Topic 3 - Electricity

Question 1

What is the definition of current?

Answer 1

Current is the rate of flow of charge

Question 2

What is the formula for curent?

Answer 2

I = Q/t

Question 3

What is potential difference?

Answer 3

Potential difference is the energy transferred per unit charge between two points in a circuit.

Question 4

What is the formula for p.d?

Answer 4

V = W/Q

Question 5

What is resistance and what is the formula?

Answer 5

Resistance is a measure of how difficult it is for charge carriers to pass through a component.

R = V/I

Question 6

What does ohm’s law state?

Answer 6

Ohm’s law states that for a ohmic conductor, current is directly proportional to p.d across it given that physical conditions e.g temperature is kept constant.

Question 7

What is the principle of charge conservation?

Answer 7

Charge conservation is the principle which states that the total electric charge in a closed system does not change.
e.g total current flowing into a junction is equal to total current flowing out of the junction.

Question 8

Due to charge conservation, how do series and parallel circuits differ?

Answer 8

In a series circuit –> current is the same everywhere
In a parallel circuit –> sum of currents in each parallel set of branches is equal to total current.

Question 9

What are Kirchoff’s laws?

Answer 9

1st first law
-total current flowing into a junction is equal to total current flowing out of junction.
Second law
- Sum of all voltages in a series circuit is equal to the battery voltage.

Question 10

What is the principle of energy conservation?

Answer 10

Energy conservation is the principle which states that energy cannot be formed or destroyed; total energy in a system stays constant.

Question 11

Due to energy conservation, how do series circuits and parallel circuits differ?

Answer 11

Series
- Battery p.d is shared across all elements in the circuit; total sum of voltages across all elements = supply p.d
Parallel
- p.d across each branch is the same.

Question 12

How do you work out total resistance in a series circuit?

Answer 12

Add resistances of all components together.
R₁ + R₂ + R₃ + … = Rₜ

Question 13

What is the definition and formula of power?

Answer 13

Power is the rate of transfer of energy.
P = VI

Question 14

How do you work out total resistance in a parallel circuit?

Answer 14

1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ + …

Question 15

What will the graph for a ohmic conductor look like?

Answer 15

Ohmic conductors’ current is proportional to voltage; straight line through the origin.

Question 16

What is the formula for energy transferred using power?

Answer 16

Power x time = energy transferred
W = Pt

Question 17

What would the IV graph for a diode look like?

Answer 17

Diode only allows current to flow in one direction, forward bias is the direction in which it will allow current to flow easily past the threshold voltage
Before point of threshold voltage, line will be near x axis; when threshold voltage is met, current increases sharply.

Question 18

What is the definition and formula of resistivity ?

Answer 18

Resistivity is a measure of how easily a metal conducts electricity.
ρ = RA/l

Question 19

What other factors affect current?

Answer 19

• Charge carrier density (n)
• Speed at which charged particles are travelling. (drift velocity)
• The charge a single charged particle carries

Question 20

What is drift velocity?

Answer 20

Charged particles are constantly colliding with other particles in the conductor; they do not travel in a straight line. The average speed they move along the conductor is the drift velocity.

Question 21

What formula can be used to calculate current passing through a conductor?

Answer 21

I = nqvA

Question 22

What is a potential divider?

Answer 22

Potential divider is a circuit with several resistors in series connected across a voltage source.

Question 23

What happens to LDR resistance as light intensity increases?

Answer 23

As light intensity increases, LDR resistance decreases

Question 24

Why do voltmeters have high resistance?

Answer 24

Limit current draw

Question 25

How does Ω change in a wire as temperature increases?

Answer 25

As temp increases, KE of lattice ions increases
Frequent collisions with electrons and lattice ions
Drift velocity of electron decreases so current decreases
Resistance increases

Question 26

How does resistance change when temperature increases in a solid?

Answer 26

• As temp increases, intensity of vibration of lattice structure increases
• More intense lattice vibrations; likely to collide with vibrating atoms
• Harder for electrons to pass through solid
• Resistance increases

Question 27

How does resistance change when temperature increases in a metal?

Answer 27

• Temp of metal increases, atoms gain energy
• Once they gain enough energy, they release electrons (thermionic emissions)
• Increases number of charge carriers
• Resistance decreases

Question 28

How do negative temperature coefficient thermistors work?

Answer 28

They release a large amount of charge carriers when temp increases; outweighs effect of lattice vibrations.
Temp increase = Resistance decrease

Question 29

How does resistance change in a metallic conductor?

Answer 29

• As temperature increases, lattice vibrations increase
• Electrons released but not quick enough to counter disruptive effect of lattice vibration.
• As temp increases, resistance increases.

Question 30

How does resistance change in a light dependant resistor?

Answer 30

LDR made of photoconductive materials
- As light intensity increases, LDR releases electrons
- Increases number of charge carriers available to conduct electricity
- Resistance of LDR decreases