Potential difference and
electromotive force.
Starter
True/false
Ω is the measure of an objects resistance
𝐼
𝑉=
𝑅
Semiconductors have a high number
density
𝐼 = 𝐴𝑛𝑒𝑣
𝑃 = 𝐼2𝑅
Starter
True/false
Ω is the measure of an objects resistance
𝐼
𝑉=
𝑅
Semiconductors have a high number
density
True
𝐼 = 𝐴𝑛𝑒𝑣
True
𝑃 = 𝐼2𝑅
True
False
False
Learning outcomes
• Recall key facts about circuits
• Define: potential difference, volt and
electromotive force.
• Describe how you would find the energy in an
electrical circuit
• Apply the equation 𝑊 = 𝑉𝑄
Energy in electrical circuits
Check the relationship between potential
difference, current, time and energy.
• What equipment would you need?
• What reading would you take?
• What formula would you use?
You need to work in two
groups to collect these results
1.
2.
3.
4.
5.
6.
7.
Set up the circuit shown with a 10 Ω
resistor for R.
Set the d.c. supply to 10 V.
Check that the joulemeter reading
increases by 10 J each second when
the current is kept constant at 1 A.
Record the current and energy
dissipated in the load resistor as a
function of time for different supply
p.d.s.
Replace the 10 Ω resistor with a 12 V,
24 W lamp. Adjust the supply to 12 V.
Check that the joulemeter reading
increases by 24 J each second when
the ammeter displays 2 A.
Measure the p.d. across the lamp,
the current in it and the energy
supplied to it for a period of 10 s for
a range of supply p.d.s.
What’s wrong with this circuit? Draw it
correctly
A
A
V
B
On the figure above add an LED with a 100  resistor in series, an ammeter and a
voltmeter to complete the circuit between terminals A and B.
[3]
Learning outcomes
• Recall key facts about circuits
• Define: potential difference, volt and
electromotive force.
• Describe how you would find the energy in an
electrical circuit
• Apply the equation 𝑊 = 𝑉𝑄
Questions
1. What is the relationship between electrical
energy, p.d., current and time?
Answers
Question 1
Using the formulae for power and power = rate
of transfer of energy gives ΔW = VIΔt.
Definitions
Potential difference is the electrical energy
transferred per unit charge when electrical energy is
converted into another form of energy.
Potential difference is energy per unit charge. Its unit the volt, is the
joule per coulomb.
𝐼=
𝑄
𝑡
𝑊
𝑉=
𝑄
𝑃=
𝑃=
𝑊
𝑉𝑄
=
= 𝑉𝐼
𝑡
𝑡
1 volt = 1 joule per coulomb = 1 watt per ampere
𝑊
𝑡
Definitions
Electromotive force (e.m.f.) is the energy transferred per unit
charge when one other type of energy is converted into
electrical energy.
𝑒. 𝑚. 𝑓. =
𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑒𝑑
𝑐ℎ𝑎𝑟𝑔𝑒
𝐸=
This definition tells us two things:
• That e.m.f. is measured in joules per coulomb
• That it always deals with the supply of electrical energy
1 volt = 1 joule per coulomb
𝑊
𝑄
e.m.f. and p.d.
The voltage shown on a cell or ………… tells you
its e.m.f. From this, you can tell the amount of
energy given to each ………………… of ……………..
Passing around the circuit, including through the
cell itself. In practice the useful p.d. across an
appliance is …… than the e.m.f. because some
voltage is …………… pushing the ……………….
through the cell.
e.m.f. and p.d.
The voltage shown on a cell or battery tells you
its e.m.f. From this, you can tell the amount of
energy given to each ………………… of ……………..
Passing around the circuit, including through the
cell itself. In practice the useful p.d. across an
appliance is …… than the e.m.f. because some
voltage is …………… pushing the ……………….
through the cell.
e.m.f. and p.d.
The voltage shown on a cell or battery tells you
its e.m.f. From this, you can tell the amount of
energy given to each coulomb of charge passing
around the circuit, including through the cell
itself. In practice the useful p.d. across an
appliance is …… than the e.m.f. because some
voltage is …………… pushing the ……………….
through the cell.
e.m.f. and p.d.
The voltage shown on a cell or battery tells you
its e.m.f. From this, you can tell the amount of
energy given to each coulomb of charge passing
around the circuit, including through the cell
itself. In practice the useful p.d. across an
appliance is less than the e.m.f. because some
voltage is …………… pushing the ……………….
through the cell.
e.m.f. and p.d.
The voltage shown on a cell or battery tells you
its e.m.f. From this, you can tell the amount of
energy given to each coulomb of charge passing
around the circuit, including through the cell
itself. In practice the useful p.d. across an
appliance is less than the e.m.f. because some
voltage is ‘lost’ pushing the ………………. through
the cell.
e.m.f. and p.d.
The voltage shown on a cell or battery tells you
its e.m.f. From this, you can tell the amount of
energy given to each coulomb of charge passing
around the circuit, including through the cell
itself. In practice the useful p.d. across an
appliance is less than the e.m.f. because some
voltage is ‘lost’ pushing the current through the
cell.
• A 1.5V cell gives 1.5 J to each coulomb
• A 6V battery gives……………………………..
• The 230 V mains gives ………………………
Questions
1.
2.
3.
4.
5.
6.
7.
How much energy is transferred to each coulomb of charge by a 9V
battery?
Which types of voltage are described here?
a) a measure of energy transferred to electrical charges by a supply
b) a measure of the energy transferred from electrical charge to a
component in a circuit
10C of charge flows through a p.d. of 6.0V. How much energy is
transferred?
How much work is done by the 230V mains supply in pushing 1C of
charge round a circuit?
A current of 2.5A flows through a resistor for 1 minute. It transfers 600J
of energy to the resistor. What is the p.d. across the resistor?
How many 1.2V rechargeable batteries must be connected in series to
provide an e.m.f. of 6.0V
What is the e.m.f. provided by the combination of cells shown in the
diagram?
2.0V
1.5V 2.5V 3.0V
Plenary
Circuit symbols
Resistance
Energy and power
e.m.f.