Find the

a. internal resistance of the cell

b. e.m.f. of the cell

Try it!

Test voltages to see if we have 6V & 3V.

Why are we not worried about the internal resistances of the batteries?

**Yr 12 Electricity AQA**

Resistivity

Understand and use the equation for resistivity.

Circuit Basics

Be able to define Charge, Current, Potential Difference, Resistance and use the equation:

Resistance

Be able to use and apply the equations for resistors in series and in parallel

Internal Resistance

With your knowledge of resistor in series and parallel, what is the current in the following circuits?

Parallel & Series Circuits

Basic Circuits

Potential Dividers

IV Characteristics

**All the lessons...**

Internal Resistance

Resistivity

Potential Dividers

AC Theory

Kirchhoff Laws - Extra

Understand how to work out the energy and power of a circuit. Be happy to apply all of these equations to both parallel and series circuits.

Be able to describe the effects of temperature on resistance of metal conductors and thermistors.

Be able to describe the characteristic I-V graph for an ohmic resistor, filament bulb and diode.

Understand and use the equation given and define the EMF of a cell.

Be able to interpret a graph of potential difference and current.

http://tinyurl.com/llzbgfk

**The Power Points**

Circuit Components

The headlamps are connected in parallel across a twelve-volt battery with internal resistance of 0.1 ohm. The starter motor (which requires 60A) is also in parallel controlled by the ignition switch.

What happens to the headlamps when a car

is started?

sudden demand for more current

large lost volts (around 0.01 Ω x 60 A = 6 V)

terminal voltage drops to 12 V – 6 V = 6 V

headlamps dim

When the engine fires, the starter motor switch is opened and the current drops. The terminal voltage rises and the headlamps return to normal. It’s better to turn the headlamps off when starting the car.

What would happen if you touched the leads of a High Voltage Supply together?

In the case where internal resistance is:

High

Low?

These supplies are designed to provide a high voltage to a high resistance load (e.g. cathode ray tube) but if the terminals or wires connected to them were accidentally touched this could provide a nasty shock (lower resistance in the load and higher current). One way to deal with this is to connect a large resistance in series with the output (positive) terminal. If the terminals are shorted (e.g. by contact through a person) the current drawn is limited to I = E / r. A typical EHT supply (up to 5000 V) is protected by a 5 MΩ resistor so the maximum current if shorted is just 1 mA. That shouldn’t kill you! Be aware however that HT supplies (0-300 V) have a much lower internal resistance, and could kill you, so special shrouded leads should be used.

EHT supplies often have a further ‘safety resistor’ (e.g. 10 MΩ) to reduce the maximum current still further. This resistor can be by-passed when necessary. No school EHT supply is allowed to provide more than 5 mA.

What do you think the currents will be in this circuit will be?

(circuits can be superimposed)

Kirchhoff current law (KCL):

The algebraic sum of currents in a network of conductors meeting at a point is zero.

Kirchhoff voltage law (KVL):

The algebraic sum of the products of the resistances of the conductors and the currents in them in a closed loop is equal to the total emf available in that loop.

Use Kirchhoff law, set up simultaneous equations to work out the currents in the circuit

ANS

ANS

ANS

ANS

ANS

Question:

What is the average Emf over one cycle?

Would this help us find, for example, the power dissipated as heat in a resistor?

AC Theory

http://tinyurl.com/mgtjb23

AC Theory

Be able to describe the properties of superconducting materials including the critical temperature.

As temperature decreases, resistance decreases

At the critical temperature

Resistance become zero

Superconductivity

Alfred Leitner

Liquid Nitrogen Bomb

Levitating Train

10 Liquid Nitrogen Experiments

Kirchhoff Laws

GCSE

A Level

https://goo.gl/pRKyyx

Direct Current

Did you hear of the man who drowned in his muesli?

He got pulled under by a strong current....

IV Characteristics Practical

Results: https://goo.gl/RrChGL

Resistivity Practical

Results: https://goo.gl/gx1BNm

Internal Resistance Practical

Results: https://goo.gl/4xX5oO

What is wrong with the value for the current?