1. So how does an ammeter work?
Current in a coil of wire in the ammeter causes a magnetic field.
These repel a permanent magnet and cause the dial to rotate slightly
The higher current, the stronger the magnetic field and therefore the larger the amount of rotation.
Changing the scale on an ammeter, changes the fraction of current flowing through the coil of wire inside the ammeter.

2. On the white boards Circuit symbols for the following; Filament bulb
Lead
Fixed resistor
Rheostat
Heater
LDR
LED
Thermistor
Earth
Loud speaker

3. Electrical Charge 18/06/2018 Define the Coulomb
Describe how an ammeter works
Recall and use the elementary charge
Relate lesson to Kirchhoff’s 1st Law

4. Last lesson we looked at Kirchhoff's Law that the sum of currents going into junction = the sum of currents coming out of the junction.
Current can be defined as the rate of flow of charge. So Kirchhoff’s law simply states that what flows into the branches must flow out once they are re-joined.
Think of water in pipes…
Or traffic on a motorway

5. Current = Rate of flow of charge
We could say that if there is a steady current its magnitude is equal to the quantity of charge which is flowing past a particular point in a circuit in 1 second.
Can you change our definition for current into a formula? Don’t forget the symbol for charge = Q

6. ΔQ = IΔt Charge = Current x Time
Where Δ = ‘the change in’
Charge = Current x Time
Charge is measured in Coulombs (C) where 1 Coulomb is the total charge supplied by a current of __ ampere in __second.
Units: 1C = 1Ax1s= 1As

7. Q = It
The base unit for current (I) is the Ampere (A), often abbreviated to amp, an SI base unit.
1A is the rate of flow when 1C of charge is deposited in 1 second.
But if 1C is equal to the charge deposited when a current of 1A flows for 1 second then isn’t this all a bit circuitous?!
We need a new base for the amp.

8. SI Units
The seven SI base units are those which are based on purely physical measurements.
They are the;
s___________
m__________
kg__________
K___________
A___________
Mol_________
cd__________

9. SI Units From the seven base units we can derive all other units.
Speed is defined as the distance travelled (in metres) in a set time (in seconds)
Speed = distance / time
m/s = m / s
A newton can be defined as the force needed to accelerate a mass of 1kg at a rate of 1 metre per second squared.
Force = m x a
N (kgm/s2) = kg x m/s/s

10. Q = It Charge (Q) is measured in coulombs (C) a derived SI unit.
The coulomb is defined as the charge deposited when a current of 1A flows for 1s (C = As)
One coulomb is approximately equivalent to the charge carried by 6.25 × 1018 electrons.
The Charge on one electron is -1.6 x C
1 / 1.6 x = 6.25 × 1018

11. Millikan’s Oil Drop
Quick Vid

12. Millikan’s Oil Drop
Millikan’s experiment demonstrates, by the careful addition of charge to small particles, that charge can only be added in discrete amounts.
The amount of charge is either 1.6 X 10-19C or a multiple of this number – it is quantised.
This value is known as the fundamental charge.
Any charged particle must obey the simple law;
Q = ±ne

13. Q = It Since we’re looking at SI units how do we define a second?
Previously - 1/ of the mean solar day.
The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

14. Examples…
In a cathode ray tube (old style TV) a charge of x 10-5 C is deposited on the screen every 1 second. What is the current in each tube?
How many tubes are required for the total charge in the TV to exceed 1C?
Assuming the TV has the above number of tubes and is left on for 15 minutes what was the total charge deposited?

15. Examples… Q = It or I = Q / t I = 8 X 10-5 / 1
Total tubes to make 1C = 1 / 8 X 10-5
= 12500
Q = It
Q = Q1 X (15 x 60) X 12500
Q = 900C

16. Current in Metals
When a metal e.g. Copper is examined in detail it is found to be crystalline; the atoms are regularly packed together in long neat rows.
Each copper atom has one electron free to move
These delocalised electrons are known as conduction electrons.
When conducting electricity, the conduction electrons drift slowly along the wire.
Diagrams:

17. Charge and Current Worksheet
Questions 1-7 Page 125