Topic 5 - Electricity and Magnetism

Question 1

Electric Field Strength

Answer 1

The force per unit charge acting on a small charge placed at that point in the field

Question 2

Electromotive force (emf)?

Answer 2

A measure of the power that a cell or battery is able to supply
Measured in volts.

Question 3

What is meant by power dissipation?

Answer 3

Power dissipation is a measure of how much power inn a circuit is converted into heat

P = I x V

Question 4

Definition of “Electronvolt”

Answer 4

The energy gained by an electron as it moves through the potential difference of one volt

= 1.6 x 10 ^-19 J

Question 5

How should an ammeter and a voltmeter be connected in order to measure the current and resistance across a resistor

Answer 5

Ammeter = in series

Voltmeter = in parallel

Question 6

What energy changes take place in a cell or battery?

Answer 6

Energy is coverted from chemical energy to electrical energy.

Question 7

What is the right-hand corkscrew rule?

Answer 7

If you point your right thumb in the direction of the conventional current, your curling fingers will show the direction of the magnetic field created due to the current.

Question 8

By what factor will the area of a circle change when the radius is changed?

Answer 8

When the radius of a circle is change by a certain factor its area will change by the square of this factor

Question 9

What does Coulomb’s Law tell us

Answer 9

The size of the force between two charged particles

Question 10

What is meant by “conventional current” ?

Answer 10

The direction in which positive charge flows

Also opposite to the direction negative charge flows

Question 11

Fleming’s Left Hand Rule

Answer 11

Thumb: direction of the (electromagnetic) force
Index: direction of the field
Middle: direction of the conventional current

Question 12

Define: ideal ammeter

Answer 12

An ideal ammeter does not affect the circuit, so the ammeter should have no resistance.

Question 13

Define: ideal voltmeter

Answer 13

An ideal voltmeter has infinite resistance so that there is no effect on the circut. (/ so that no current flows through it.)

Question 14

What happens to the kinetic energy of a particle when it is accelerated through a certain potential difference?

Answer 14

Its kinetic energy is equal to the work done by the electric field.

V = W/ q  
(V = potential difference, W= work done, q =charge)

E (kinetic) = W = 0.5mv^2

Question 15

What is meant by “the capacity of a cell”? and how is it calculated?

Answer 15

The amount of energy that a cell or battery can deliver in a single discharge

Capacity = current x time

Question 16

What do all the letters represent in the equation

I = nAvq ?

Answer 16

I = current
n = number of charge carriers per unit volume (also the density)
A = cross-sectional area
q = the charge of each charge carrier

Question 17

What is significant about the values of I,n and q when two wires of the same material are connected in series?

Answer 17

When two wires of the same material are connected in series, I, n and q are the same in both wires.

Question 18

What happens to the current when it enters a junction in a parallel circut

Answer 18

When current enters a junction it splits and flows through each branch. The magnitude (of current) in each branch depends on the resistance in that branch. The larger the resistance, the smaller the current.

Question 19

What is meant by internal resistance of a cell?

Answer 19

The resistance due to the components/materials in the cell that leads to energy losses.

Question 20

What happens to the resistance in a wire if the crosse-sectional area increases?

Answer 20

The resistance decreases

Question 21

The temperature of a metal cylinder is decreased. Explain, referring to the electrons in the metal, what happens to the resistance of the cylinder.

Answer 21

As temperature goes down, the vibration of the lattice ions in the cylinder decreases.

This leads to fewer collisions with the electrons.

As a result, drift speed (current) increases, so resistance decreases.

Question 22

What happens to the resistance when the diameter of a metal cylinder is halved and its length is doubled?

Answer 22

When diameter is halved, cross-sectional area (A) decreases to a quarter of its original value.

According to the equation R = (resistivity x L)/ A
​
when A decreases to a quarter of its original value and length is doubled, resistance increases eightfold.

Question 23

Why it is useful for scientists to know such quantities as resistivity?

Answer 23

It helps scientists compare properties of different materials.

OR

Correct materials can be selected for a certain experiment or project.

Question 24

What assumption is made when calculating the power dissipated in a cell?

Answer 24

We assume that the wiring has no internal resistance.

As a result, the calculated value of the internal resistance is larger than its actual value, so the calculated value of the power dissipated in the cell is also larger than its actual value.

Question 25

What does Theta in the magnetic force equation represent ?

Answer 25

The angle between the direction of the magnetic field and the direction of the conventional current.

Question 26

What is the difference between a conductor and an insulator?

Answer 26

A conductor has (a large amount of) free electrons that can move through it to create a current.

An insulator does not have (a large amount of) free electrons, so it will not have a current flowing through it.

Question 27

What is meant by emf of a cell?

Answer 27

The energy provided by the cell per coulomb of charge that passes through it.

OR

The work done per unit charge when moving a certain quantity of charge completely around a circuit.