1. G482 Electricity, Waves & Photons
2.2 Resistance
G482 Electricity, Waves & Photons
2.2.2 EMF
Ks5 OCR Physics H158/H558
Index
Mr Powell 2012

2. 2.1.1 Prior Learning

3. 2.2.2 Electromotive Force and Potential Difference
Assessable learning outcomes
(a) define potential difference (p.d.);
(b) select and use the equation W = VQ;
(c) define the volt;
(d) describe how a voltmeter may be used to determine the p.d. across a component;
(e) define electromotive force (e.m.f.) of a source such as a cell or a power supply;
(f) describe the difference between e.m.f. And p.d. in terms of energy transfer.
Students can use a simple circuit consisting of a cell and filament lamp to illustrate the differences between p.d. and e.m.f.

4. (a) define potential difference (p.d.);
If we think of potential difference in terms of energy transfer;
1) To bring two like charges near each other work must be done.
2) To separate two opposite charges, work must be done.
3) Whenever work gets done, energy changes form.
Monkey example; Imagine two positive spheres in space and a monkey. Imagine that a small monkey does some work on one of the positive charges. He pushes the small charge towards the big charge; + +
Work

5. (b) select and use the equation W = VQ;
This is all about a separation of charge between two points. The closer he brings it, the more potential energy it has since the two charges want to repel
When he releases the charge, work gets done on the charge. This is a change in energy;
Electrical potential energy to Kinetic energy.
If the monkey brought the charge closer to the other object, it would have more potential energy.
If he brought 2 or 3 charges instead of one, then he would have had to do more work.
Since the potential energy can change when the amount of charge you are moving changes, it is helpful to describe the potential energy per unit of charge. This is known as electrical potential or potential difference.
V = potential difference in volts, V, JC -1
W = Energy or work done in Joules, J
Q = charge in coulombs, C

6. (c) define the volt & electron volt
This is all about a separation of charge between two points.
To understand this we should remember that the charge on one electron is;
1e = 1.6 x 10-19C
Inversely to this the number of electrons that makes up 1 Coulomb of charge is
1C = 6.25 x 1018 electrons
The volt (V) defines the work done per coulomb of charge transferred between two points where;
1V = 1 JC-1
We can also derive a smaller unit of energy (J) for an electron which is the electron volt. Sub 3 into 1 i.e. x each side directly and multiply out the units
1eV = 1.6 x 10-19J 1 2 3 4
NB: Remember this definition for Unit 1 as well!

7. (c) define the volt & electron volt
Potential Difference:
* is defined as the work done (or energy transfer) per unit charge
V (Volt) = W (work done, J)
Q (charge, C) + B
If 1J of work is done in moving 1 C
of positive charge from A to B then the
Pd is 1V + A
1V = 1 J / C

8. (d) describe how a voltmeter may be used to determine the p. d
(d) describe how a voltmeter may be used to determine the p.d. across a component;
1.5 V
Voltage or pd:
* is electrical pressure
* causes current to flow
* is measured in Volts (V)
with a voltmeter in parallel
1.5 V
1.5 V
1.5 V
1.5 V
1.5 V V
1.5 V

9. d) Series PD
3 V 4
3 V 4 V V V A
3.0 V
0.1 A
In a series circuit the PD is shared among the components
In a series circuit the current is the same everywhere V V
3 V 4
3 V 4

10. (e) define electromotive force (e. m. f
(e) define electromotive force (e.m.f.) of a source such as a cell or a power supply;
e.m.f or electromotive force describes the force required to separate two charges at a given distance.
This terminology dates back to the early research into electric charges. The e.m.f is the available push from a cell or source and is the maximum theoretical open circuit push available.
However, as soon as you connect a component in the circuit you will not get all of the e.m.f as some push is lost inside the cell itself (due to internal resistance)

11. (f) describe the difference between e. m. f. And p. d
(f) describe the difference between e.m.f. And p.d. in terms of energy transfer.
Potential difference is simply a voltage differential between two points in a circuit (or in free space).
Such a potential difference can be a source of emf if it is used to move charges. However, as said previously some voltage or push is lost inside a cell due to internal resistance in the real world
EMF is a gain in energy per unit charge and is what drives a current
PD is a Loss in energy per unit charge

12. Practice Questions….
Select and use the equation W = VQ
If 50 J of work is done (or energy is transferred) when
5 C of charge passes through a component, the p.d. across the component is :
If an electrical supply has an e.m.f. of 12 V, it means that each coulomb of charge which passes through the
supply is given :

13. Practice Questions….
If 50 J of work is done (or energy is transferred) when
5 C of charge passes through a component, the p.d. across the component is :
If an electrical supply has an e.m.f. of 12 V, it means that each coulomb of charge which passes through the
supply is given :
W = QV = 1 x 12 = 12 J
(of electrical energy)
V = W/Q = 50/5 = 10 V

14. Connection
Connect your learning to the content of the lesson
Share the process by which the learning will actually take place
Explore the outcomes of the learning, emphasising why this will be beneficial for the learner
Demonstration
Use formative feedback – Assessment for Learning
Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex
Offer different ways for the students to demonstrate their understanding
Allow the students to “show off” their learning
Consolidation
Structure active reflection on the lesson content and the process of learning
Seek transfer between “subjects”
Review the learning from this lesson and preview the learning for the next
Promote ways in which the students will remember
A “news broadcast” approach to learning
Activation
Construct problem-solving challenges for the students
Use a multi-sensory approach – VAK
Promote a language of learning to enable the students to talk about their progress or obstacles to it
Learning as an active process, so the students aren’t passive receptors