C8 (Charge and Current) Flashcards
Electrical charge is measured in..
Amperes (amps)
Electrical current is defined as…
The rate of flow of charge
Equation for current
I= change in Q/ change in t
One ampere is equivalent to what passing a given point per second
One coulomb of charge
What is electrical charge
A physical property
Some particles are charged like protons and electrons others like neutrons aren’t
Any object that isn’t charged is called…
Neutral
What are the two types of forces
And how do they interact
Positive and negative
Like charges repel, opposite charges attract
Electric charge is measured in…
Coulombs (C)
One coulomb is defined as….
The electric charge flowing past a point in one second when there is an electric current of on ampere
Equation for charge
Change in Q= I x change in t
Any particle that has electric charge is a…
Charged carrier
What is the value of the elementary charge
1.6 x 10^19 C
If we know the value of current in a metal wire we can calculate…
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).
The charge of most objects is usually due to a..
Loss or gain of electrons by the object
How can the size of charge on a particular object be expressed
Q=+/- ne
Q is the net charge
We describe the charge of an object as being what
And why
Quantised
This is because charge can only have certain values (that are integer multiples of e)
In a metal an electric current is usually what
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.
How do electrons move
From a negative terminal to a positive one
The greater the rate of charge flow the greater…
The electric current in the wire
A larger current may be due to:
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
Conventional current
The flow of current from the positive terminal to a negative terminal
Electron current
The flow of electrons from the positive terminal to the negative one
Electrolytes
Liquids that can carry electric current
What is the electric current in electrolytes
Not a flow of electrons but a flow of ions
Ammeter
Measures the electric current at any point in a circuit
Always placed directly in series
Once ammeters are placed in series they must have what
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
Conservation of charge
States that electric charge can neither be created nor destroyed. The total amount of electric charge in the universe is constant.
Kirchoff’s first law
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
What’s is Kirchoff’s first law based on
Conservation of charge
Where the charge is a product of the current and the time
What does current depend on
The speed at which free electrons are moving
What is the number density (n)?
Number of free electrons per unit volume
The higher the number density the greater the number of…
Free electrons per m^3 and so the better the electrical conductor
What are the three groups we can classify groups according to their number density?
Conductors (very high n)
Semiconductor (middle)
Insulators (much lower the semiconductors)
As semiconductors have a much lower density number then conductors what must be altered so they can match the same current?
The electrons must move much faster
This leads to an increase in temperature of semiconductors
What is drift velocity
Repeated collisions between positive ions and free electrons (causing the random motion) known as drift velocity
Additional equation for current (using drift velocity)
I= A n e v
What does the components of I= A n e v
Stand for and what are their units?
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
Is the equation for current homogeneous
Yes
What happens if the cords sectional area of a wire changes
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
Drift velocity is inversely proportional to what?
Drift velocity is inversely proportional to cross sectional area
