7 Electric and Magnetic Fields

Question 1

Electric field

Answer 1

A region in which a charged object will experience a force

Question 2

In which direction do electric fields exert a force?

Answer 2

Positive to negative

Question 3

Radial field

Answer 3

A field which forms around a point charge

Question 4

Uniform field

Answer 4

A field typically formed between two plates

Question 5

What happens to field strength in a radial field?

Answer 5

As you go further from the centre the field becomes weaker because the field lines become more spread out

Question 6

Field strength in a uniform field

Answer 6

The same throughout because the lines are equally spaced

Question 7

How is electric field strength shown in a drawing

Answer 7

The closer the field lines are the stronger the field is at that point

Question 8

Equation linking electric field strength, force on a positive charge and the charge

Answer 8

E = F/Q

Electric field strength = force acting on the charge/ charge

Question 9

Equation to work out electric field strength between two plates

Answer 9

E = V/d

Electric field strength = voltage between two plates/ distance between plates

Question 10

Lines of equipotential

Answer 10

Lines joining points of equal electric potential in an electric field

Question 11

Equation for work done in moving an object of charge between two lines of equipotential (learn)

Answer 11

W= Qx (change in)V

Question 12

Uses of electrostatics

Answer 12

• microwaves
• precipitators in chimneys
• crop spraying
• spray painting

Question 13

Equation for electric field strength in a radial field

Answer 13

E = Qx 8.99x10^9/r^2

Electric field strength = charge of point/ permittivity of free space x 4pi x radius squared

Question 14

Equation for force between two charges in a radial field

Answer 14

F = Q1 x Q2 x 8.99x10^9/r^2

Force between charges = charge of point1 x charge of point2 / permittivity of free space x 4pi x radius squared

Question 15

Electric potential

Answer 15

The work done per unit charge when a particle is moved in a field

Question 16

Equation for electric potential in a uniform field

Answer 16

V =Ed

Electric potential = electric field strength x distance

Question 17

Equation for electric potential in radial field

Answer 17

V = Q x 8.99x10^9/r

Question 18

Differences between G fields and E fields

Answer 18

•gravitational forces affect all particles with a mass, electrostatic forces only
affect particles carrying a charge
•gravitational forces are always attractive but electrostatic can attract or repel
•it isn’t possible to shield a mass from a gravitational field but this can be done in an electric field

Question 19

What happens when you apply a p.d. to the terminals of a capacitor?

Answer 19

The plates become charged and an electric field is created between the plates

Question 20

Equation linking capacitance, charge and voltage

Answer 20

C =Q/V

Question 21

What does the area under a voltage against charge graph represent

Answer 21

Energy transferred

Question 22

Equation linking work done, voltage and charge in a capacitor

Answer 22

W =1/2 Vmax x Q
or
W=Vaverage x Q

Question 23

Equation linking work done, capacitance and voltage

Answer 23

W = 1/2 C x V^2

W= 1/2 Q^2/C

Question 24

Uses for capacitors

Answer 24

• camera flash
• keys on a laptop
• inside the cable from a satellite dish to a TV

Question 25

How do capacitors charge and discharge?

Answer 25

Exponentially

Question 26

Equation linking I, Q, R and C in a discharging capacitor (learn)

Answer 26

I= Q/RC RC = time constant

Question 27

What gives the half life of a discharging capacitor?

Answer 27

RC x ln(2) Time constant x ln(2)

Question 28

Equation linking starting value of charge, voltage or current and and the value at any point when discharging

Answer 28

Q = Qo x e^(-t/RC) V = Vo x e^(-t/RC) I = Io x e^(-t/RC)

Question 29

When a capacitor discharges for 1 time constant what value of the original charge, voltage or current is given?

Answer 29

37% of the original value Q= 0.37Qo V= 0.37Vo I= 0.37Io

Question 30

Magnetic field

Answer 30

A region in which a magnetic force will be experienced by a magnetic material

Question 31

In which direction do magnetic field lines travel?

Answer 31

North to south

Question 32

What force is felt at neutral points in a field?

Answer 32

Nothing since the forces cancel out

Question 33

If current is travelling into the page which way is the magnetic field acting?

Answer 33

Clockwise

Question 34

Equation linking force in current carrying wire, magnetic flux density, current in a wire, length of wire, angle between the field and wire

Answer 34

F = BILsin(x)

Question 35

Equation linking force on a charged particle, magnetic flux density, charge of particle and velocity of particle

Answer 35

F = BQVsin(x) ``` F= force on charged particle B= magnetic flux density Q= charge of particle V= velocity of particle X= angle between magnetic field and movement of particle ```

Question 36

Equation to work out magnetic flux density (learn)

Answer 36

B = I/A ``` B= magnetic flux density I= magnetic flux A= area of surface perpendicular to field ```

Question 37

Equation for flux linkage (learn)

Answer 37

Flux linkage = N x I Flux linkage = number of turns of coil x magnetic flux

Question 38

Electromagnetic induction

Answer 38

The generation of a current in a conductor by the interaction of a changing magnetic field with the conductor

Question 39

How can the magnitude of an induced electromagnetic voltage be increased

Answer 39

* stronger magnet * moving the magnet faster * increasing the turns if the coil

Question 40

What happens when a magnet falls through a coil

Answer 40

* a current is induced in the coil * the coil opposes the movement of the magnet due to Lenz’s law * as the magnet falls out the other end of the coil the “poles” of the coil flip in order to oppose the direction of the magnet * this causes the current to change direction

Question 41

Faraday and Lenz’s law

Answer 41

E= -d(NI)/dt E= -d(NBA)/dt emf= - change in number of coils x magnetic flux/ change in time

Question 42

Uses of electromagnetic induction

Answer 42

* large rotating coils generating electricity * step up or step down transformers * charging phones * braking on trains * braking on electric cars

Question 43

Transformers

Answer 43

Transformers increase or decrease the voltage of alternating current electrical power supplies

Question 44

How do transformers work?

Answer 44

* primary coil produces changing magnetic flux * this magnetic flux is linked to the secondary coil by iron core * therefore the magnetic flux linkage in the secondary coil varies continuously * the change of this flux linkage induces a varying emf in the secondary coil

Question 45

Transformer equation (learn)

Answer 45

Vsecondary/ Vprimary= Nsecondary/ Nprimary

Question 46

Equation to work out average voltage from peak voltage

Answer 46

Vrms = Vo/ route 2 Irms = Io/ route 2

Question 47

If three identical magnets are dropped through a cylinder of cardboard, copper and plastic, which will hit the floor first and why?

Answer 47

Cardboard and plastic first then copper | The copper opposes the direction of motion of the magnet so slows it down