Physics__Electric and Magnetic Fields

Question 1

What is an electric field?

Answer 1

A region where a charged particle experiences a force.

Question 2

Equation for electric field strength E?

Answer 2

E = Force/Charge

Question 3

What’s electric potential?

Answer 3

Energy per unit charge needed to reach a certain point in space from infinity (where there’s no force of attraction or repulsion)

Question 4

How to calculate electric field strength between 2 parallel plates?

Answer 4

E= V/d

Question 5

What is the time constant?

Answer 5

RC The time for a capacitor to discharge to 1/e (~37%) of original value.

Question 6

What’s magnetic flux density?

Answer 6

The strength of a magnetic field - unit Tesla

Question 7

How to calculate energy transferred when charging a capacitor?

Answer 7

1/2 Vmax Q

Question 8

Capacitor energy equations?

Answer 8

W= 1/2 x CV2 W= Q2/2C

Question 9

What’s magnetic flux?

Answer 9

The magnetic field or magnetic field lines passing through a given area
When the field is perpendicular to the area phi = BA
Where B is magnetic flux density,
and A is the area.

Question 10

What’s magnetic flux linkage?

Answer 10

The magnetic flux multiplied by the number of turns N , of a coil.
N*PHI = BAN.
B is the magnetic flux density,
A is the area
and N is the number of turns

Question 11

How does a magnetic field effect a moving charged particle?

Answer 11

It exerts a force on charged particles.
This force can be calculated with F = Bqvsin(theta)¸
theta is angle between the velocity of the particle and the direction of the magnetic field

Question 12

How can we determine the direction of the force exerted on a charged particle and the direction of the magnetic field?

Answer 12

fleming’s left hand rule.
ThuMb - Motion /force,
First finger - direction of Field,
SeCond finger - direction of the Conventional Current

Question 13

What causes charged particles to follow a circular path when in a magnetic field?

Answer 13

> Force exerted is always perpendicular to the motion of travel (FLHR)
This provides centripetal force
So charged particles follow a circular path

Question 14

How does a magnetic field effect a current carrying conductor

Answer 14

It exerts a force on the current carrying conductor.
This force can be calculated with F = BilsinTHETA¸
THETA¸ is angle between the wire and the direction of the magnetic (B) field

Question 15

What factors effect induced e.m.f. in a coil when there is relative motion between the coil and a permanent magnet

Answer 15

> Moving coil/magnet faster increases e.m.f.
Adding more turns to the coil increases e.m.f.
Increasing strength of magnetic field/bar magnet increases induced e.m.f.

Question 16

What is electromagnetic induction?

Answer 16

The process in which an e.m.f is induced in a closed circuit due to changes in magnetic flux (linkage)

Question 17

How does electromagnetic induction work?

Answer 17

> When a conductor moves through a magnetic field, there is a change of magnetic flux/+linkage.
Mechanical work is transformed into electrical energy.
If attached to a complete circuit, current is induced in the conductor

Question 18

What factors affect the e.m.f. induced in a coil when there is a change of current in another coil linked with this one?
I.e. how is induced emf in secondary coil affected by changing current in the primary coil of a transformer

Answer 18

Think: transformers
> Alternating current producing an alternating voltage on the primary coil is applied.
> This creates an alternating magnetic field inside the iron core, and thus a changing magnetic flux linkage.
> A changing magnetic field passes through the secondary coil through the iron core,
> Resulting in a changing magnetic flux linkage in the secondary coil and (from Faraday’s Law) an e.m.f. is induced.
> E.m.f. produces alternating output voltage of the same frequency as the input voltage

Question 19

What is Lenz’s law?

Answer 19

> The (induced) e.m.f is such as to oppose the change creating it
Consequence of conservation of energy - Electrical energy gained by the induction of a current is offset by an equal amount of energy being removed.

Question 20

How do we know Lenz’s law is true?

Answer 20

Assume the opposite to Lenz’s law were true.
> The direction of the induced current supports the motion of the magnet
> so the opposite pole to the one of the magnet which is approaching the coil is induced, in order to attract the magnet.
> This causes the magnet to speed up, due to electrostatic forces of attraction ,
> so the magnet would gain kinetic energy from no where! THIS VIOLATES THE CONSERVATION OF ENERGY!!!!

Question 21

How does Faraday’s Law work?

Answer 21

Induced emf = no. of turns x rate of change of magnetic flux

Question 22

How can Lenz’s law and Faraday’s law be combined?

Answer 22

e = - d(NPHI)/dt

Question 23

Why might a magnet take longer to fall through a copper tube than a plastic tube, as a consequence of electromagnetic induction?

Answer 23

When magnet falls, there’s a rate of change of magnetic flux with the copper tube.

The change in flux linkage induces an emf (Faraday’s law).

The induced emf causes current to flow in the tube,

the induced emf is in such a direction as to oppose the change in flux linkage (Lenz’s law).

A force is exerted on the magnet opposing its motion.

As plastic is not a conductor, no current/emf is induced, so shorter time to fall through tube