Electric Circuits Flashcards
What is 1W in SI base units?
Watts = Power (P) P = VI = WI / Q =FsI / Q =FsI / It =masI / It
=kgm²s⁻²A / As
=kgm²s⁻³
What is charge?
A property that particles or objects can have.
Can be described as positive (+), negative (-), or neutral.
What is the symbol for charges?
Charge (C)
Positive (+)
Negative (-)
Define electrical current.
The rate of flow of charge.
What direction does conventional current flow in?
Positive → Negative
How does the flow of the actual electrons differ from the conventional current flow?
Opposite to conventional current flow.
How can we increase the current through a material?
Increase the rate of charges.
Define potential difference.
The energy transferred per unit charge.
Define resistance.
Opposite to the flow of charge.
Define power.
The energy transferred per unit time.
Define work done.
The energy transferred when a force is acting.
How is current quantified?
The amount of charge passing a point in a second:
I = ΔQ / Δt
One electron has a charge of ______ C.
e = -1.60 x 10⁻¹⁹C
How many electrons pass a point in a second if there is a current of 1A?
1 = ΔQ / 1 ΔQ = 1 1 electron = - 1.6 x 10⁻¹⁹ C x electrons = 1 C x = 1 / (1.6 x 10⁻¹⁹) = 6.25 x 10¹⁸
Where do the electrons come from in a circuit?
The metal from the wire.
Can we increase or decrease the amount of charges?
No, the number of charges in a circuit is a constant.
Due to the conservation of charge:
The number of charges that enter a cell or battery must be equal to the number of charges leaving a cell or battery.
In a series circuit, the current is ____ at every point.
Equal.
In a parallel circuit, the current is __1__ across __2__.
1) Split.
2) Each branch.
What is the total resistance in series?
Rₜ = R₁ + R₂ + …
What is the total resistance in parallel?
¹/Rₜ = ¹/R₁ + ¹/R₂ + …
Derive the equation for total resistance in series?
Due to the conservation of energy: the Pd supplied bu the cell is the sum of the Pd across the circuit
∴ Vₜ = V₁ + V₂ + …
Due to the conservation of charge: the current is the same across the circuit
∴ Iₜ = I₁ = I₂ = …
V=IR
∴ IₜRₜ = IₜR₁ + IₜR₂ + IₜR₃ + …
(÷ Iₜ)
Rₜ = R₁ + R₂ + R₃ + …
Derive the equation for total resistance in parallel?
Due to the conservation of charge: the total current is the sum of the currents on each branch
∴ Iₜ = I₁ + I₂ + …
Due to the conservation of energy: the Pd across the circuit must be the same
∴ Vₜ = V₁ = V₂ = …
V = IR (but I must be the subject ∵ V is a constant), I = V/R
∴ Vₜ /Rₜ = Vₜ /R₁ + Vₜ /R₂ + Vₜ /R₃ + …
(÷ Vₜ)
¹/Rₜ = ¹/R₁ + ¹/R₂ + ¹/R₃ + …
What is Ohm’s Law?
The current through a conductor between two points is directly proportional to the Pd across the two points.
What is the electromotive force?
The amount of energy a source supplies to one coulomb of charge.
What is the difference between voltage, Pd, and e.m.f?
Voltage is a general term referring to the amount of energy transferred per amount of charge.
Pd is the same as voltage but specifically into other forms of energy.
e.m.f is the same a voltage but specific to one coulomb of charge.
Derive power = force x velocity.
P = E / t = W/t = Fs/t = F x s/t [s/t = v] ∴ P = Fv
Express Pd in terms of work done and charge.
V = W/Q
What is resistivity?
Resistance is associated with a specific component, like a resistor or bulb or wire.
Resistivity is a property of a material.
(Think of this like mass and density: you can know the mass of an object, but if you change its shape its mass will change. Its density will remain constant as that is a property of the material that the object is made from)
How does resistance relate to:
- Resistivity
- Length
- Area
Resistance ∝ Resistivity
higher resistivity = higher resistance
Resistance ∝ Length
longer wire = higher resistance
Resistance ∝ 1/Area
larger area = smaller resistance
How would you work out this question?
A basketball is launched from a height of 1.6 m with an initial speed of 6 ms-1 at 50o to the horizontal. There is a horizontal distance of 3.1 m between the basketball at the hoop base. The hoop itself is 2.5 m above ground. Does the ball land in the hoop?
- Sketch
- Find time to peak.
- Find horizontal displacement.
- Find time between peak and hoop.
- Find speed at hoop
- Find distance dropped
Describe the flow of electrons and conventional current in a DC circuit.
electron flow: negative to positive
conventional current: positive to negative
What are the quantities in this equation:
I = nqvA
I = nqvA
I = Current, A n = Number of charges per cubic metre, m⁻³ q = Electron charge, C v = Drift velocity, ms⁻¹ A = Cross sectional area, m²
