What is current?

Rate of flow of charge

What is the direction of conventional current flow?

Flows from +’ve to -‘ve

What is the current equation (with Q and t)

I = Q/t

I - current

Q - charge

t - time

What is one coulomb defined as?

Defined as the the amount of charge that passes in 1 second when current is 1 ampere

What is the symbol for a coulomb?

C

How do you attach an ammeter to a circuit?

You attach it in series

What is Potential Difference defined as?

1 V is when you do 1 Joule of work moving 1 coulomb of charge through a component

What is the Potential Difference equation (with E & Q)

V = E/Q

V - Potential Difference

E - Energy transferred/work done

Q - Charge

How do you attach a Voltmeter to a circuit?

In parallel

What is the maximum value a voltmeter or ammeter can read called?

Full scale deflection

What are charge carriers in liquid and gases called?

Ion

In a circuit what are the flow of charges particles?

Charge carriers (free electrons)

Does a flow of positively charged particles produce a current?

Produces the same current as if it were negative but in the opposite direction

Ionic crystals are [……]

Insulators

Once a ionic Crystal is molten they […….] electricity

Conduct

What is the mean drift velocity?

Is the average velocity of the charge carriers

What is the current equation with drift velocity?

I = nqvA

I = electrical current (A) n = number density of charge carriers (m-3) q = charge in each carrier (C) v = mean drift velocity (ms-1) A = cross sectional area (m*2)

What is the charge of an electron?

-1.6 x 10*19

So what does current depend on?

- Number of charge carriers
- velocity of charge carriers
- area of the median the current is travelling through
- the amount of charge a carrier has

Different materials have different numbers of […..][…..]

Charge carriers

What are free electrons?

The electrons on the outer wheel of an atom.

Semiconductors have […….] charge carriers

Fewer

What do the charge carriers in a semiconductor need to have in order to give the same current than a metal?

Have a greater drift velocity

How many charge carriers would a perfect insulator have?

0 so there would be no current

If you put a 1-[………][……] across an electrical component a 2-[…..] will flow.

1 - Potential Difference

2 - Current

How much 1-[……..] you get for a particular p.d depend on the 2-[………] of the component

1 Current

2 Resistance

What is the resistance equation with (V & I)

R = V/I

Define resistance?

1 ohm is if a Potential Difference of 1V make a current of 1 A flow through it.

What three things determine resistance?

- Length
- Area
- Resistivity

How does length affect resistance?

The longer the wire the more difficult it is to make current flow

How does area affect resistance?

The wider the wire the easier it is to make a current flow

How does resistivity determine resistance?

- Depends on the material it is made from

- Temperature

Define resistivity?

The resistance of a 1m length with a 1 m*2 cross sectional area

What is resistivity measured in?

Ohm meters (Ωm)

What is the resistivity equation?

R = pl/A

R - Resisitance

p - Resistivity

l - Length (Meters)

A - cross sectional area (m*2)

How do you know if you have gone way off when calculating resistivity?!

Typical values of resistivity conductors are really small.

But if you get 1 x 10*-7 go back and check you’ve converted area into m*2

What is the resistivity of a material related to?

- Number density of charge carriers ( and their mean drift velocity which varies with temperature)
- the higher the current the lower the resistance
- therefore the lower the materials resistivity

For a [……][…….] R is constant on a P.D (X axis) Current (y axis) graph

Ohmic conductor

What are conductors that obey Ohm’s Law called?

Ohmic conductors

What is Ohm’s Law?

Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the Potential Difference across it ( I ∝ V )

For an Ohmic Conductor what does doubling the p.d do to the current?

Doubles it too!

Finding the Resistivity of Wire - what do you need to find out first about the wire before finding out the resistivity?

Resistance

Finding the Resistivity of Wire - What is the first measurement we take?

Cross sectional area of a wire (we assume that the wire is cylindrical so the wire is circular)

Using the equation AREA = πr*2

Finding the Resistivity of Wire - What instrument do you use to find the diameter of the wire?

Micrometer

Finding the Resistivity of Wire - Wen finding the diameter of the wire how many measurements do you take along the wire?

3 measurements at different points along the wire

What is micrometer also called?

Micrometer caliper

Finding the Resistivity of Wire - What is step 1?

Clamp the test wire to a ruler connected ruler to the circuit at the point where the ruler reads 0

Finding the Resistivity of Wire - What is step 2?

Attached the flying lead to the test wire

Finding the Resistivity of Wire - What is step 3?

Record the leg the of the test wire connected and the voltmeter reading and the ammeter reading

Finding the Resistivity of Wire - What is step 4?

Use the readings on the voltmeter and the ammeter to calculate the resistance - R = V/I

Finding the Resistivity of Wire - What is step 5?

Repeat the experiment for several different lengths within a sensible range

Finding the Resistivity of Wire - What is step 6?

PLOT A GRAPH - of resistance against length and draw a line of best fit

When calculating the resistivity of wire experiment, on the graph of resistance against length what does the gradient of the line equal?

Gradient is R/I = p/A

- So multiply the gradient of the line of best fit by the Cross Sectional Area to find the resistivity of the wire!

Finding the Resistivity of Wire - What could cause results to be inaccurate?

Resistivity depends on TEMPERATURE!