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DC Physics

on 28 February 2016

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Transcript of Electricity

When a 1Ω resistor is connected to the terminal of a cell, the current that flow through it is 8A. When the resistor is replaced by another resistor with resistance 4Ω, the current becomes 2.67A.

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
Understand and use the equation for resistivity.
Circuit Basics
Be able to define Charge, Current, Potential Difference, Resistance and use the equation:
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
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.
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:

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
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
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
Alfred Leitner
Liquid Nitrogen Bomb
Levitating Train
10 Liquid Nitrogen Experiments
Kirchhoff Laws
A Level
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?
Full transcript