1. 2.2 Electromotive force and potential difference
Analogy of energy conversion in a simple circuit
Energy change in a circuit
Voltage‚ potential difference and electromotive force
Check-point 3
Check-point 4 1 2
Book 4 Section 2.2 Electromotive force and potential difference

2. Analogy of energy conversion in a simple circuit
A (‘cell’) moves the hula hoop. B and C (‘bulbs’) lightly grip it. The others (‘wires’) provide a pathway with minimal resistance to its movement.
Book 4 Section 2.2 Electromotive force and potential difference

3. Analogy of energy conversion in a simple circuit
The ‘bulbs’ feel the heat.
Energy transfer: ‘cell’  ‘bulbs’
Moving hula hoop
Electric circuit
Energy input
cells
Energy output
light bulbs
Conduction loop
hands holding hoop
conducting wire
What is/are driven to move?
electrons in wire A
B and C
hula hoop
Book 4 Section 2.2 Electromotive force and potential difference

4. 1 Energy change in a circuit
energy transfer system
transfers energy from the source to the load (e.g. from dynamo to light bulb)
Simulation
2.1 Model of simple circuit
Book 4 Section 2.2 Electromotive force and potential difference

5. 1 Energy change in a circuit
Battery
drives free e– round the circuit at once
supplies energy to light bulbs R and S simultaneously.
Book 4 Section 2.2 Electromotive force and potential difference

6. 1 Energy change in a circuit
Total energy supplied depends on flow rate of electrons
 depends on total opposition from bulbs
Simulation
2.2 Energy changes in a circuit
Book 4 Section 2.2 Electromotive force and potential difference

7. 1 Energy change in a circuit
The bakery-bread-lorry-supermarket analogy:
bakery
Every lorry
gets four trays of bread at a time
unloads 3 trays in supermarket S
unloads the remaining tray in supermarket R
heads back to the bakery for another cycle of delivery
supermarket S
supermarket R
Book 4 Section 2.2 Electromotive force and potential difference

8. 1 Energy change in a circuit
Bakery-bread-lorry-supermarket analogy
Electric circuit
Bakery
Battery
Bread
Energy
Lorries
Electrons
Loads (e.g. electric appliances)
Supermarkets
Book 4 Section 2.2 Electromotive force and potential difference

9. 1 Energy change in a circuit
A ‘hill diagram’ of the circuit:
For every 1 C of e–, electric PE increases by 4 J
3 J of energy changes to heat and light energy
1 J of energy changes to heat and light energy
Book 4 Section 2.2 Electromotive force and potential difference

10. 2 Voltage‚ potential difference and electromotive force
Voltage across two points
= change of electric PE per unit charge
Voltage =
energy
charge
V = E Q
1 V = 1 J C–1
Unit: volts (V)
Book 4 Section 2.2 Electromotive force and potential difference

11. 2 Voltage‚ potential difference and electromotive force
Electromotive force (e.m.f.)
 Voltage across a source of electricity from which no current is being drawn
The electromotive force (e.m.f.) of a source is the energy imparted by the source per unit charge passing through it.
Book 4 Section 2.2 Electromotive force and potential difference

12. 2 Voltage‚ potential difference and electromotive force
Potential difference (p.d.)
 Voltage across two points in external circuit
P.d. is the electric PE converted to other forms per unit charge passing from one point to another outside the source.
Example 2
Energy change
Book 4 Section 2.2 Electromotive force and potential difference

13. Book 4 Section 2.2 Electromotive force and potential difference
Example 2
Energy change
When iPod operates,
voltage = 6 V, current = 0.35 A
(a) How much electric PE is converted to sound and heat when 1 C of charge passes through the circuit?
6 V  6 J of electric PE is converted into other forms of energy for 1 C of charge
Book 4 Section 2.2 Electromotive force and potential difference

14. Book 4 Section 2.2 Electromotive force and potential difference
Example 2
Energy change
(b) How much electric PE is converted to sound and heat when the iPod has been switched on for 10 min?
Charge flowed through in 10 min
Q = It
= 0.35  600 = 210 C
Total energy converted
E = QV
= 210  6 = 1260 J
Book 4 Section 2.2 Electromotive force and potential difference

