C8 (Charge and Current)

Question 1

Electrical charge is measured in..

Answer 1

Amperes (amps)

Question 2

Electrical current is defined as…

Answer 2

The rate of flow of charge

Question 3

Equation for current

Answer 3

I= change in Q/ change in t

Question 4

One ampere is equivalent to what passing a given point per second

Answer 4

One coulomb of charge

Question 5

What is electrical charge

Answer 5

A physical property

Some particles are charged like protons and electrons others like neutrons aren’t

Question 6

Any object that isn’t charged is called…

Answer 6

Neutral

Question 7

What are the two types of forces

And how do they interact

Answer 7

Positive and negative

Like charges repel, opposite charges attract

Question 8

Electric charge is measured in…

Answer 8

Coulombs (C)

Question 9

One coulomb is defined as….

Answer 9

The electric charge flowing past a point in one second when there is an electric current of on ampere

Question 10

Equation for charge

Answer 10

Change in Q= I x change in t

Question 11

Any particle that has electric charge is a…

Answer 11

Charged carrier

Question 12

What is the value of the elementary charge

Answer 12

1.6 x 10^19 C

Question 13

If we know the value of current in a metal wire we can calculate…

Answer 13

The charge passing through it in the a given time and the number of electrons (divide the charge by the charge e on each electron, the elementary charge).

Question 14

The charge of most objects is usually due to a..

Answer 14

Loss or gain of electrons by the object

Question 15

How can the size of charge on a particular object be expressed

Answer 15

Q=+/- ne

Q is the net charge

Question 16

We describe the charge of an object as being what

And why

Answer 16

Quantised

This is because charge can only have certain values (that are integer multiples of e)

Question 17

In a metal an electric current is usually what

Answer 17

A flow of electrons
Because of the way atoms are bonded, most electrons in metal atoms are fixed to their atom. However a small number of electrons from each atom are free to move.

Question 18

How do electrons move

Answer 18

From a negative terminal to a positive one

Question 19

The greater the rate of charge flow the greater…

Answer 19

The electric current in the wire

Question 20

A larger current may be due to:

Answer 20

Greater number of electrons moving past a given point each second (i.e wire with large cross sectional area).

Same number of electrons moving faster through the metal

Question 21

Conventional current

Answer 21

The flow of current from the positive terminal to a negative terminal

Question 22

Electron current

Answer 22

The flow of electrons from the positive terminal to the negative one

Question 23

Electrolytes

Answer 23

Liquids that can carry electric current

Question 24

What is the electric current in electrolytes

Answer 24

Not a flow of electrons but a flow of ions

Question 25

Ammeter

Answer 25

Measures the electric current at any point in a circuit

Always placed directly in series

Question 26

Once ammeters are placed in series they must have what

Answer 26

The lowest possible resistance to reduce the effect they have on the current.
The ideal (perfect) ammeter has zero resistance and so has no effect on the current it measures

Question 27

Conservation of charge

Answer 27

States that electric charge can neither be created nor destroyed. The total amount of electric charge in the universe is constant.

Question 28

Kirchoff’s first law

Answer 28

Deals with electric current
States that for any point in an electrical circuit the sum of currents into that point is equal to the sum of currents our of that point

Question 29

What’s is Kirchoff’s first law based on

Answer 29

Conservation of charge

Where the charge is a product of the current and the time

Question 30

What does current depend on

Answer 30

The speed at which free electrons are moving

Question 31

What is the number density (n)?

Answer 31

Number of free electrons per unit volume

Question 32

The higher the number density the greater the number of…

Answer 32

Free electrons per m^3 and so the better the electrical conductor

Question 33

What are the three groups we can classify groups according to their number density?

Answer 33

Conductors (very high n)
Semiconductor (middle)
Insulators (much lower the semiconductors)

Question 34

As semiconductors have a much lower density number then conductors what must be altered so they can match the same current?

Answer 34

The electrons must move much faster

This leads to an increase in temperature of semiconductors

Question 35

What is drift velocity

Answer 35

Repeated collisions between positive ions and free electrons (causing the random motion) known as drift velocity

Question 36

Additional equation for current (using drift velocity)

Answer 36

I= A n e v

Question 37

What does the components of I= A n e v

Stand for and what are their units?

Answer 37

I= electric current in conductor, measured  in Amps
A= cross sectional area in m^2
n= number density
e= elementary charge (1.60 x 10^-19) in Coulombs 
v= mean drift velocity of charge carriers, in m/s

Question 38

Is the equation for current homogeneous

Answer 38

Yes

Question 39

What happens if the cords sectional area of a wire changes

Answer 39

So must the mean drift velocity

Narrower the wire the greater the drift velocity must be in order to supply the same level of current

Question 40

Drift velocity is inversely proportional to what?

Answer 40

Drift velocity is inversely proportional to cross sectional area