Electromagnetism

Question 1

Define electric field strength

Answer 1

The force per unit positive charge

Question 2

State whether electric field strength is a vector or scaler quantity

Answer 2

Vector quantity

Question 3

State if electric field strength obeys the inverse square law

Answer 3

Yes it does

Question 4

Explain how Millikan was able to keep oil droplets stationary in his field of view when looking through a microscope

Answer 4

Millikan applied an electric field between two metal plates between which the oil droplets fell. This produced an upward force to counter the downward force of gravity. By varying the voltage applied to the plates it was possible for him to exactly balance the two forces and keep the drop stationary

Question 5

X-rays were fired into the chamber that Millikan used. What effect did this have on the oil droplets?

Answer 5

The x-rays fired into the chamber ionised the air. Electrons would attach themselves to the oil droplets thus varying the charge in them

Question 6

Some of the droplets in Millikans experiment were unnaffected by a change in the strength of the electric field the droplets where in. Why would this be the case?

Answer 6

These droplets would not carry any excess charge

Question 7

Provide a defenition of the electronvolt

Answer 7

An electron volt is the kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt

Question 8

State coloumbs law

Answer 8

The force between two point charges is proportional to the product of their charge and inversely proportional to the square of their distance appart

Question 9

Explain what is meant by a ferromagnetic material

Answer 9

A ferromagnetic material is one which can become permanently magnetised

Question 10

Describe how ferromagnetic material can become aligned

Answer 10

They will become aligned if placed in a strong magnetic or electromagnetic field

Question 11

State one difference and one similarity of the magnetic fields around a solenoid and a bar magnet

Answer 11

Similarity - same shape of magnetic field/both have a north and south pole
Difference - magnetic field produced by a current in a solenoid/magnetic field can be switched off in a solenoid

Question 12

A permenant magnet is dropped on the floor by a teacher. Explain why this might cause its magnetic field to weaken

Answer 12

A knock can cause the dipoles to randomise so reducing the strength of the magnetic field

Question 13

An unmagnetised iron bar is placed within a solenoid which is attached to a direct current supply and a steady current is passed through the solenoid. The current is then switched off.

a) State what happens to the magnetic field around the solenoid
b) State what will happen to the magnetic field around the iron bar giving reasons for your answer

Answer 13

a) The magnetic field disapears
b) The magnetic field around the iron bar remains as the dipoles have now become aligned creating a permenant magnetic field

Question 14

An unmagnetised iron bar within a solenoid is attached to an alternating current supply. The iron bar is slowly withdrawn from the solenoid and found to be unmagnetised. Explain this observation

Answer 14

As the current alternates it creates opposing alignment in the dipoles which cancel each other out

Question 15

An electron, a neutron and a proton all enter a uniform magnetic field at right angles with the same velocity. State and compare how the magnetic field will affect the paths of each particles.

Answer 15

The electron will follow a curved path with a certain radius. The neutron will be unaffected as it has no harge. The proton will follow a curved path in the opposite direction to the electron and with a larger radius as it has more mass.

Question 16

Charges particles enter the Earths magnetic field. The particles travel with an initial velocity at an angle of theta to the direction of the magnetic field.

a) What name is given to the type of path followed by the charged particle?
b) Why is the path followed by the charged particle this shape?

Answer 16

a) Helical
b) There is a centripetal force acting on the charged particle due to the component of its velocity in the magnetic field. This produces a circular motion. The particle still has a forward component to its velocity which creates the helical path.

Question 17

A permanent magnet can be made from ferromagnetic material. Explain how ferromagnetic material can be made magnetic and the changes wih take place within it when this happens.

Answer 17

Ferromagnetic material can be made magnetic by placing it within a magnetic or electromagnetic field. The magnetic dipoles in the atom will align themselves with the magnetic field they are in rather than being randomly aligned

Question 18

A student writes in his notebook “electrostatic force and gravitational force obey the inverse square law with electrostatic force requiring two opposite charges.” Explain what is correct and what is incorrect in his statement

Answer 18

It is correct that both obey the inverse square law but the electrostatic force acts between both like and unlike charges

Question 19

Why can magnetic field intensity not be described in the same way as gravitational or electrostatic field intensity?

Answer 19

Magnetic fields are not point sources

Question 20

Which fundamental forces act in the nucleus of an atom?

Answer 20

Strong and weak forces

Question 21

Why do strong and weak forces not have an effect outside the nucleus of an atom?

Answer 21

They only have a very short range

Question 22

What is meant by time constant?

Answer 22

The time constant is the time taken to increase the charge stored by 63% of the difference between initial charge and full charge. If the capacitor is discharging it is the time taken for the capacitor to discharge to 37% of its initial charge

Question 23

Explain what is meant by the term ‘capacitive reactance’ and state the unit is measured in

Answer 23

Capacitive resistance is the opposition to the current flow that a capacitor provides in an a.c circuit. Xc varies inversely with frequency and is measured in ohms

Question 24

Explain why frequency has an effect on the capacitive reactance of a capacitor

Answer 24

Current stops flowing in a circuit when the capacitor becomes fully charged. The higher the frequency, the more often the direction of current changes the less time there is for the capacitor to become fully charged

Question 25

Explain what is meant by a ‘back e.m.f’ and how it is produced

Answer 25

A back e.m.f is the potential difference induced across an inductor when the current flowing through it is changing. Its direction is against the e.m.f which is producing the current

Question 26

State Lenzs Law

Answer 26

Lenzs law states that the induced e.m.f always opposes the change in current which causes it

Question 27

Explain what is meant by the term ‘inductive reactance’ and state the unit it is measured in

Answer 27

Inductive reactance is the opposition to current flow that an inductor provides in an a.c circuit. Xl varies in proprtion with frequency and is measured in ohms

Question 28

Describe James Clerk Maxwells contribution to the unification of electric and magnetic fields

Answer 28

Macwell predicted that elecric and magnetic fields would sustain themselves, free from charges and currents, if they took the form of an electromagnetic wave. The electromagnetic wave is formed when an electric field combines with a mgnetic field. The fields are perendicular to each other and to the direction of travel of the wave