Chapter 13 - DC circuits Flashcards
What is conservation of charge?
As charge flows through a circuit, it doesn’t get used up or lost. Whatever charge flows into a junction will flow out again.
What is Kirchoff’s first law?
total current entering a junction = total current leaving it
What are the 2 current rules for components in series?
- Current entering a component is the same as the current leaving the component.
- The current passing through 2 or more components in series is the same through each component
What is Kirchoff’s second law?
Total emf around a series circuit = sum of the pds across each component
What are the 2 Pd rules?
- For 2 or more components in series, the total PD across all the components is equal to the sum of the potential differences across each component.
- PD across components in parallel is the same
How do you work out total resistance of resistors in series?
Rt = R1 + R2 +R3
How do you find total resistance of resistors in parallel?
1/Rt = 1/R1 + 1/R2 + 1/R3 ….
What is internal resistance?
Loss of potential difference per unit current in the source when current passes through the source
What is electromotive force (emf)?
Give equation
electrical energy per unit charge produced by the source
emf = E/Q
Give equation for emf using internal resistance and external resistance?
emf = IR + Ir
r = internal resistance R = external resistance
What is the equation for power supplied by the cell?
Power = I x emf
What is a potential divider?
2 or more resistors in series. potential of the source is divided between components in the circuit. Supplies constant or variable PD from a power supply.
How do you find the resistance of 2 resistors from the PD across them?
Ratio of the PDs across each resistor is equal to the resistance ratio of the two resistors.
Give examples of potential dividers
- Variable resistors
- thermistors
- light dependent resistors
Draw the circuit used to measure internal resistance?
What could this circuit be used for?
A temperature sensor
(potential divider made using a thermistor and a variable resistor - when temperature of thermistor changes, its resistance changes so the pd across it changes)
Calculate the current through the ammeter when the switch is closed
When the switch is closed the 60 ohm resistor is ‘shorted out’. It is in effect a resistor in parallel with a piece of connecting wire. The ‘short’ wire and resistance parallel combination have less resistance than the wire itself - therefore can be ignored. It is as if there is just a piece of connecting wire at that point in the circuit.
R = 20 ohms
I = V/R = 6/20 = 0.3A
