Fields & their consequences

Question 1

Capacitance

Answer 1

The charge stored per unit pd of a capacitor. The unit of capacitance is the farad (F), Equal to 1 coulomb per volt. For a capacitor of capacitance C at pd V, the charge stored, Q = CV

Question 2

Capacitor discharge

Answer 2

through a fixed resistor of resistance R, Time Constant = RC; exponential decrease for charge and/or p.d.;
x=x0*e^(-t/RC)

Question 3

Capacitor energy

Answer 3

Energy stored by the capacitor.

E = QV/2 = CV²/2 = Q²/2C

Question 4

Coulomb’s law of force

Answer 4

for two points Q1 and Q2 at distance apart r, the force F between the two charges is given by the equation F = Q1Q2/4πε0*r², where ε0 is the permittivity of free space.

Question 5

dielectric

Answer 5

material that increases the capacity of a parallel-plate capacitor to store charge when placed between the plates of a capacitor. Polythene and waxed paper are examples of dielectrics.

Question 6

dielectric constant

Answer 6

ratio of the charge stored by a parallel- plate capacitor with dielectric filling the space between its plates to the charge stored without the dielectric for the same pd

Question 7

dynamo rule

Answer 7

rule that relates the directions of force, magnetic field and current on a current carrying conductor in a magnetic field

Question 8

eddy currents

Answer 8

induced currents in the metal parts of ac machines.

Question 9

electric field strength, E

Answer 9

at a point in the electric field, is the force per unit charge on a small positively charged object at a point in the field

Question 10

electric potential, V

Answer 10

at a point in the electric field is the work done per unit charge on a small positively charged object to move it from infinity to that point in the field

Question 11

electromagnetic induction

Answer 11

the generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines

Question 12

equipotential

Answer 12

a line or surface in a field along which the electric or gravitational potential is constant

Question 13

escape velocity

Answer 13

the minimum velocity an object must be given to escape from the planet when projected vertically from the surface

v=(2GM/r)^1/2
(^NOT GIVEN^)

Question 14

exponential change

Answer 14

exponential change happens when the change of a quantity is proportional to the quantity itself. For an exponential decrease of a quantity x, dx/dt = -λx (λ = decay constant). The solution to this equation is
X = X₀*e^-λt, where x₀ is an initial value of x

Question 15

Faraday’s Law of electromagnetic induction

Answer 15

the induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit. For a changing magnetic field in a fixed coil of area A and turns N, the induced emf = -NA(change in B/ change in t)

Question 16

field line

Answer 16

the direction of a line of force indicates the direction of the force. An electric field line is the path followed by a free positive test charge. The gravitational field lines of a single mass point towards that mass

Question 17

Fleming’s left hand rule

Answer 17

rule that relates the directions of force, magnetic field and current on a current carrying conductor in a magnetic field

Question 18

Fleming’s right hand rule

Answer 18

rule that relates the directions of the induced current, magnetic field and velocity of the conductor when the conductor cuts across magnetic field lines and an emf if induced in it

Question 19

geostationary satellite

Answer 19

a satellite that stays above the same point in the earth’s equator as it orbits the earth because it’s orbit is exactly 24 hours and it orbits in the same direction as the earth’s direction of rotation.

Question 20

gravitational constant (G)

Answer 20

the constant of proportionality in Newton’s law of gravitation = 6.67x10⁻¹¹.

Question 21

gravitational field

Answer 21

the region surrounding an object in which it exerts a gravitational force on any other object.

Question 22

gravitational field strength (g)

Answer 22

the force per unit mass on a small mass placed in the field.

1. g = F/m, where F is the gravitational force on a small mass m.
2. at distance r from a point mass M, and at or beyond the surface of a sphere of mass M, g = GM/r² where r is the distance to the centre of the sphere.

Question 23

gravitational potential (V)

Answer 23

at a point in a gravitational field is the work done per unit mass to move a small object from infinity to that point. At distance r from the centre of a spherical object of mass M,V = -GM/r

Question 24

gravitational potential energy

Answer 24

at a point in a gravitational field is the work done to move a small object from infinity to that point. the change of GPE of a mass m moved through height h near the earth’s surface. Eₚ = mgh

Question 25

grid system

Answer 25

the network of transformers and cables that is used to distribute electrical power from power stations to users.

