Electricity

Question 1

What is electric current?

Answer 1

is the rate of flow of charge

Question 2

What is electric charge formula?

Answer 2

Q = I t
where Q = N e
(i.e. number of charged particles x charge per particle)

Question 3

What is the formula for power?

Answer 3

Electrical Power P = IV = I2R = V2/R

Question 4

How to determine brightness of bulb?

Answer 4

Brightness of bulb is determined by the power dissipated, and not by I, V or R alone.

Question 5

What is formula for e.m.f.? What is terminal p.d.?

Answer 5

For a cell with internal resistance r,
e.m.f. = terminal p.d. + p.d. across internal resistance
= p.d across external resistor + p.d across internal resistance
i.e. E = I R + I r

Question 6

What is resistance of a circuit component? Formula?

Answer 6

Resistance R of a circuit component is defined as the ratio of the potential difference across the component to the current flowing through it,
ie R = V / I

Question 7

What is the effect of temp. on resistance for metallic/semiconductors?

Answer 7

a. For metallic conductors: resistance increases with increasing temperature. At higher temperatures, amplitude of vibration of metal lattice ions increases and thus increases the rate of collision (ie collision frequency) of charge carriers with lattice ions. Thus the degree of scattering of the carriers increases
b. For semiconductors: resistance decreases with increasing temperature, due to increased number of electrons and holes.

Question 8

What is the I-V characteristic of ohmic resistor/filament lamp/NTC thermistor/semiconductor diode?

Answer 8

1) For an ohmic resistor (eg metallic conductor maintained at constant temperature): Resistance is constant. Hence gradient of I-V graph is linear AND passes through origin
2) Filament lamp: Resistance increases as temperature increases.
3) NTC thermistor: Resistance decreases as temperature increases.
4) Semiconductor diode: In forward bias, a diode has low resistance. {If ideal, R = 0} In reverse bias, the diode has very high resistance {until the breakdown voltage is reached} ∙ Need to sketch the resistance-temperature characteristic of an NTC thermistor

Question 9

What is resistivity?

Answer 9

Resistivity ρ is defined as the resistance of a material of unit cross-sectional area and unit length.

Question 10

What are the 2 conservation principles for circuits?

Answer 10

1) {Kirchoff’s 1st law} Because charge is always conserved, the sum of currents entering any junction in an electric circuit is always equal to the sum of currents leaving that junction,
i.e. Σ(Iin) = Σ(Iout)
2) {Kirchoff’s 2nd law Because energy is always conserved, in any closed loop in an electric circuit, sum of e.m.f.s in a loop equals the sum of the pd across all the components in that loop.
i.e. Σ(e.m.f.) = Σ(I R)
{Need to know sign convention for e.m.f., & I R when substituting in their numerical values}

Question 11

What is the formula for potentiometer? Why potentiometer is more accurate?

Answer 11

p.d. ‘being balanced’ = p.d. per unit length of potentiometer wire x balance length.
To obtain an accurate value of the emf of a source, using a potentiometer would be better than using a voltmeter connected across the source. This is because a voltmeter has a finite resistance and draws some current which causes the voltage reading to be smaller than the emf (by an amount equal to the pd across the internal resistance of the emf source).
{For a voltmeter to be ideal, it needs to have an infinite resistance.}

Question 12

What is a field of force?

Answer 12

A field of force is a region of space where a particle experiences a force due to the interaction between the particle’s and the field’s properties.

Question 13

What is an electric field?

Answer 13

An electric field is a region of space where any charged particle in it experiences an electric force.

Question 14

WHat are the characteristic of electric field lines?

Answer 14

1) Indicate the direction of the force a small positive (test) charge would experience if it is placed at that point in the electric field. {N10/ P1/Q24}.
2) Arrow of a field line must point from a positive to a negative charge.
3) Spacing between the lines indicates the strength of the field. Equally-spaced (parallel) lines indicate a uniform electric field.
4) Lines can never cross one another {as that would mean that at the intersection, the electric field vector has 2 directions}

Question 15

What is coulomb’s law? What is the formula?

Answer 15

The (mutual) electric force F acting between 2 point charges, Q1 and Q2, separated by a distance r is given by:

Question 16

What is electric field strength? Formula?

Answer 16

Electric field strength at a point is defined as the electric force per unit positive charge acting on a small positive (test) charge placed at that point.
E = F / q

Question 17

Why E= 0 inside a charged metal conductor?

Answer 17

∙ Charge carriers (i.e. free electrons) of metal sphere are mobile and so they can distribute themselves to reach electrostatic equilibrium within conductor
∙ ⇒ Net force on charge carriers = 0; therefore E = 0 since E = F/Q

Question 18

What is electric potential(V) at a point? Formula?

Answer 18

Electric potential (V) at a point is defined as the work done per unit positive charge (by an external  agent) in bringing a small test charge from infinity to that point (without a change in KE).    
i.e. V = W/Q scalar; S.I. unit: volt (V)

Question 19

What is electric potential energy (U)? Formulas?

Answer 19

electric potential energy (U) of a charge (at a point) in an electric field is defined as the work done (by an external agent) in moving the charge from infinity to that point (without a change in KE).
i.e. U = q V

Question 20

What is formula for change in U?

Answer 20

∆U = q ∆V

Question 21

What is the r/s between E and V?

Answer 21

E = - dV/dx

Electric field strength at a point is numerically equal to the potential gradient

Question 22

What is equipotential surface?

