Topic 7 - Electric and Magnetic Fields

Question 1

What is a force field?

Answer 1

A region in which a non-contact force is experienced by a corresponding interacting particle

Question 2

What is an electric field?

Answer 2

A region in which a charged particle will experience a non-contact force

Question 3

What is electric field strength?

Answer 3

The force per unit charge experienced by a charged particle when placed at that point in the field

Force per unit charge acting on a small positive charge

Question 4

State the equation used to calculate the force a charge experiences in an electric field.

Answer 4

Force = electric field strength x charge

F = EQ

Question 5

What law determines the magnitude of the electric force between two charges?

Answer 5

Coulomb’s Law

Question 6

State Coulomb’s law in words.

Answer 6

The force between two charges is directly proportional to the product of their charges and inversely proportional to the square of their separation

Question 7

Give Coulomb’s law in equation form.

Answer 7

F = (kQq) / r^2

Where k = 1 / 4πε0

Question 8

Describe the electric field pattern around a positive point charge.

Answer 8

A radial field, acting outwards

Question 9

Describe the electric field pattern around a negative point charge

Answer 9

A radial field, acting inwards

Question 10

Describe the uniform electric field between parallel plates.

Answer 10

• straight lines of equal spacing
• from positive to negative

Question 11

What equation is used to calculate the electric field between parallel plates?

Answer 11

E = V/d

Question 12

What is electric potential?

Answer 12

The amount of energy required to move a positive test charge from infinity to a given point in an electric field

Question 13

How do you calculate electric potential for a radial field?

Answer 13

F = kQ / r

Where k = 1 / 4πε0

Question 14

What are equipotentials?

Answer 14

Equipotentials are lines along which the electric potential remains the same

Question 15

How do you calculate the work done in a moving charge along an equipotential?

Answer 15

No work is done when moving a charge along an equipotential since the electric potential doesn’t change

Question 16

What does a capacitor do?

Answer 16

Capacitors are used to store charge in a circuit

Question 17

What two factors determine how much charge can be stored by a capacitor?

Answer 17

• The potential difference across it
• The capacitance

Question 18

What is capacitance?

Answer 18

Capacitance is a measure of how much charge can be stored by a capacitor per unit potential difference across it

Question 19

State the equation used to calculate the capacitance from the charge and potential difference.

Answer 19

Capacitance = charge / potential difference

C = Q/V

Question 20

What is the unit of capacitance?

Answer 20

CV^-1

F (farad)

Question 21

What is represented by the area under a capacitor’s charge-potential graph?

Answer 21

The energy stored by a capacitor

Question 22

What is represented by the gradient of a capacitor’s charge-potential graph?

Answer 22

Capacitance

Question 23

Give three equations to calculate the energy stored by a capacitor.

Answer 23

W = 1/2 QV
W = 1/2 CV^2
W = (1/2 Q^2) / C

Question 24

Describe a charge-time graph for charging a capacitor. What does the gradient represent?

Answer 24

• positive curve (like top left corner of circle)
• plateaus off
• gradient = current

Question 25

Describe a current-time graph for charging a capacitor. What does the area represent?

Answer 25

- negative curve (like bottom left corner of a circle) - exponential - area = charge

Question 26

Describe a voltage-time graph for charging a capacitor.

Answer 26

- positive curve (like top left corner of a circle) - plateaus off

Question 27

What value is given by the product of resistance and capacitance?

Answer 27

The time constant of the circuit

Question 28

What does the time constant of a circuit tell you?

Answer 28

The time the capacitor will take to reach 63% of its total charge, and the time taken for it to discharge to 37% of its full charge

Question 29

What is a magnetic field?

Answer 29

A magnetic field is a region in which a magnetic pole will experience a non-contact force

Question 30

In which direction do magnetic field lines point?

Answer 30

From north to south

Question 31

What is magnetic flux density?

Answer 31

Magnetic flux density is a measure of the strength of a magnetic field. It can be viewed as the number of magnetic field lines that pass through a given area

Question 32

What three factors determine the force exerted on a charge moving through a magnetic field?

Answer 32

- the magnetic flux density - the charge of the particle - the velocity of the particle perpendicular to the field

Question 33

State the equation used to calculate the force exerted on a charge moving through a magnetic field.

Answer 33

F = Bqvsinθ

Question 34

What is the relationship between the direction of a charge’s motion and the direction of the magnetic force it experiences?

Answer 34

The force is always perpendicular to the charge’s motion

Question 35

Describe and explain the path taken by a charge in a magnetic field.

Answer 35

The charge will move in a circular path. This is because the magnetic force always acts perpendicular to the charge’s motion and so acts as a centripetal force

Question 36

What is produced by a current-carrying wire?

Answer 36

A magnetic field is produced in concentric circles around a wire when a current passes through it

Question 37

What occurs when a current-carrying wire is placed in a magnetic field?

Answer 37

The wire will experience a force due to the permanent magnetic field interacting with a the wire’s magnetic field

Question 38

What three factors affect the force experienced by a current-carrying wire placed in a magnetic field?

Answer 38

- the magnetic flux density of the wire - the current passing through the wire - the length of the wire

Question 39

State the equation used to calculate the force experienced by a current-carrying wire.

Answer 39

F = BILsinθ

Question 40

What is Fleming’s left-hand rule used for?

Answer 40

To determine the direction of the force experienced by a current-carrying wire or moving charge in a magnetic field

Question 41

What does the thumb represent when using Fleming’s left-hand rule?

Answer 41

The thumb represents the direction of the force

Question 42

What does the first finger represent when using Fleming’s left-hand rule?

Answer 42

The direction of the field

Question 43

What does the second finger represent when using Fleming’s left-hand rule for a moving charge?

Answer 43

The direction that a positive charge would move. This means that if it is a negative charge, you must point your second finger on the opposite direction to its motion

Question 44

What does the second finger represent when using Fleming’s left-hand rule for a current-carrying wire?

Answer 44

The direction of conventional current flow

Question 45

What is electromagnetic induction?

Answer 45

When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force (as they are charged particles), and build up on one side of the rod, causing an emf to be induced in the rod

Question 46

What is Faraday’s law?

Answer 46

The magnitude of induced emf is equal/directly proportional to the rate of change of flux linkage ε = N (ΔΦ) / (Δt)

Question 47

What is Lenz’s law?

Answer 47

The induced emf is always in such a direction as to oppose the change/motion that caused it ε = -d(NΦ) / dt