1. Year 12 Current Electricity Lesson 3 Potential difference and power
Miss O’Donnell

2. What questions are we looking at today?
What do we mean by potential difference?
How can we calculate electrical power?
How do energy transfers take place in electrical devices?

3. What do we mean by potential difference?
Defined as … the work done (or energy transferred) per unit charge
Unit of pd is the VOLT
Volt is equal to 1 Joule per Coulomb
V= W/Q
Work is done when charge Q flows through a component
W = QV
V = I x R

4. Quick Questions
30J of work is done when 5C of charge pass through a component – what is the pd?
If the pd is 12V across a component and 3C of charge is passed through it – what is the energy transfer from the battery to the component?
W = QV
V = W/Q = 30/5 = 6V
W = QV = 3 x 12 = 36J

5. Electrical power & current
A component has a pd V across its terminals and a current I passing through it.
In time ∆t
the charge through the component Q=I ∆t
the work done by the charge carriers W=QV
By substitution W= (I ∆t)V = IV∆t
Because Power = Energy/Time = IV∆t/ ∆t
Power = IV

6. Questions
Calculate the energy transfer in 1200s in a component when the pd across it is 12V and the current is: 2A
0.05A
A 6V, 12W light bulb is connected to a 6V battery. Calculate:
The current through the bulb
The energy transfer to the light bulb in 1800s
ANSWERS:
29kJ
720J
22kJ

7. Questions 2
A 230V electrical appliance has a power rating of 800W. Calculate:
The energy transfer in the appliance in 1 minute
The current taken by the appliance
Which of the following fuse values would be suitable for this appliance? 3A, 5A & 13A
ANSWERS:
48kJ
3.5A 5A

8. Energy Transfer in Electrical Devices
The emf of a source of electricity is defined as the electrical energy produced per unit charge passing through the source.
Unit is the VOLT – same as pd
For a source of emf ε in a circuit, the electrical energy produced when a charge Q pass through the source is Qε
This energy is transferred to other parts of the circuit and some may be dissipated by the source itself due to internal resistance – we will explore this further next lesson

9. Electric current effects
What effects do electric currents have when they pass through components and appliances?
Heating – when they pass through a component with resistance. WHY?
Magnetic – in electric motors and loudspeakers
Kinetic – force the charge carriers though wires

10. Questions 3
A battery has an emf of 9V and negligible internal resistance. It is capable of delivering a total charge of 1350C. Calculate:
The maximum energy the battery could deliver
The power it would deliver to the components of the circuit if the current through it was 0.5A
How long the battery would last for, if it were to supply power at the rate calculated in the last part of the question
ANSWERS:
12kJ
4.5W
2700s
W=Q x V
P = V x I
Q = I x dt