Chapter 8

Question 1

Electric current definition

Answer 1

Rate of flow of charge carries

Question 2

Current formula

Answer 2

I=q/t

Question 3

What is conventional current

Answer 3

The flow of positive charge from the positive terminal to the negative terminal. This is opposite to electron flow as the electron hadn’t yet been discovered

Question 4

How to use an ammeter to measure current is a circuit

Answer 4

• Must be connected in series
  -Have a very low resistance in order to reduce the effect on the current

Question 5

What is one coulomb

Answer 5

1 coulomb = The quantity of electricity conveyed by 1 amp of current flowing in 1 second
C=As

Question 6

What is the elementary charge

Answer 6

Refers to the actual charge of a proton/electron
Proton: 1.6x10^-19
Electron: -1.6x10^-19
It is denoted by (e)

Question 7

Give to examples of an electric current

Answer 7

Electron flowing through a metal wire
Ions moving through an electrolyte

Question 8

Define electron flow

Answer 8

Electron flow is the flow of electron from the negatively charged terminal to the positively charged terminal. The electrons are repelled from the negatively terminal and are attracted to the positive terminal

Question 9

What is the movement of electrons in a metal

Answer 9

In metallic bonding, the metals form positively charged ions, leaving a sea of delocalised electrons which are free to move. (Also known as an conduction metals)
To make the delocalised electrons move give each end of the wire a positive and negative charge. The electrons in the wire will be attracted to the positive terminal generating an electric current.

Question 10

How to generate a larger electric current

Answer 10

-Increase the cross-sectional area of the wire
(A greater number of charge carries will move past a given point)
-Decrease the length of the wire
(The same number of electrons moving faster through the metal as they have less distance distance to travel)

Question 11

Electric currents in electrolytes

Answer 11

Cathode is negatively charged, so when a cation reaches the cathode it accepts an electron from the cathode to become neutral. And an anion donated its electron to the anode so electrons can flow through the metal part of the wire

Question 12

State Kirchoff’s first law

Answer 12

Kirchoff’s first law stated that the sum of the current going into a junction must equal the sum of the current coming out of a junction, this is due to conservation of charge (as charge can’t be created or destroyed)
Therefore the total amount of electric current in the universe is constant.

Question 13

Define number density

Answer 13

The number of free charge carries per unit volume
Units (m^-3)
The higher the number density, the greater the amount of free electrons per unit volume, so the better the electrical conductor

Question 14

Number density of 1) insulators 2) Semi-conductors 3) conductors

Answer 14

1) insulators such as rubber or wax have very low number densities
2) Semi-conductors such as silicon or graphite have a number density around x10^17
3) Conductors (metals) have a high number density around x10^28

Question 15

New electric current equation

Answer 15

I=nAVe
Current = number density x cross-sectional area x mean drift velocity velocity x elementary charge
Units : A=m^-3x m^2xms^-1 x As

Question 16

How do semi-conductors generate the same current as metals

Answer 16

As the number density of semi-conductors is much lower compared to metals, the charge carries travel at a much faster means drift velocity compared to metals in order to maintain the same current. Semiconductors mean drift velocity is roughly 2.9x10^8m/s (close to the spread of light). Whereas metals mean drift velocity is usually measured in mm /s due to how slow the charge carries move. This increases the temperature of the semi-conductor

Question 17

Charge flow equation

Answer 17

Charge flow = number of charge carriers x elementary charge
Q=ne

Question 18

The effect of changing the cross-sectional area if current MUST remain constant

Answer 18

When increasing the cross-sectional area of a wire it increase the number density as there are more charge carries per unit volume. Therefore in order to maintain the same current, mean drift velocity must decrease. If the wire gets narrower , there are fewer charge carriers per unit volume, therefore to maintain the current mean drift velocity must increase.
Velocity is inversely proportional to area

Question 19

If area doubles and length doubles what happens to mean drift velocity if current remains constant

Answer 19

V/2

Question 20

If radius halves what happens to mean drift velocity if current remains the same

Answer 20

Velocity must quadruple as cross-sectional area was decreased by 1/4

Question 21

Why do lights turn on so quickly

Answer 21

The electrons don’t rush to the switch, the electrons actually move very slowly, but they all move at once. Therefore they repeatedly collide with one another as well as the positive metal ions inside of the wire, giving them each a random velocity. Therefore they repeatedly free electrons drift towards the positive terminal of the wire. The distance per unit time a long the length of the wire is known as the mean drift velocity