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Electric circuits - Topic 2 > General notes > Flashcards

General notes Flashcards

1
Q

What is electric current?

A

Rate of flow of charged particles

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2
Q

What is the equation for current?

A

I=ΔQ/Δt

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3
Q

What is potential difference?

A

Energy transferred per unit charge

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4
Q

What is the equation for p.d.?

A

V=E/Q

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5
Q

What is resistance

A

How difficult it is for charge carriers to pass through a component

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6
Q

What is the equation for resistance?

A

R=V/I

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7
Q

What is ohm’s law?

A

For an ohmic conductor, current is directly proportional to the p.d across it, given that other conditions are kept constant.

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8
Q

What is the principle of charge conservation?

A

The total electric charge in a closed circuit does not change, only if other conditions remain constant

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9
Q

What is Kirchoff’s first law?

A
  • Uses principle of charge conservation
  • The current flowing into a junction is equal to the current flowing out of a junction
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10
Q

How is current distributed in a series circuit?

A

The current is the same everywhere in the circuit

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11
Q

How is current distributed in a parallel circuit

A

The sum of the currents in each parallel branch is equal to the total current.

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12
Q

What is the principle of energy conservation?

A

Energy cannot be created or destroyed, but can be transferred from one form to another. Therefore the total energy of a closed system remains constant.

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13
Q

What is Kirchoff’s second law?

A

In any closed loop, the sum of p.ds in the loop is the same as the sum of pds around the loop.

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14
Q

How does pd work in a parallel circuit?

A

The pd accross each branch is the same

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15
Q

How does pd work in a series circuit?

A

The pd across each component is shared.H

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16
Q

Derive the equation for sum of resistances in series.

A
  • V1=I1R1, V2=I2R2
  • Using kirchoff’s second law, VT = V1+V2,
  • We get ITRT = I1R1 + I2R2
  • IT = I1 = I2, kirchoff’s first law. Divide equation by I
  • We get RT = R1 + R2
17
Q

Derive the equation for sum of resistances in parallel.

A
  • I1 = I2 + I3
  • VT/RT = V1/R1 + V2/R2
  • 1/RT = 1/R1 +1/R2
18
Q

What is power(elec)?

A
  • Energy transferred over time
19
Q

What is the electrical equation for power

A

P = IV

20
Q

What are the electrical equations for power with I and V

A

P = I^2*R
P = V^2/R

21
Q

Describe ohmic conductor graph

A
  • Follows ohm’s law
  • looks like straight line through origin
22
Q

Describe diode graph

A
  • Past the threshold voltage, current will flow easily.
  • In direction of negative bias, little no current will flow as the resistance is very high
23
Q

Describe filament bulb graph

A
  • At low currents, ohm’s law is obeyed.
  • At higher currents, the current increases at a lower rate than p.d. due to the resistance of the component increasing as the wire heats up
24
Q

Describe a thermistor graph

A
  • At low currents, ohm’s law is obeyed
  • At higher currents, currents increase at a higher rate than p.d., this is because increasing the temperature of the thermistor causes electrons to be released from the atoms to the conduction band
25
Q

What is resistivity

A

A measure of how easily a material conducts electricity

26
Q

What is resistivity a measure of?

A

Resistance of 1 metre cube of a material

27
Q

What effects resistivity?

A

Temperature

28
Q

What is the formula for resistivity

A

R = ρl/A

29
Q

How do you get to the transport equation for current?

A

I = ΔQ/Δt
I = Nq/Δt
I = nVq/(l/v)
- I = nVvq/l
I = nAqv

30
Q

What is each part of the transport equation for current?

A

I: current
n: charge carrier density
A: area
q: charge of one electron
v: drift velocity

31
Q

Describe the potential along a uniform current-carrying wire

A

Take a uniform wire with constant surface area and resistivity. As resistance length increases, resistance increases. Linking this to V=IR, we can see that V increases uniformly with the distance along it.

32
Q

What is a potential divider circuit?

A

A circuit with several resistors in it which is used to produce a required fraction of the source potential difference, which remains constant.

33
Q

What is internal resistance?

A

Caused by electrons colliding with each-other inside the battery. Causes some energy to be lost even before electrons leave the battery.

34
Q

What is emf

A

energy transferred by a cell per coulomb of charge that passes through it.

35
Q

What is the p.d. in a circuit minus the internal resistance called

A

terminal resistance

36
Q

How can the emf of a battery be measured, not cp.

A

Using a voltmeter when there is no current in the circuit

37
Q

How does temperature affect the resistance of a metal

A

As the temperature of the metal increases, the metal ions vibrate with a higher amplitude. Due to the higher amplitude of vibrations, electrons have more frequent collisions with the ions therefore leading to a lower drift velocity of the electrons. Due to I = naqv, as drift velocity decreases, the current will also decrease proportionally. Current and resistance are inversely proportional. So as current decreases, resistance increases.

38
Q

What happens as the temperature of a semiconductor increases?

A
  • Atoms gain energy
  • Once they gain enough energy, electrons are released by thermionic emission
  • Moves electrons to the conduction band
  • Increases number of charge carriers, n, in the semiconductor
  • Due to I = nAqv, as n increases, the current increases, outweighing the effect of lattice vibrations
  • Due to Ohm’s law, as current increases, resistance decreases.
39
Q

What happens as the light intensity on an LDR increases?

A
  • Atoms gain energy from light
  • Once the electrons have enough energy, they are released from the atom to join the conduction band
  • Increases number of charge carriers, n, in the semiconductor
  • Due to I = nAqv, as n increases, the current increases, outweighing the effect of lattice vibrations
  • Due to Ohm’s law, as current increases, resistance decreases.

Electric circuits - Topic 2 (3 decks)

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