8 : Charge and current

Question 1

Electrical current

Answer 1

• Measured in amperes
• the rate of flow of charge
• l= change in charge / change in time
• the amount of charge passing a given point in a circuit per unit time

Question 2

Electrical charge

Answer 2

• It is a physical property much like mass volume or temperature.
• Any object with no charge is called neutral.
• There are two types of charge positive and negative objects with charge, interact and exert forces on each other
• like charges repel an opposite charges attract.

Question 3

Measuring charge

Answer 3

• Measured in coulombs
  Electric charge flowing past a point in one second when there is an electric current of one ampere
• charge= current / change in time
• any particle that has an electric charge is a charge carrier
• relative charges = +1 or -2
• elementary charges, e=1.6 x 10^ -19 C

Question 4

Net charge

Answer 4

• Charge on object as being quantised
• Q = + or- ne
• Q is the net charge

Question 5

Electric current in metals

Answer 5

• Because of way atoms are bonded in a metal most electrons remain fixed but small number from each atom are free to move
• structure ora metal can be thought of as a regular crystal structure or lattice of positive ions surrounded by a number of free electrons
• positive ions are not free to move but vibrate around fixed points
• to make electrons move is to make one end of a wire positive and the other negative - electrons will be attracted towards positive end

Question 6

A larger current may be due to

Answer 6

The greater rate of flow of charge the greater the electric current
- a greater number of electrons moving past a given point each second (greater cross-sectional area )
- same number of electrons moving faster through metal

Question 7

Conventional current and electron wheels

Answer 7

• Conventional current is the current from the positive terminal towards a negative terminal
• in metals elections travel from the negative terminal to the positive terminal - but this flow of electrons is in the opposite direction to the conventional current

Question 8

Electrical current in electrolytes

Answer 8

Electrolytes are liquids that can carry an electric current - not a flow of electrons but flow of ions
- positive electrode (anode) and negative electrode (cathode) are placed in solution -the ions are attracted to the electrodes

Question 9

Conservation of charge

Answer 9

• electric charge can neither be created nor destroyed
• the total amount of electric charge in the universe is constant

Question 10

Kirchhoff first law

Answer 10

• Any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point
• charge carriers into point in a given time must be equal to the total number of charge carriers lowing that same point during that time

Question 11

Classification of materials

Answer 11

• The number density is in the number of free electrons per cubic metre of material. the higher the number density the greater the number of free electrons per m^3 and so the better the electrical conductor
• conductors have very high number density
• semiconductors have much lower number density than metals so in order to carry same current electrons must travel much faster which increases the temperature

Question 12

How fast do charge carriers move

Answer 12

-Most charge carriers move slowly
- free electrons repeatedly collide with the positive metal ions as they drift through the wire towards the positive terminal
- all the free elections in the wire start moving almost at once

Question 13

Mean drift velocity

Answer 13

I= Anev
- A = cross sectional area
-I = electrical current
- v= mean drift velocity

Question 14

Effect of changing cross sectional area

Answer 14

• Area changes so must the right velocity the narrower the wire the greater the drift velocity must be in order for the current to be the same