Question 26

Hall probe

Answer 26

a device used to measure magnetic flux density.

Question 27

induced emf

Answer 27

the generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines.

Question 28

Kepler’s third law

Answer 28

For any planet, the cube of its mean radius of orbit (r) is directly proportional to the square of its time period (T)

Question 29

Lenz’s law

Answer 29

When a current is induced by an electromagnetic induction, the direction of the induced current is always such as to oppose the change that causes the current

Question 30

line of force or a field line

Answer 30

indicates the direction of the force

Question 31

magnetic flux

Answer 31

Φ = BA for a uniform field of flux density B perpendicular to the area A. Measured in Wb.

Question 32

magnetic flux density

Answer 32

the magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. Its unit is the Tesla (T)

Question 33

magnetic flux linkage

Answer 33

through a coil of N turns, = ΦN = BAN perpendicular to the area A. Measured in Wb, T m^2 or V s.

Question 34

magnetic force

Answer 34

F = BILsin(θ); F = BIL if perpendicular to field
F = BQvsin(θ); F = BQv if perpendicular to field

Question 35

Motor Effect

Answer 35

The force on a current-carrying conductor due to a magnetic field

Question 36

Newton’s Law of Gravitation

Answer 36

The gravitational force F between two point masses m1 and m2 at distance r apart is given by : F = Gm1m2/r^2

Question 37

Permittivity of free space

Answer 37

The charge per unit area in coulombs per square metre on oppositely charged parallel plates in a vacuum when the electric field strength between the plates is 1 volt per metre

Question 38

Potential Gradient

Answer 38

at a point in a field is the change of potential per unit change of distance along the field line at that point. The Potential Gradient= - the field strength at any point.

Question 39

Radial Field

Answer 39

a field in which the field lines are straight and converge or diverge as if from a single point

Question 40

Relative Permittivity

Answer 40

ratio of the charge stored by a parallel- plate capacitor with dielectric filling the space between its plates to the charge stored without the dielectric for the same pd

Question 41

Satellite

Answer 41

A small object in orbit round a larger object

Question 42

Satellite motion

Answer 42

for a satellite moving at speed v in a circular orbit of radius r round a planet, its centripetal acceleration, g= (v^2)/r

Question 43

Time constant

Answer 43

The time taken for a quantity that decreases exponentially to decrease to 0.37 (=1/e) of its initial value. For the discharge of a capacitor through a fixed resistor, the time constant = resistance x capacitance.

Question 44

Transformer

Answer 44

Converts the amplitude of an alternating pd to a different value. It consists of two insulated coils, the primary coil and the secondary coil, wound round a soft iron laminated core; step-down transformer: a transformer in which the rms pd across the secondary coil is less than the rms pd applied to the primary coil; step-up transformer: a transformer in which the rms pd across the secondary coil is greater than the rms pd applied to the primary coil.

Question 45

Transformer efficiency

Answer 45

For an ideal transformer (i.e., one that is 100% efficient), the output power (= secondary voltage x secondary current) = the input power (=primary voltage x primary current). Transformer inefficiency is due to: resistance heating of a current in each coil; the heating effect of eddy currents (i.e., unwanted induced currents) in the core; and heating caused by repeated magnetisation and demagnetisation of the core.

Question 46

Transformer rule

Answer 46

The ratio of the secondary voltage to the primary voltage is equal to the ratio of the number of secondary turns to the number of primary turns.

Question 47

Uniform field

Answer 47

A region where the field strength is the same in magnitude and direction at every point in the field.

1. The electric field between two oppositely charged parallel plates is uniform. The electric field strength E = V/d, where V is the pd between the plates and d is the perpendicular distance between the plates.
2. The gravitational field of the Earth is uniform over a region which is small compared to the scale of the Earth.
3. The magnetic field inside a solenoid carrying a constant current along and near the axis.