General notes

Question 1

What is electric current?

Answer 1

Rate of flow of charged particles

Question 2

What is the equation for current?

Answer 2

I=ΔQ/Δt

Question 3

What is potential difference?

Answer 3

Energy transferred per unit charge

Question 4

What is the equation for p.d.?

Answer 4

V=E/Q

Question 5

What is resistance

Answer 5

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

Question 6

What is the equation for resistance?

Answer 6

R=V/I

Question 7

What is ohm’s law?

Answer 7

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

Question 8

What is the principle of charge conservation?

Answer 8

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

Question 9

What is Kirchoff’s first law?

Answer 9

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

Question 10

How is current distributed in a series circuit?

Answer 10

The current is the same everywhere in the circuit

Question 11

How is current distributed in a parallel circuit

Answer 11

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

Question 12

What is the principle of energy conservation?

Answer 12

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.

Question 13

What is Kirchoff’s second law?

Answer 13

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

Question 14

How does pd work in a parallel circuit?

Answer 14

The pd accross each branch is the same

Question 15

How does pd work in a series circuit?

Answer 15

The pd across each component is shared.H

Question 16

Derive the equation for sum of resistances in series.

Answer 16

• 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

Question 17

Derive the equation for sum of resistances in parallel.

Answer 17

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

Question 18

What is power(elec)?

Answer 18

• Energy transferred over time

Question 19

What is the electrical equation for power

Answer 19

P = IV

Question 20

What are the electrical equations for power with I and V

Answer 20

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

Question 21

Describe ohmic conductor graph

Answer 21

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

Question 22

Describe diode graph

Answer 22

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

Question 23

Describe filament bulb graph

Answer 23

• 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

Question 24

Describe a thermistor graph

Answer 24

• 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

Question 25

What is resistivity

Answer 25

A measure of how easily a material conducts electricity

Question 26

What is resistivity a measure of?

Answer 26

Resistance of 1 metre cube of a material

Question 27

What effects resistivity?

Answer 27

Temperature

Question 28

What is the formula for resistivity

Answer 28

R = ρl/A

Question 29

How do you get to the transport equation for current?

Answer 29

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

Question 30

What is each part of the transport equation for current?

Answer 30

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

Question 31

Describe the potential along a uniform current-carrying wire

Answer 31

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.

Question 32

What is a potential divider circuit?

Answer 32

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

Question 33

What is internal resistance?

Answer 33

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

Question 34

What is emf

Answer 34

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

Question 35

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

Answer 35

terminal resistance

Question 36

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

Answer 36

Using a voltmeter when there is no current in the circuit

Question 37

How does temperature affect the resistance of a metal

Answer 37

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.

Question 38

What happens as the temperature of a semiconductor increases?

Answer 38

- 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.

Question 39

What happens as the light intensity on an LDR increases?

Answer 39

- 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.