C10 Electrical Circuits Flashcards
10.1 Kirchoff’s second law
In any circuit, the sum of the electromotive forces is equal to the sum of the potential differences around a closed loop.
10.2 How to calculate the total resistance of resistors in series?
R= R1 + R2 + …
How to calculate the total resistance of resistors in parallel?
1/RT = 1/R1+ 1/R2 + …
10.4 Why are car batteries and AA batteries different?
Car batteries have a very low internal resistance compared to AA batteries
What are lost volts?
When a load resistance is connected, current flows through the cell and a voltage develops across the internal resistance. This voltage is not available to the circuit so it is called the lost volts, . can also be calculated as using Ohm’s Law .
What is the lost energy caused by when the e.m.f is supplied to the cell?
It is the internal resistance
10.5 What is a potential divider?
A potential divider is a simple circuit that uses resistors (or thermistors/LDR’s) to supply a variable output potential difference.
What is the equation of a potential divider?
Vout = R2/R1+r2 X Vin
What is loading in a potential divider?
Loading refers to connecting a component or circuit to Vout, that is, placing a component in parallel with R2. This lowers the resistance of this part of circuit and so lowers Vout.
Note: Adding a large load has little effect on the Vout, but if the load has a small resistance Vout is significantly reduced.
10.6 What happens when you use a variable resistor on potential divider circuits Vin = 9.0V, R1= 2.0 Kohms, R2 = 2.0Kohms
A variable resistor can be used to vary the output voltage in a circuit. The variable resistor has a maximum resistance 2kohms and a minimum resistance of 0ohms. This means the output voltage, Vout, varies between 4.5V and 0V.
What happens when a thermistor can be used on a potential divider circuit? Vin = 9.0V R1 = 4.0kohms
A thermistor can be used in a potential divider circuit in a heating element. Thermistors have a high resistance at low temperatures, therefore the voltage will be highest at 0 degrees celsius and will switch the heater on. As the thermistor heats up the resistance decreases and will reach a minimum value at 50 degrees celsius for example. This will turn the heater off. Data-logger can be used if a continuous record of temperature is needed.
What is a potentiometer?
A potentiometer works in the same way as a potential divider but rather than using two resistors in series it uses a length of resistance wire with a sliding contract.
What happens when you move the potentiometer?
When the contact is moved towards A, Vout increases, until at A it equals to Vin. When the contact is moved towards B, V out decreases until at B it is zero.
