Chapter 16

Question 1

What are electric fields created by?

Answer 1

Charged particles. And following the golden rule that what ever creates a field, the field will act on, charged particles are effected by electric fields.

Question 2

What is the strength of an electric field effected by?

Answer 2

Higher potential difference

Distance

Question 3

How do you calculate the electric field strength?

Answer 3

E=V/d
E=F/Q
units are Vm^-1 or NC^-1

Question 4

What is the potential gradient?

Answer 4

V/d How quickly the potential changes with respect to position.

Question 5

How do you calculate the force due to an electric field?

Answer 5

F=QE where Q is charge on particle and E is electric field strength

Question 6

What are the rules for drawing field lines?

Answer 6

Spacing of field lines indicates the strength of the field
Direction of field lines is direction of a force of a positively charged particle
Therefore, field is a VECTOR
Field lines must never cross
Field lines must start and end on charges

Question 7

What is potential difference?

Answer 7

The energy per unit charge between 2 points (rearrangement of E=QV, where E is energy)

Question 8

How do you calculate electrical potential energy?

Answer 8

EPE=QEd where Q is charge, E is strength of electric field and d is distance
EPE=QV (replacing E with V/d)

Question 9

What is the definition of electric potential?

Answer 9

The electric potential at a point in the field is the potential energy acquired per unit charge by a small positive test charge brought from infinity to that point in the field.

Question 10

What is the definition of electric field strength?

Answer 10

The force, per unit charge, on a small positive test charge placed at that point in the field.

Question 11

What are equipotentials?

Answer 11

Lines that join together points of the same potential. Always perpendicular to field lines. They can cross each other

Question 12

How does a charged particle act when placed in a uniform electric field?

Answer 12

Accelerates in the direction of the field (with if positive, against if negative). If particle is moving perpendicular to the field, a parabola path is followed

Question 13

What is the maximum deflection possible in an electric field?

Answer 13

180 deg/ pi radians. There is a constant magnitude force in a constant direction

Question 14

How does a stationary charged particle act when placed in a uniform magnetic field?

Answer 14

Doesn’t move: if stationary, no current, so no force (think Flemming’s left hand rule)

Question 15

How does a moving charged particle act when placed in a uniform magnetic field?

Answer 15

Moves in a circular motion. Can use Flemming’s left hand rule (being mindful of conventional current) to find force
As particle moves, force changes direction as the current changes direction. Force is always perpendicular to motion, resulting in a circular pathway.

Question 16

What is the maximum angle of deflection in a magnetic field?

Answer 16

360 deg/ 2pi radians as many times as you like. Constant magnitude of force, always perpendicular to velocity

Question 17

Why do things move in a circular motion?

Answer 17

Because of a centripetal ‘centre seeking’ force.

Question 18

What is the equation giving the strength of a centripetal force?

Answer 18

F=mv^2/r where m is mass, v is velocity and r is radius of circle

Question 19

What is the equation giving the force on a charged particle in a magnetic field?

Answer 19

F=BQv where B is the strength of the magnetic field, Q is the charge on the particle and v is the velocity of the particle

Question 20

What is the equation giving the radius of the pathway followed by a charged particle in a magnetic field?

Answer 20

r=mv/BQ where m is mass, v is velocity, B is strength of magnetic field and Q is charge on particle

Question 21

How does a velocity selector work?

Answer 21

Uses electric fields and magnetic fields. Electric field has a force acting in one direction, magnetic field has a force acting in the other direction. Particles will only go straight out the hole in the end if F(e)=F(b)

Question 22

How do we know what velocity is being selected for in a velocity selector?

Answer 22

Particles selected for will have F(b)=F(e)
so BQv=QE
so Bv=E
so v=E/B (=V/dB)

Question 23

What is the electron gun equation?

Answer 23

v=sqrt(2QV/m) where v is velocity, Q is charge, V is potential difference and m is mass.

Question 24

What is the difference between Newtonian and relativistic momentum?

Answer 24

N: p=mv
R: p=(gamma)mv
where gamma=1/sqrt(1-v^2/c^2)

Question 25

When does gamma become significant?

Answer 25

At approximately 1/5 of the speed of light (gamma is 1.02)

Question 26

What is rest energy?

Answer 26

When thinking about energy in relativistic terms, we have to consider time. Rest energy is the energy a particle has just from moving through time, not from moving through space. As there is no movement, gamma is 1. Rest energy, E, = mc^2

Question 27

What is the difference between the Newtonian and relativistic version of kinetic energy?

Answer 27

R: E(k)=E(total)-E(T) (rest energy)
= (gamma)mc^2 - mc^2
= ((gamma)-1(mc^2)
N: E(k) = 1/2 mv^2 (relativistic tends to this as v decreases)

Question 28

What is the equation for calculating the strength of an electric field around a point charge?

Answer 28

E=kQ/r^2 where Q is the charge on the particle, r is the distance from the charge and k=1/4(pi)e(0) where e(0) is the permittivity of free space

Question 29

What is the equation for calculating the force between 2 point charges?

Answer 29

F=kQ(1)Q(2)/r^2 where Q(1) and Q(2) are the charges on the particles, r is the distance between the 2 charges and k=1/4(pi)e(0)

Question 30

What can be said about a uniformly charged sphere?

Answer 30

It can be treated as a point charge at the centre of the sphere

Question 31

What is the equation for calculating the electric potential around a point charge?

Answer 31

V=kQ/r where Q is the charge, r is the distance and k=1/4(pi)e(0)

Question 32

What is the equation for calculating the electrical potential energy between 2 point charges?

Answer 32

NOTE: this card is a bit of a guess…..
E=QV
so that implies that:
E=kQ(1)Q(2)/r where the Q’s are the charges, r is the distance between them and k=1/4(pi)e(0)