Circuits Flashcards
Ohm’s Law
- The current (I in Ampere or A) is proportional to voltage (U in Volt or V)
Voltage (U)
- Voltage is a specific measure of potential energy that is always relative between two nodes.
- Measure in Volts
- A difference in potential.
Voltage Source
- Provides a fixed voltage between two nodes.
- Ideal voltage source has an infinite supply of power, provides as much current as necessary to maintain the voltage.
- Ideal voltage sources imply a capability of providing infinite power while they also have zero internal resistance.
Resistance (Ω)
-Measure of the opposition to current flow in an electrical circuit.
- Due to friction and collisions between the free electrons and other atoms in the wire.
- Where R = U/I (voltage/current).
Resistivity (rho-ρ)
R = ρl/A
where R is R is resistance, ρ is resistivity, l is length, A is area. Unit is ohm meters.
- Resistance depends on several parameters, such as the size (length and cross-sectional area) of the wire, its material and its temperature.
Kirchhoff’s Voltage Law
- The sum of voltages in a closed circuit is always 0.
- Ensure the correct polarity (positive or negative) for each reading of voltage between points.
Resistors in Series
R(Series) = R(1) + R(2)…
Current Law in Series
i(series)= i(1) = i(2)…
Voltage in Series
u(series)= u(1) + u(2)…
Voltage Divider
- A voltage source connected in series with two resistors of different values
- Measure the voltage over only one resistor
- U(in) =U1 + U2
- U(out)/U(in) = R1/ (R1 + R2)
- Only valid if no current or power is drawn out, assumes only measures the voltage
Kirchhoff’s Current Law
- The algebraic sum of the currents entering and leaving a node (junction) of a network is zero.
- ∑ i(n) = 0
- Positive are considered the currents that enter the junction, while negative are the currents that leave.
- Applies to any part of the circuit.
Branch
A series connection of elements that has the same current throughout its length.
Node/Junction
A point in a circuit where three or more branches meet.
Loop
Any closed path in a circuit. A large loop may contain parts of smaller loops.
Charge (q)
- Electric charge is the physical property of matter that causes it to experience a mechanical force when placed inside an electric field.
- Charges of the same type repel.
- The net charge of a body is the algebraic sum of positive and negative charges.
- Unit is Coulombs
Current (I)
- Continuous movement of electric charge through the conductors of a closed circuit.
- Current is defined as the time-rate of flow of charge through a cross section of a conductor. I = dq/dt
- Current is “flow”, measured in Ampere (A).
Current Source
- A current source is a source that will always provide a constant current that is a predefined function of time.
- Symbol is a circle with an arrow in it.
- Current sources provide a constant current flow (DC).
- The direction of flow is that of the flow of positive charge. In electric circuits what actually flows are electrons (negatively charged), so negative charge flowing to the left this is equivalent to positive charge flowing to the right.
Resistance in Parallel
1/R(total) = 1/R1 + 1/R2 + 1/R3…
Current in Parallel
i(Total) = i1 + i2…
Voltage in Parallel
U(Total) = U1 = U2…
Conductance
- Inverse of resistance, consider how easy it is for
current to flow.
G = 1/R - Where G is conductance, unit of Siemens (S)
Potential
- The Voltage at a node.
- Voltage is a difference in the potential.
- Every node has potential.
- Potential field defines how much energy a charge carries.
Ground
- A reference for measuring potential, where voltage is 0 volts.
Power (P)
- It takes energy to move electrons against an electric field. Unit of Watts
- The higher the potential difference the more power.
- Product of voltage and current
P = UI
P = Ri^2 - If you double the current, and the voltage you quadruple the power.
- Sources provide or absorb the power.
Loaded Supply
- A real voltage supply has resistance.
U = U(0) - IR
