Electric Fields

Question 1

how do we accelerate electrons in x-rays

Answer 1

by putting them in the appropriate electric field

Question 2

how do we visualise the forces particles feel when in an electric field

Answer 2

by drawing electric field lines

Question 3

what is a fundamental law about all field lines

Answer 3

• the closer the field lines are
• the stronger the field
• so the stronger the force exerted

Question 4

what things can only be affected in an electric field

Answer 4

charged particles

Question 5

what is the equation for calculating the force a charged particle feels

Answer 5

F = EQ

Question 6

what are each of the variables in that equation

Answer 6

• F = force (N)
• E = electric field strength (Vm-1)
• Q = coulombs of charge

Question 7

how would you use that equation to calculate the acceleration of a charged particle within an electric field

Answer 7

• F = ma
• ma = EQ
• a = EQ / m

Question 8

what force will an electron feel when it is in an x-ray machines electric field which has a strength of 4.5x10^5 Vm-1

Answer 8

• F = EQ
• Q of an electron = -1.6x10-19
• (4.5x10^5)x(-1.6x10-19) = -7.2x10-14 N

Question 9

why do we use a negative sign for the charge of an electron

Answer 9

• because the force is trying to accelerate the electron to the more positive end of the field
• this is in the opposite direction to the conventional field
• which goes from positive to negative (like field lines)

Question 10

how would you calculate the velocity of the electron if it accelerates from rest for a distance of 10cm

Answer 10

• you need a kinematics equation with all the variables you need
• without time, its s = (v^2 - u^2) / 2a (v^2 = u^2 + 2as)
• as u = 0, youre left with v = the root of 2as
• as we know a = EQ / m
• root 2as = root (2EQs / m)
• inputting the numbers gives you a negative velocity
• as the electron is moving in the opposite direction to the field

Question 11

what is electric potential

Answer 11

• the energy a charged object posses
• by virtue of its position within an electric field
• per unit charge

Question 12

what transfer of energy occurred with the electron in the x-ray machine

Answer 12

• a transfer of its electric potential

- to its kinetic energy stores

Question 13

what is the equation for electric potential

Answer 13

V = E(p) / Q

Question 14

what similarity does this equation have to the equation for electric field strength

Answer 14

• F = QE goes to E = F / Q

- which tells us that electric field strength is the force felt by a particle within an electric field per unit charge

Question 15

what equation can you use to work out the kinetic energy a particle gains by moving through an electric field

Answer 15

E(k) = QV

Question 16

what is the equation similar to

Answer 16

• the energy supplied to a component within a circuit with a given charge and pd
• E = QV

Question 17

what is the mass of an electron

Answer 17

9.11x10-31 kg

Question 18

what is a uniform electric field

Answer 18

• an electric field between two plates with opposite charges

- in which the field lines connecting them are parallel and equidistant

Question 19

what is the definition of the strength of a uniform electric field

Answer 19

a measure of how rapidly it changes the potential over a distance

Question 20

what is the equation for the electric field strength of a uniform field

Answer 20

E = V / d

Question 21

what are equipotentials

Answer 21

positions within a field with no potential difference between them

Question 22

how would you draw the equipotentials in a uniform field

Answer 22

• vertical lines parallel to the plates

- equidistant from each other

Question 23

why would you draw them like this

Answer 23

• because the pd across the plates changes the closer to get to one or the other
• so to maintain the same distance between both plates
• you would need to be parallel to them

Question 24

what is the general rule for drawing equipotential field lines

Answer 24

• they will always be perpendicular to the field lines
• as field lines are indicating the directions that pd across the charged bodies would change
• so going perpendicular to that would give you the opposite effect