Chapter 13 - Direct current circuits Flashcards
What are the current rules?
- At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction.
Current total = Current 1 + Current 2
where component 1 and 2 are in parallel. - The current entering a component is the same as the current leaving it.
- For components in SERIES, the current in them is the same.
What are the potential difference rules?
- For components in SERIES, the total pd across all the components is equal to the sum of pds across each component.
Vtotal = V1 + V2 + V3 - The pd across components in PARALLEL is the same.
- For any complete loop of a circuit, Σℰ = ΣV
What is the total resistance of resistors in series?
Resistors in series pass the same current.
Rtotal = R1 + R2 + R3 +…
What is the total resistance of resistors in parallel?
Resistors in parallel have the same pd. The current through a parallel combination of resistors is equal to the sum of the individual currents.
1 1 1 1
—- = —– + —– + —– + …
Rtotal R1 R2 R3
What is the electromotive force of a source?
The electrical energy per unit charge produced by the source.
What is the internal resistance of a source?
The loss of potential difference per unit current in the source when current passes through the source.
How does the terminal pd compare to the emf?
The terminal pd is less than the emf whenever current passes through the source.
The difference is due to the internal resistance of the source.
What is the equation for cell current of cells in series?
Cell emf
Cell current = ————————————
Total circuit resistance
For a circuit with n identical cells in parallel:
- internal resistance =
- The current through each cell =…
- Terminal pd of each cell =
resistance of each resistor r
- Internal resistance = ——————————————— = —-
n n
I
- Current through each cell = —–
n
where I = total current supplied by cells
- Terminal pd of each cell = ε - Ir/n
where r = r of each cell
I = total current
What assumptions should be made about a silicon diode in circuits?
- Forward pd of 0.6V whenever a current passes through it.
- Infinite resistance in reverse direction or at pds less than 0.6V in the forward direction.
What is a potential divider used for?
A potential divider is used to supply a constant or variable potential difference from a power supply.
What is the advantage of using a potential divider instead of a variable resistor?
The potential divider can vary the pd across a component from zero, therefore it can stop any current from flowing.
The variable resistor can only reduce the current to a minimum (non-zero) value.
What is the electromotive force (emf), ε, of a source of electricity?
The amount of energy it transfers from other forms of energy to electrical energy per unit charge
(i.e. the energy transferred per unit charge).
When charge carriers travel through a battery, the battery does work on them and so they gain energy.
Chemical energy in battery → Electrical energy
What is Kirchhoff’s Second Law?
Σℰ = ΣV
Energy is conserved in a circuit.
The total energy transferred to the charges in a circuit is equal to the total energy transferred from the charges to the components.
What is the equation for the rate of heat transfer in a circuit?
2
Rate of heat transfer, P = I R
