice.
- Sinner slice, larger voltage.
Why current in Hall probe must be constant to measure magnetic flux density [1]
s17 42 Q9
- Hall voltage depends on current in slice.
Why Hall Voltage varies between a maximum positive value and a maximum negative value when rotating Hall probe [3]
VHdepends on the sine of angle between (plane of) probe and B-field.
Why Hall Voltage is constant between surfaces [4]
w17 42 Q8
- Charge carriers moving normal to field.
- Charge carriers experience a force normal to
I. - Charge build-up sets up electric field across the slice.
- Charge stops building up when
Capacitance
Define capacitance [2]
C = Q/V- Ratio of charge and its potential.
Why capacitors store energy but no resultant charge [3]
It’s suggested to remember and write them all as questions varies.
- Charge on plates are equal and opposite,
- … so no resultant charge.
- On a capacitor, there’s charge separation.
- To separate charges, work must be done.
Why Q = CV is not applicable to a plastic sphere [3]
- Plastic is an insulator
- …, charges don’t move
- …, so charge can’t be considered at the centre.
Functionality of capacitors [2]
w20 43 Q6 [3]
- Block direct current.
- Smoothing.
- Temporary power supply.
- Timing delay.
Combining capacitors
- Parallel: C1 + C2
- Serial:
\frac{1}{\frac{1}{C_1}+\frac{1}{C_2}}
Alternating current
Basics:
- Number of turns is proportional to e.m.f.
What is meant by root-mean-square(r.m.s) value of an alternating current [3]
Always write these 3 points.
- The value of the direct current
- … that dissipates energy in a resistor
- … at the same rate
What is meant by smoothing [1]
- Reduction in the variation of the output voltage.
- or Output voltage does not fall to zero.
The effect of value of the capacitance of the smoothing capacitor in relation to smoothing [2]
- Larger capacitance produces more smoothing
- … for the same load.
Why the coils are wound on a core made of iron [1]
- To prevent flux losses
Why use iron core [1]
m17 42 Q9 [1]
- To reduce flux loses.
Why the iron core is laminated [2]
- To reduce power loss in the core
- … due to eddy currents.
What is meant by an ideal transformer [1]
- no power loss in transformer
Why thermal energy is generated in the core [2]
- Changing flux induces current in the core
- …, causing heating effect
Why alternating current in the primary coil gives rise to an induced e.m.f. in the secondary coil [3]
w20 43 Q9 [3]
- Alternating current creates changing flux.
- Core links flux with secondary coil.
- Changing flux causes induced e.m.f.
Why alternating current in the primary coil is not in phase with the alternating e.m.f induced in the secondary coil [3]
- Flux in core is in phase with current in the primary coil
- Secondary coil cuts flux so induced e.m.f.
- Flux and rate of change of flux not in phase
Why the p.d. across the load and the e.m.f. of the supply are not in phase [2]
- Magnetic flux in phase with e.m.f. in primary coil.
- e.m.f. across secondary coil is proportional to the rate of change of flux.
Why output p.d. of a transformer is alternating [3]
s20 42 Q10
- e.m.f. varies as rate of change of flux changes.
- Direction of e.m.f. changes when direction of change of flux reverses.
- Flux is continuously increasing and decreasing.
Direct Sensing
Diagram of strain gauge [3]
State properties of an ideal operational amplifier [4]
- Infinite gain
- Infinite input impedance
- Zero output impedance
- Infinite bandwidth
What is meant by zero output inpedance [2]
w19 43 Q7 [1]
- No drop in output voltage.
State what is meant by infinite bandwidth [1]
w20 Q8 [1]
m18 42 Q8 [1]
w19 43 Q7 [1]
- Gain is the same for all frequencies.
State what is meant by infinite slew rate [1]
w20 Q8 [1]
m18 42 Q8 [1]
- No time delay in change in output when input is changed.
What is meant by (negative) feedback [2]
s18 42 Q7 [2]
w17 42 Q7 [2]
- Output potential is combined with the input potential
- Output potential is subtracted from input potential
State two effects of negative feedback on the gain of an amplifier [2]
s18 42 Q7 [2]
- Smaller gain
- Greater bandwidth
Benefits of feedback in an amplifier circuit [2]
w17 42 Q7 [2]
- Increased stability.
- Increased bandwidth of frequencies over with gain is constant.
- Less distortion.
What is meant by virtual earth [3]
s20 42 Q8
- Infinite gain.
- Feedback loop ensures
V+ ≈ V-(any difference betweenV+andV-results in saturated output) - Non-inverting input is earthed.
State two effects of negative feedback on an amplifier incorporating an op-amp [2]
- Reduces gain
- Increases bandwidth
- Greater stability
State the function of a comparator [2]
- To compare two potentials.
- Output depends upon which is greater.
State the function of a non-inverting amplifier [2]
s20 43 Q7 [2]
- Output signal proportional to input signal.
- Output signal has same sign as input.
Calculating gain
s18 42 Q7
s20 42 Q8
G = \frac{V_{out}}{V_{in}}
- normal amplifier
G = \frac{R_{out}}{R_{in}} - non-inverting amplifier
G = \frac{R_{out}}{R_{in}} + 1 - inverting amplifier
G = -\frac{R_{out}}{R_{in}}