1. Calculate the current at point A
To get you thinking…. 6V
Calculate the current at point A A
200mΩ
200 Ω
200 kΩ
The measured current is actually 2.8x10-5 Amps. What factors could be responsible for this lower current?

2. Internal Resistance 11/09/2018
LO:
Explain what is meant by internal resistance and apply in circuits.
Describe why internal resistance occurs and use a simple circuit to determine its value.

3. Recap: e.m.f - Energy transferred ___ the charge
p.d – Energy transferred ____ the charge to the components.
Cell gives the charge energy but the charge loses a small amount of energy moving through the cell itself, overcoming the resistance offered.
So total p.d measured across terminals is _____ than the e.m.f.

4. All cells resist the flow of charge to some extent because, like any material, they too are made of imperfect conductors.
Any cell can therefore be drawn as a “perfect battery” in series with a resistor.

5. Supply across the terminal will only equal ideal emf when current is 0.
The moment the circuit is loaded, a current flows, resistance occurs and the emf from the battery will be less than the ideal emf.
The cheaper the battery the greater the discrepancy!

6. emf which is supplied by the battery is given by Ohms Law
V = Ir
We can therefore see that the higher the current used the greater the discrepancy between the ideal battery and the emf supplied.

7. Deriving Internal Resistance
V = Ir
emf = Ir
Rt = R + r
emf = I (R + r) I R
Write down an expression for the voltage which will be supplied from the ‘components’ within the circle.
Sub V for emf.
Write down the expression for finding total R in the circuit
Combine the above
Now write down the expression for p.d. at R

8. R
Practice Question
A cell of emf 12V has an internal resistance of 0.5 Ω. It is connected to an external resistor, R. The current in the circuit is 2A. What is the value of R?

9. IR is known as the terminal potential difference
Ir is known as the ‘lost volts’
So Terminal pd = Emf – Lost volts or V = emf – Ir
Lost volts tell us how much energy is transferred to the cell itself.
If we have a short circuit:
The current becomes very _______
This leads to a large amount of L____ v____.
This means a large amount of energy is transferred to..
The power supply _____ up.

10. A battery has an emf of 9.0V and an internal resistance of 2.0Ω.
Further Practice Q
A battery has an emf of 9.0V and an internal resistance of 2.0Ω.
What current does it supply to an external resistor of 13Ω?
What power is used in the external resistor?
What percentage of the total power is wasted in the internal resistor?
What is this as a fraction?
Calculate the ratio of internal resistance to the total resistance – This is the efficiency
0.6A
4.68W
13.3%
2/15
2/15

11. Measuring the internal resistance of a Cell
Circuit Set-up
Connect a cell in series with a variable resistor and an ammeter.
Add a voltmeter across the cell
Record values of V & I.
Plot a graph of V (y-axis)
Against I (x-axis).
Relate gradient to our formula to find a value for internal resistance and emf.

12. V = emf – Ir
y = mx + c
(y) Y-axis =
(x) X-axis =
(m) Gradient =
(c) Intercept =

13. Conclusion
The terminal potential difference (pd over the cell) decreases when current increases because of internal resistance leading to more ‘lost volts’.
The energy lost by the charge is transferred to heat in the cell.

14. 1. Current at A is found to be 3.10A. What must internal r be?
To get you thinking…. 6V
1. Current at A is found to be 3.10A. What must internal r be? A
60mΩ
1.5 Ω
0.2Ω
Explain why the cell is likely to be flat in a short period of time.
If the cell can supply 250 Amp-hours how long will it last?

15. P-P-P-Practice! EMF and Internal Resistance worksheet
Questions page123
AS Level Physics - DC Electricity sheet

16. Homework – Create a half A4 summary sheet of the topic below
Homework – Create a half A4 summary sheet of the topic below. Your half a page should include key definitions, a worked example of a question and a few practice Q’s to try.
Name
Topic
Rajat
Electrical Current and charge
Harry
Kirchhoff's Law
Sam
Drift Velocity
Hugo C
e.m.f.
Toby C
Series, parallel and circuits
Felix
p.d.
Toby B
Ohms Law
Stu
IV-characteristics of diode + fixed R
Sam H
IV-characteristic of filament lamp
Name
Topic
Resistance of LDR
Jeffery
Resistance of Thermistor
Jack A
Resistivity of wire
Electrical power
Jack W
Paying for electricity
Vince
Resistors in parallel and series
Dan
Internal R
Ben
Potential Dividers
Thermistors and LDR’s as potential dividers

17. Homework – Create a half A4 summary sheet of the topic below
Homework – Create a half A4 summary sheet of the topic below. Your half a page should include key definitions, a worked example of a question and a few practice Q’s to try.
Name
Topic
Jake A
Electrical Current and charge
Joe B
Kirchhoff's Laws
Adrian S
Drift Velocity
Will
e.m.f.
Ollie D
Series, parallel and circuits
Hugh D
p.d.
Clayton B
Ohms Law
Munv.
IV-characteristics of diode + fixed R
Alex M
IV-characteristic of filament lamp
Name
Topic
Tim G
Resistance of LDR
Nick E
Resistance of Thermistor Ed
Resistivity of wire
Dan E
Electrical power
Ruari
Paying for electricity
Femi O
Resistors in parallel and series
Mr T
Internal R
Tom M
Potential Dividers
Matt
Thermistors and LDR’s as potential dividers

18. Pg123 Answers
1ai) 2.0A
1aii) 1.4V
1aiii) Straight line with negative gradient including points (0,2) & (1.4,0.14)
Solar Cell Power,P (W)
Resistance,R (Ω)
0.20
3.20
0.38
1.52
0.42
1.17
0.39
0.92
0.34
0.75
0.14
0.29

19. Pg123 Answers
1bii) Graph
1biii) 0.14
2a) i) Series 12V; r=4.0Ω ii)Parallel 3V; r=0.25Ω
2b) i) 3.0A ii) 12A
Go flat at same time
2c) i)2.0A ii) 1.3A
Batteries in parallel would go flat more slowly.