Chapter 10 Electrical Circuits Flashcards
Kirchhoff’s first law
For any point in an electrical circuit, the sum of the currents entering that point is equal to the sum of the currents out of that point (Conservation of charge)
Kirchhoff’s second law
In any circuit, the sum of the electromotive forces is equal to the sum of the p.d.s around a closed loop.
What happens to the current around a series circuit?
It remains constant.
How is the distribution of Potential difference distributed across 2 resistors in series?
The potential difference is proportional to the resistance across the resistors.
What is the total EMF if two batteries are connected together but one of them is pointing the wrong way?
(correct direction one EMF) - (the incorrect direction one EMF)
In parallel what and how is the current distributed?
The current is distributed in proportion to the resistance across each wire.
What is the general formula for total resistance in series?
R1 + R2…… = Rtotal
What is the general formula for resistors in parallel?
1/R1 + 1/R2 + 1/R3……= 1/Rtotal
When would a high internal resistance be useful to us?
In a high voltage power supply in schools as this lowers the current and so can no longer give fatal doses of current.
When would low internal resistance be useful?
Car batteries as they need to supply very large currents.
Why do all power sources have internal resistance?
Whenever there is a current in a power source work has to be done by the charge as they move through the power source.
What is lost volts and how does one calculate it?
Lost volts is the difference between the expected EMF and the actual EMF. This is because some of the energy is lost via heat transfer.
lost volts = I x r(internal resistance)
Terminal P.D
The Electromotive force of the cell - lost volts
EMF
Electromotive force
Work done per unit charge when energy is converted to electrical from other forms.
When work done is on the charge carriers.
Potential difference
Potential difference is a measure of the transfer of energy by charge carriers and is measured in volts just like EMF. Transferred from electrical to other forms.
Resistance
Resistance is the potential difference across a component / the current through the component
The unit is ohms
If a filament lamp is glowing at maximum brightness and the output voltage from the power supply is halved, why does the current not half?
the resistance of the filament lamp depends on its temperature and so at maximum brightness, the resistance of the lamp is a maximum. As the component is non-ohmic then the current will not half.
What needs to happen for a component to obey Ohms law?
Must be directly proportional
Must pass through the origin
What is internal resistance?
Energy is lost thermally when passing through the battery and this behaves as resistance.
