1. Physics 2.6 Demonstrate understanding of electricity and electromagnetism
Credits 5
This achievement standard involves knowledge and understanding of phenomena, concepts, principles, and relationships related to electricity and electromagnetism, and the use of appropriate methods to solve related problems.

2. Achievement Criteria Achievement Merit Excellence
Identify or describe aspects of phenomena, concepts or principles.
Give descriptions or explanations in terms of phenomena, concepts, principles and/or relationships.
Give concise explanations, that show clear understanding, in terms of phenomena, concepts, principles and/or relationships.
Solve straightforward problems where the concept will be obvious and the solution involves a single process.
Solve problems where the relevent concept is not immediately obvious and may involve rearranging a formula.
Solve complex where two different concepts are needed and the solution involves more than one process.

3. D.C. Electricity
Parallel circuits with resistive component(s) in series with the source,
Circuit diagrams;
Voltage, current, resistance, energy, power;
Ohm’s Law;
Electrical properties of diodes.

4. Equations

5. Conductors
For charges to move through a material it needs a supply of charge carriers which can move freely. In most solid conductors the carriers are the loosely held outer electrons, which are not attached to a particular atom. Such materials are called conductors.
Atomic nucleus and inner electrons.
Outer electrons free to move throughout the metal.

6. Insulators
Insulators are materials whose outer electrons are required to form bonds (covalent or ionic) that bind the atoms of the material together. This means that charges cannot move through them.
Atomic nucleus and inner electrons.
Outer electrons tightly bound and unable to move.

7. Semiconductors
Semiconductors have only a small proportion of charge carriers that are free to move. The number of free electrons can be controlled and hence the conductivity of the material can be controlled.
Atomic nucleus and inner electrons.
Some free outer electrons.
When light hits an LDR it releases more electrons
Outer electrons tightly bound and unable to move.

8. Charge, Q
Charge is a property of some elementary particles that causes them to experience a force of attraction and repulsion in an electric field.
Charge occurs in two forms: positive and negative.
The charge on an electron is extremely small and it is therefore much too small to use as a practical unit. Instead the unit of electric charge Q is the coulomb (C).

9. Charge, Q An electron has a charge, e of: e = 1.60 × 10-19 C
In 1 coulomb of charge there are:
= 6,240,000,000,000,000,000 electrons
1.60 × 10-19

10. Conventional Current I
By convention charges flow from positive to negative.
We refer to the flow of electrical charges around a circuit every second as a current.
It is measured in amperes (A) using an ammeter.
If a quantity of charge, Q (coulombs) passes around a circuit in time, t (seconds) then a steady current, I (amperes) flows:
I = Q 1.0 t

11. Potential difference, V
The potential difference, V between two points is the amount of electrical energy changed, ΔE to other forms of energy when unit charge, Q passes from one point to the other.
V = ΔEP Q
The unit of potential difference is the volt (V), and is measured using a voltmeter.
1 V = 1 JC-1

12. Potential difference, V
If two points are at the same potential, no current can flow between them.
When ever a current flows from one point to another it does so because the electrical potentials at the two points are different.
9 V
9 V
9 m
9 m
4 m
9 V
4 V
Potential difference = 5 V
Current flows from high potential to low potential

13. Potential difference, V
If a charge, Q (coulombs) flows across a potential difference, V (volts) then the energy changed, ΔE (joules) is given by:
ΔEp = QV
If a steady current of I (amperes) flows then from equation 1.0:
ΔEp = ItV a
By conventional current we consider that when charges flow from positive to negative they are flowing from a point of higher potential to one of lower potential.

14. Resistance, R
All materials SLOW DOWN the flow of charges through them due to collisions with the atoms in the material.
Resistance is dependent upon the width, length…
Thick
Thin - + - +
Short
Long - + - +

15. Resistance, R … type and temperature of the conductor. - + - + - + - +
Copper
Nichrome - + - +
Cold
Hot - + - +

16. Ohm’s Law
The resistance, R of a conductor at a constant temperature is defined as the ratio of the potential difference, V across it to the current, I flowing through it:
R = V I
The unit of resistance is the ohm, Ω (the Greek letter omega).
The resistance of a metal can be regarded as arising from the interactions that occur between the crystal lattice and the free electrons.

