10 - Electrical circuits Flashcards
What is Kirchoff’s second law?
In any closed loop circuit, the sum of the electromotive forces is equal to the sum of the potential differences.
What is Kirchoff’s first law?
Sum of currents into a point = sum of currents out of point.
What is a closed loop circuit?
A single possible path for current where everything is connected in series.
What do you do to the resistances when you add resistors in series?
You add the resistances.
R1 + R2 = R
What do you do when you add the resistances of resistors in parallel?
You add the reciprocal of the resistances.
1/R1 + 1/R2 = 1/R
(Remember to flip final fraction)
What is internal resistance?
The resistance of the power supply / battery / cell.
What causes internal resistance?
Electrons colliding with atoms inside the battery.
What is meant by lost volts?
The energy lost per unit charge (coulomb) due to the internal resistance of the battery.
How do you measure the lost volts?
The difference between the terminal voltage and the actual emf of the cell.
What is the terminal voltage?
The p.d that is measured across the cell.
What happens to the energy that is lost as lost volts?
It is transferred to heat energy.
What does an increase in current cause for the lost volts?
More current flowing through means more charge flows through the power supply per second and so more work is done, increasing the lost volts.
What happens when the lost volts increase?
The terminal voltage decreases.
The temperature increases.
What is the equation that links emf, terminal voltage and lost volts?
E = IR + Ir
r = internal resistance , Ir = lost volts
How can a graph be used to find the internal resistance and emf of a cell?
Graph of V against I
Y intercept = emf
-gradient = r
