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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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