17. Ohmic conductor - Resistor
A resistor wire placed in water will maintain a constant temperature and so obey Ohm’s law.
Ammeter
Voltmeter ΔV
Voltage ΔI
Current
Resistor Ohmic conductor
Constant temperature

18. Non-ohmic conductor - Lamp
The temperature of the lamp increases as the current increases and so the resistance increases.
Ammeter
Voltmeter
Voltage
Current
Resistor Ohmic conductor
Increasing temperature

19. Semi-conductor - Diode
In FORWARDS BIAS the diode doesn't start conducting until the voltage exceeds ~ 0.7 V.
Ammeter
Voltmeter
Voltage
Current
Resistor Ohmic conductor

20. Current - Voltage Relationships
If the potential difference V across a component is varied and the corresponding current I measured then a graph of V against I shows that the relationship between the two quantities and is called the characteristic of the component.
Voltage
Voltage
Voltage
Switch on voltage 0.7V
Current
Current
Current
Resistor Ohmic conductor
Lamp Non-ohmic conductor
Diode Non-ohmic conductor

21. Kirchoff's First Law
At a junction in a circuit, the current arriving equals the current leaving.
I = I1 + I2 + I3 1.5
hence
I - I1 - I2 - I3 = 0
When dealing with steady currents charge is conserved and flows in a circuit without being destroyed or accumulating at any point. I I1
Junction I2 I3

22. Kirchoff’s Second Law VB1
In any closed loop, the sum of the voltage gains is equal to the sum of the voltage drops, i.e.
VB1 + VB2 = VR1 + VR2
1.6
VB1 I
VR2
VR1
VB2 I

23. Energy and power
The rate at which a device can change energy ΔE (joules) from one form to another is referred to as the power P. The power of a device is given by:
P = ΔE t
Current flow I (amperes) due to a potential difference V (volts) across a device is accompanied by the conversion of electrical energy, ΔE into other forms of energy. (Equation 1.2a)
ΔE = ItV

24. Energy and power
The rate at which a device can change energy ΔE (joules) from one form to another is referred to as the power P. The power of a device is given by:
P = ΔE t
Both a stadium lamp and a candle can give out the same amount of light energy but it will take the candle times longer!
Key Power words:
Brighter Louder Faster Hotter

25. Energy and power We know that: ΔE = QV (Equation 1.2) and
Q = It (Equation 1.0) hence ΔE = (It)V.
Substituting P = ItV/t hence
P = IV
The unit of power is the watt (W) where
1 W = 1 Js-1

26. Power, P
If all the electrical energy is converted into heat by the device it is called a passive resistor and the rate of production of heat will also be equal to IV. If its resistance is R then rearranging equation 1.4, R = V/I and substituting, we have:
P = V × V hence P = V
R R or
P = I × IR hence P = I2R 1.10

27. Resistors in series V I V1 V2 V3
Resistors are in series if the same current, I passes through each of them in turn. V I R1 R2 R3 V1 V2 V3
The electrical energy changed in passing through all the resistors equals the sum of that changed in each resistor,
therefore V = V1 + V2 + V3
by equation 1.5 we have V = IR, V1 = IR1, V2 = IR2, V3 = IR3 hence:
IR = IR1 + IR2 + IR3
so R = R1 + R2 + R

28. Resistors in Parallel
When resistors are in parallel the current is provided with several alternative routes and we would therefore expect the resistance to be less than the smallest individual resistance.
If I is the total current through the circuit and I1, I2, I3, are the currents in the separate branches, then since current is not used up:
I = I1 + I2 + I3 V I1 R1
In a parallel circuit the potential difference, V across each parallel branch is the same.
1 =
R R R R3 I I2 I R2 I3 R3