1. cell
energy ☺
electron
lamp

2. Coulomb of charge (electrons)
Think of it as a “bag of electrons” (containing electrons!) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺

3. I’m counting how many coulombs of electrons go past me every second
Current
I’m counting how many coulombs of electrons go past me every second ☺ ☺ ☺
The rate of flow of electric charge (number of Coulombs flowing past a point in the circuit every second).
I = ΔQ/Δt ☺ ☺ A ☺ ☺ ☺ ☺ ☺ ☺
1 Amp = 1 coulomb per second

4. In a series circuit Current is the same at any point in the circuit

5. In a parallel circuit
The current splits (total current stays the same)
2.5 A
2.5 A
1.25 A
1.25 A

6. Voltage(emf) ☺ V
I’m checking the difference in energy (per coulomb) between the 2 red arrows
1 Volt = 1 Joule per coulomb

7. Voltage (p.d.) ☺
I’m checking the difference in energy (per coulomb) before and after the lamp V
1 Volt = 1 Joule per coulomb

8. p.d. and e.m.f
Electric potential difference between two points is the work done per unit charge to move a small positive charge between two points.
Electromotive force is the total energy difference per unit charge around the circuit (it is the potential difference when no current flows in a circuit).

9. In a series circuit
The sum of the p.d.s across the lamps equals the emf across the cells
9 V
3 V
3 V
3 V

10. In a parallel circuit
In a simple parallel circuit, p.d. across each lamp equals the e.m.f. across the cells
5 V
5 V
5 V

11. Resistance
Measures how difficult it is for current to flow. Measured in Ohms (Ω) A V
Resistance = voltage/current R = V/I

12. Ohm’s Law
V = IR V R I X

13. Resistance R is proportional to the length of wire – WHY? R α L
R is inversely proportional to the cross sectional area of wire – WHY?
R α 1/A
R depends on the type of material – WHY?

14. Resistivity R = ρL A where R = resistance in Ohms
L = Length of conductor in metres
A = cross sectional area of conductor in m2
ρ = resistivity of the material in Ohms.meters

15. Example
The resistivity of copper is 1.7 x 10-8 Ωm. What is the resistance of a piece of copper wire 1 m in length with a diameter of 0.1mm?
radius = 0.05mm = 5 x 10-5m
cross sectional area = πr2 = 3.14x(5 x 10-5)2
= 7 x 10-9 m2
R = ρL/A = (1.7 x 10-8 x 1)/ 7 x 10-9 = 2.42 Ω

16. Resistance of a lamp A V Resistance = voltage/current R = V/I
Vary the voltage and current using a variable resistor (rheostat). Plot a graph of resistance against current A V
Resistance = voltage/current R = V/I

17. Resistance of a lamp
As the current in a lamp increases, its resistance increases. Why?

18. Ohmic behaviour Metal wires at constant temperature
p.d. is proportional to the current
Metal wires at constant temperature

19. Non-Ohmic behaviour
p.d. is not proportional to the current

20. Power
The amount of energy used by a device per second, measured in Watts (Joules per second) A V
Power = voltage x current P = VI

21. Power dissipated in a resistor/lamp
P = VI
From Ohm’s law, V = IR
So P = VI = I2R
From Ohm’s law also, I = V/R
So P = VI = V2/R

22. Total energy
So the total energy transformed by a lamp is the power (J/s) times the time the lamp is on for in seconds,
E = VIt
E = energy transformed (J)
V = Voltage (also called p.d.)
I = current (A)
t = time (s)

23. Electronvolt
Electronvolt – the energy gained by an electron when it moves through a potential difference of one volt.

24. Internal resistance
Connecting a voltmeter (VERY high resistance) across the terminals of a cell measures the EMF of the cell (no current flowing) V

25. Internal resistance
We have assumed so far that the power source has no resistance…….not a good assumption!

26. Internal resistance
In actuality the p.d. across a cell is less than the EMF due to energy lost in the INTERNAL RESISTANCE

27. Internal resistance
To help us visualize this, a cell is represented as a “perfect” cell attached in series to the internal resistance, given the symbol r.

28. Internal resistance
The p.d. across a cell (V) is then equal to the EMF (ε)minus the voltage lost across the internal resistance (=Ir)
V = ε - Ir

29. Example
A cell of emf 12V and internal resistance 1.5 Ω produces a current of 3A. What is the p.d. across the cell terminals?
V = ε - Ir
V = 12 – 3x1.5
V = 7.5 V

30. Adding resistances

31. In series and parallel

32. Ideal meters Voltmeters – infinite resistance!
Ammeters – Zero resistance!

33. The potential divider
The potential divider is at its simplest two series resistors connected to a battery and the voltage is measured between the negative terminal and a point between the two resistors.
However if we use battery the
principle will be the same but formula
will be different

34. Light depended resistor (LDR)
A Light Dependent Resistor is a device which has a resistance which varies according to the amount of light falling on its surface.
High resistance in the dark but a low resistance in light

35. Light depended resistor (LDR)
Suppose the LDR has a resistance of 500 , 0.5 , in bright light, and 200  in the shade
When the LDR is in the light, Vout will be:
In the shade, Vout will be: