Transcript Slide 1

2.2 Resistance
G482 Electricity, Waves & Photons
2.2.2 EMF
Mr Powell 2012
Index
2.1.1 Prior Learning
Mr Powell 2012
Index
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.
Mr Powell 2012
Index
(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
Mr Powell 2012
Index
(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
Mr Powell 2012
Index
(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

1
Inversely to this the number of electrons that makes up 1 Coulomb of charge is
2
1C = 6.25 x 1018 electrons

The volt (V) defines the work done per coulomb of charge transferred between two
points where;
3
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
NB: Remember this definition for Unit 1 as well!
4
Mr Powell 2012
Index
(c) define the volt & electron volt
Potential Difference:
* is defined as the work done (or energy transfer) per unit charge
V (Volt)
=
W
Q
(work done, J)
(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
Mr Powell 2012
Index
(d) describe how a voltmeter may be used to determine the
1.5 V
p.d. across a component;
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
V
1.5 V
1.5 V
1.5 V
1.5 V
Mr Powell 2012
Index
d) Series PD
3.0 V
3 V
4
3 V
4
V
V
A
V
In a series circuit
the current is the
same
everywhere
V
V
3 V
4
3 V
4
0.1 A
In a series circuit
the PD is shared
among the
components
Mr Powell 2012
Index
(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)
Mr Powell 2012
Index
(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
Mr Powell 2012
Index
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 :
Mr Powell 2012
Index
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
V = W/Q = 50/5 = 10 V
(of electrical energy)
Mr Powell 2012
Index
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
Activation
Consolidation
• 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
• 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
Mr Powell 2012
Index