Electric Circuits

Question 1

What is 1W in SI base units?

Answer 1

Watts = Power (P)
P = VI
   = WI / Q
   =FsI / Q
   =FsI / It
   =masI / It

=kgm²s⁻²A / As
=kgm²s⁻³

Question 2

What is charge?

Answer 2

A property that particles or objects can have.

Can be described as positive (+), negative (-), or neutral.

Question 3

What is the symbol for charges?

Answer 3

Charge (C)
Positive (+)
Negative (-)

Question 4

Define electrical current.

Answer 4

The rate of flow of charge.

Question 5

What direction does conventional current flow in?

Answer 5

Positive → Negative

Question 6

How does the flow of the actual electrons differ from the conventional current flow?

Answer 6

Opposite to conventional current flow.

Question 7

How can we increase the current through a material?

Answer 7

Increase the rate of charges.

Question 8

Define potential difference.

Answer 8

The energy transferred per unit charge.

Question 9

Define resistance.

Answer 9

Opposite to the flow of charge.

Question 10

Define power.

Answer 10

The energy transferred per unit time.

Question 11

Define work done.

Answer 11

The energy transferred when a force is acting.

Question 12

How is current quantified?

Answer 12

The amount of charge passing a point in a second:

I = ΔQ / Δt

Question 13

One electron has a charge of ______ C.

Answer 13

e = -1.60 x 10⁻¹⁹C

Question 14

How many electrons pass a point in a second if there is a current of 1A?

Answer 14

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

Question 15

Where do the electrons come from in a circuit?

Answer 15

The metal from the wire.

Question 16

Can we increase or decrease the amount of charges?

Answer 16

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.

Question 17

In a series circuit, the current is ____ at every point.

Answer 17

Equal.

Question 18

In a parallel circuit, the current is __1__ across __2__.

Answer 18

1) Split.

2) Each branch.

Question 19

What is the total resistance in series?

Answer 19

Rₜ = R₁ + R₂ + …

Question 20

What is the total resistance in parallel?

Answer 20

¹/Rₜ = ¹/R₁ + ¹/R₂ + …

Question 21

Derive the equation for total resistance in series?

Answer 21

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 + …

Question 22

Derive the equation for total resistance in parallel?

Answer 22

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 + …

Question 23

What is Ohm’s Law?

Answer 23

The current through a conductor between two points is directly proportional to the Pd across the two points.

Question 24

What is the electromotive force?

Answer 24

The amount of energy a source supplies to one coulomb of charge.

Question 25

What is the difference between voltage, Pd, and e.m.f?

Answer 25

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.

Question 26

Derive power = force x velocity.

Answer 26

P = E / t
   = W/t
   = Fs/t
   = F x s/t
[s/t = v]
∴ P = Fv

Question 27

Express Pd in terms of work done and charge.

Answer 27

V = W/Q

Question 28

What is resistivity?

Answer 28

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)

Question 29

How does resistance relate to:

• Resistivity
• Length
• Area

Answer 29

Resistance ∝ Resistivity
higher resistivity = higher resistance

Resistance ∝ Length
longer wire = higher resistance

Resistance ∝ 1/Area
larger area = smaller resistance

Question 30

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?

Answer 30

• Sketch
• Find time to peak.
• Find horizontal displacement.
• Find time between peak and hoop.
• Find speed at hoop
• Find distance dropped

Question 31

Describe the flow of electrons and conventional current in a DC circuit.

Answer 31

electron flow: negative to positive

conventional current: positive to negative

Question 32

What are the quantities in this equation:

I = nqvA

Answer 32

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