Answer 22

a surface where the electric potential is constant.
∙ Hence no work is done when a charge is moving along this surface at const speed. W = q ∆V, where ∆V = 0
∙ Electric field lines must meet such a surface at right angles {because if the field lines are not at 90 degrees to it, it would imply that there is a non-zero component of E along the surface. This would contradict the fact that E along equipotential surface must = 0 }

Question 23

What is formula for uniform electric field strength between 2 charged parallel plates?

Answer 23

E = V / d
where d = perpendicular distance between the plates,
V = p.d. between plates

Question 24

What is magnetic field?

Answer 24

a region (of space) where a magnetic force is experienced by a current-carrying  conductor {or moving charged particle or a permanent magnet}. 
∙ The direction of a magnetic field line (i.e. the arrow) defines the direction of the magnetic force  on a north pole placed there. (N2010 P3Q3b) 
∙ Must know how to sketch the magnetic flux pattern due to a current in a straight wire, flat  circular coil and solenoid.

Question 25

What is magnetic flux density(B)? Formula?

Answer 25

Magnetic flux density (B) is defined as the force acting per unit current in a wire of unit length placed at right-angles to the field.
F = BILsinθ
θ: angle between the B and I

Question 26

What is What is formula for force on a current carrying conductor?

Answer 26

F = B I L sin θ

θ: angle between the B and I

Question 27

What is fleming’s left hand rule?

Answer 27

Direction of the magnetic force is always perpendicular to the plane containing the current I and B

Question 28

What is the formula for force acting on a moving charge?

Answer 28

F = B Q v sin θ {θ: angle between B and v}

Question 29

What are the possible paths of moving charged particle in a magnetic field for 0 or 180 / 90 / others? What happens to speed?

Answer 29

∙ If θ = 0 or 180: undeflected
∙ If θ = 90: circular (or arc of a circle)
∙ If θ is not 0, 90 or 180: helical.
{In all 3 cases, the speed remains unchanged as FB is always perpendicular to v.}

Question 30

What are crossed-fields in velocity selector? What is the formula? How to derive?

Answer 30

A setup whereby an E-field and a B-field are perpendicular to each other such that they exert equal & opposite forces on a moving charge & hence causes no deflection of the particle.
Magnetic Force = Electric Force (same magnitude but in opposite directions)
B q v = q E

Question 31

What is magnetic flux? What is the formula?

Answer 31

defined as the product of an area A and the component of the magnetic flux density B perpendicular to that area.
φ = B A sinθ

Question 32

What is the magnetic flux linkage? What is the formula?

Answer 32

The product of the magnetic flux passing through a coil and the number of turns of the coil.
Φ = N φ

Question 33

What is electromagnetic induction?

Answer 33

it refers to the phenomenon whereby an e.m.f. is induced when the magnetic flux linking a conductor changes.

Question 34

What is Faraday’s law? Formula?

Answer 34

The magnitude of e.m.f. induced in a coil is directly proportional to the rate of change of magnetic flux linking (or cutting) the coil. |E| = d(NBA)/dt

Question 35

What is Lenz’s law? Formula?

Answer 35

The direction of the induced e.m.f. is such that it produces an effect to oppose the change which causes it.
E = - d(Nφ)/dt

Question 36

How is Lenz’s law an example of conservation of energy?

Answer 36

∙ As the external agent brings the magnet towards the coil, by Lenz’s law, a current is induced in such a direction that the coil opposes, (i.e. repels) the approaching magnet.
∙ Consequently, work has to be done by the external agent to overcome this opposition {the repulsive force}, and
∙ It is this work done which is the source of the electrical energy {Not: induced e.m.f. }

Question 37

What is the 2-step guide in applying Faraday’s and Lenz’s law?

Answer 37

1. Identify the change in magnetic flux or flux linkage (whether increasing or decreasing)
2. Apply Lenz’s Law together with either Right Hand Grip Rule or Fleming’s Left Hand Rule to determine direction of induced current.
   ( In using FLH Rule, note that the direction of the induced force (represented by the Thumb) is opposite to the actual motion of the rod (since there has to be opposition to the actual motion)

Question 38

What is the formula for e.m.f. induced for a straight conductor (rod/wire) ‘cutting across’ a B-field?

Answer 38

E = B L v sinθ

θ: angle betw B and v

Question 39

What are the formula for coil rotating in a B-field with angular freq. w when initial coil is parallel/perpendicular to field?

Answer 39

Parallel: Φ = N φ = N B A sin (ω t)
Perpendicular: Φ = N φ = N B A cos (ω t)

Question 40

What is the formula for e.m.f. induced between centre and any pt. on rim for rotating disc?

Answer 40

E = B πr2 f

Question 41

What are eddy currents?

Answer 41

Eddy currents dissipate energy in a ‘bulk piece’ of metal as heat; and thus is a source of energy loss in the core of a transformer, electrical motors and generators.
Laminating the piece of metal reduces eddy currents and thus, the energy loss (as heat).

Question 42

What is r.m.s current of an a.c.?

Answer 42

the magnitude of the steady direct current that produces the same average heating effect as the alternating current in a given resistor.

Question 43

How to reduce power loss during transmission of electrical power?

Answer 43

read notes for first part
thus to reduce power loss, for a given amount of power generated, electricity is transmitted at high voltage (i.e. low current) by stepping up the a.c. voltage using a transformer. {Vi is NOT the p.d. across the cables.}