15. 2 Voltage, potential difference and electromotive force
a Measuring voltage
Voltmeter / voltage sensor connected to a data-logger: measure the voltage across any two points
Book 4 Section 2.2 Electromotive force and potential difference

16. Book 4 Section 2.2 Electromotive force and potential difference
a Measuring voltage
Voltmeter:
Like ammeter, the red terminal (+) leads to + terminal of the battery and the black one (–) to – terminal.
Wrong connection  may damage the voltmeter
Simulation
2.3 Reading an ammeter and a voltmeter
Book 4 Section 2.2 Electromotive force and potential difference

17. 2 Voltage, potential difference and electromotive force
b Voltage around a simple circuit
Electric PE gained in the battery per unit charge is completely released in the two bulbs.
P.d. across the two bulbs
= 1 V + 3 V
= 4 V (e.m.f. of the battery)
Example 3
Voltage across different points of a circuit
Book 4 Section 2.2 Electromotive force and potential difference

18. Book 4 Section 2.2 Electromotive force and potential difference
Example 3
Voltage across different points of a circuit
Find the voltage across different points and the –ve terminal of the battery O.
Take the electric potential at O as zero.
(a) AO
(b) BO
(c) CO
(d) DO
(e) EO
4 V
3 V
Book 4 Section 2.2 Electromotive force and potential difference

19. 2 Voltage, potential difference and electromotive force
Like gravitational PE, we can set any point as reference zero for electric potential.
Conventionally, earthed point = 0 V
Voltage across two points is independent of the reference zero.
Book 4 Section 2.2 Electromotive force and potential difference

20. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 3 – Q1
Voltage across a source is called ____________________________.
Voltage across two points in the external circuit is called ____________________________.
electromotive force / e.m.f.
potential difference / p.d.
Book 4 Section 2.2 Electromotive force and potential difference

21. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 3 – Q2
A light bulb is connected to a 3-V battery.
If the battery has delivered 12 J of energy to the bulb, how much charge has passed through the bulb?
A 1 C
B 4 C
C 6 C
D 12 C
Book 4 Section 2.2 Electromotive force and potential difference

22. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 3 – Q3
How much energy is supplied to each coulomb of charge that flows through a 9-V battery?
Energy supplied = 9 J
Book 4 Section 2.2 Electromotive force and potential difference

23. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 3 – Q4
How many joules of energy are stored in this battery? (1800 mA h = 6480 C)
Energy stored = QV
= 1800 mA h  1.2 V
= 6480 C  1.2 V
= 7776 J
Book 4 Section 2.2 Electromotive force and potential difference

24. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 3 – Q5
If the electric potential of D is set as the reference zero, find the potential difference across the following points.
AO =
BO =
CO =
DO =
EO =
4 V
3 V
Book 4 Section 2.2 Electromotive force and potential difference

25. 2 Voltage, potential difference and electromotive force
c Cells in series and parallel
Connect six 1.5-V cells in series:
Book 4 Section 2.2 Electromotive force and potential difference

26. c Cells in series and parallel
When a unit charge passes through the battery, it gains 9 J (6  1.5 V) of energy from the cells.
Total e.m.f. = sum of e.m.f. of all the cells
 e.m.f. of the battery is 9 V.
In a serial arrangement, the e.m.f. add up.
Book 4 Section 2.2 Electromotive force and potential difference

27. c Cells in series and parallel
Connect six 1.5-V cells in parallel:
Book 4 Section 2.2 Electromotive force and potential difference

28. c Cells in series and parallel
A unit charge gains 1.5 J of energy, as it can only pass through one cell each round.
 Same voltage as a single cell
However, the cells last longer.
In a parallel arrangement, the currents add up.
Book 4 Section 2.2 Electromotive force and potential difference

29. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 4 – Q1
Write an equation representing the relationship between V1, V2 and V3.
V1 = V2 + V3
Book 4 Section 2.2 Electromotive force and potential difference

30. Book 4 Section 2.2 Electromotive force and potential difference
Check-point 4 – Q2
Total e.m.f. of each of the following arrangements of cells = ?
(a)
(b)
3 V
1.5 V
Book 4 Section 2.2 Electromotive force and potential difference

31. Book 4 Section 2.2 Electromotive force and potential difference
The End
Book 4 Section 2.2 Electromotive force and potential difference