magnets

Question 1

magnetic field

Answer 1

any region in space where magnetic forces can be felt. the direction of the magnetic field at a point is the direction of the force on a north pole if it were placed at that point

Question 2

magnetic field line

Answer 2

a line drawn so that the tangent to it at any point shows the direction of the magnetic field at that point

Question 3

force on current carrying conductor in a magnetic field

Answer 3

will always experience a force unless the conductor is parallel to the magnetic field
direction of the force is always
-perpendicular to current
-perpendicular to magnetic field

Question 4

magnetic flux density (B)

Answer 4

F = ILB
B at a point in a magnetic field is a vector whose magnitude is equal to the force that would be experienced by a conductor of length 1m carrying a current of 1A at right angles to the field at that point and whose direction is the direction of the force on a north pole placed at that point
unit: Tesla (T)

Question 5

force on a moving charge in a magnetic field

Answer 5

F = qvB
q = charge
v = velocity

Question 6

the ampere

Answer 6

the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross section and placed 1m apart in a vacuum, would produce a force on each conductor of 2*10^-7 newtons per metre of length

Question 7

the coulomb

Answer 7

the amount of charge that passes any point in a circuit when a current of 1A flows for 1s

Question 8

electromagnetic induction

Answer 8

whenever the magnetic field passing through a coil changes an emf is induced in the coil. if the circuit is closed an induced current will flow

Question 9

magnetic flux (Φ)

Answer 9

Φ = BA
a = area
b = mag. flux density
scalar
unit: weber (Wb)

Question 10

faraday’s law

Answer 10

states that the size of the induced emf is directly proportional to the rate of change of the flux

Question 11

faraday’s law equation

Answer 11

E = (final flux - initial flux)/time taken

E = NdΦ/dt
N = no. of coils

Question 12

Lenz’s law

Answer 12

states that the direction if an induced current is always such as to oppose the change producing it
e.g. if north pole brought towards coil current will flow anticlockwise (north pole) to repel magnet
law follows from the principle of the conservation of energy

Question 13

generators

Answer 13

device that converts mechanical energy into electrical energy e.g. elec. power stations
rotate a coil in a magnetic field which induces emf and current

Question 14

mutual induction

Answer 14

where changing the magnetic field in one coil causes an induced emf to appear in a nearby coil.
size of the emf can be increased by
-having coils near each other
-winding coils together on soft iron core
-increasing no. of coils

Question 15

transformers

Answer 15

device used to change the value of an alternating voltage
-consists of 2 coils, primary and secondary, connected by a soft iron core

Question 16

step down transformer

Answer 16

if the no. of primary turns Np > than no. of secondary turns Ns then output voltage Vo < input voltage Vi

Question 17

step up transformer

Answer 17

if Np < Ns then Vo > Vi

Question 18

transformer eqns

Answer 18

Vi/Vo = Np/Ns
ViIp = VoIs

Question 19

uses of transformers

Answer 19

• power stations step up/down voltage leaving station and entering houses
• computers, tvs, radios etc. use transformers as different parts of the appliance need high or low voltages

Question 20

self induction

Answer 20

this occurs whenever the current passing through a coil changes. when the current changes the magnetic field changes causing an emf to be induced to oppose the changing current (back emf).
a coil which has the property of self induction is often called an inductor.

Question 21

alternating current

Answer 21

this is where the flow of current is constantly changing direction
e.g. 50Hz, each electron goes a full cycle in 1/50 of a second

Question 22

rms voltage

Answer 22

Vrms = Vo/root 2

where Vo = max/peak voltage

Question 23

alternating voltage

Answer 23

in a 50Hz system the voltage has a full cycle between +/- 325V in 20ms.
the electrical energy per sec on average is equivalent to 230V d.c. supply (root mean square value)

Question 23

a.c. across pure resistor

Answer 23

if an alternating current is applied across a pure resistor (no induction or capacitance) then:
instantaneous current = voltage at that instant/ resistance
i.e. I = V/R

Question 24

rms current

Answer 24

I(rms) = Io/ root 2
where Io = max/peak current

Question 25

power (a.c.)

Answer 25

P = (Irms)(Vrms)

or P = (I^2rms)(R)

Question 26

a.c. + inductors

Answer 26

from lenz’s law an induced emf always opposes the changing current hence if the current is alternating there will be a back emf induced.
the greater the frequency of the a.c. the greater the opposition of the inductor to it