Module 4 - Section 1 - E.m.f And Internal Resistance Flashcards
What is internal resistance ?
Resistance comes from electrons colliding with atoms and loosing energy to the surroundings
In a battery, chemical energy is used to make the electrons move - as they move they collide with the atoms inside the battery - so the battery must have a resistance - this is called internal resistance
Why does the battery warm up when it’s used ?
Internal resistance causes the battery to heat up and this is because the electrons inside the battery collide with the batteries atoms and this causes energy to be lost to the surroundings resorting in the battery heating up
What is load resistance ?
Load resistance is the total resistance of all the components in the external circuit - so without the resistance of the battery
Load resistance = resistance in the external circuit - internal resistance
What is the electromotive force (e.m.f) ?
EMF is the total amount of work that the battery does on each coulomb of charge - emf isn’t actually a force - it’s measured in volts
emf = W / Q
Whats the potential difference across the load resistance ?
The pd across the load resistance is the work done when 1C of charge flows through the load resistance - This pd is called terminal pd
What is terminal pd ?
The terminal pd is basically the potential difference that is on the external circuit so not including the battery - so it is the pd across the load resistance - which is the external circuits resistance - as the battery also has a resistance - the terminal pd is different from the emf because the battery also has an internal resistance and as electrons move they collide with the atoms of the battery and this causes resistance and V = IR so the voltage would be different as there would be resistance in the battery
IF there was no internal resistance, the terminal pd would be the same as the emf as for e.g. that emf could be 12V but the battery only gives out 10V because 2V were lost because of the resistance
What is ‘lost volts’ ?
It is the energy wasted per coulomb when overcoming the internal resistance - the volts that have been lost due to the battery having that internal resistance
What is the equation for terminal pd ?
The amount of energy available to the rest of the cell = E.m.f supplied to the cell - the lost volts
Whats the difference between between emf and pd ?
EMF is the energy transferred TO the charge carriers by the battery/cell
Pd is the energy transferred BY the charge carriers to the components
What are the 3 equations that you need to know for internal resistance and emf calculations ?
EMF = V + v V=terminal pd v=lost volts
EMF = I ( R + r ) R=load resistance - external resistance r=internal resistance
EMF = V + Ir V=terminal pd
Whats the equation to find the total emf for cells in series ?
And what about for identical cells in parallel ?
EMF total = emf 1 + emf 2 + emf 3 ….
EMF total = emf 1 = emf 2 = emf 3 ….
How do you measure the internal resistance and the emf of a circuit ?
First you make your circuit - from the top - there’s a battery - then parallel to it there is a voltmeter and on the right of the battery there is an ammeter - then parallel to the voltmeter ( below it ) there is your load resistance which is a variable resistor
First you start of by varying the current by changing the value of the load resistance(R) by using the variable resistor - then measure the pd for several different values of the current - use ammeter to measure the different values of current as you vary the variable resistor
Then make table of results for V and I and then plot you results in a graph where x=current and y=pd
Then you use the equation emf = V + Ir and rearrange it to V = -rI + emf and use the straight line equation y=mx + c —> so -r is the gradient and the emf is the y intercept
