Electric Circuits

Question 1

Define Current

Answer 1

Rate of flow of charge

I = Q/t

Question 2

Conventional current flows

Answer 2

from positive to negative

Question 3

Electrons flow

Answer 3

from negative to positive

Question 4

Electric Current occurs when

Answer 4

a charged particle, which is free to move experiences an electric force.

Question 5

Define Voltage

Answer 5

Energy transferred per unit charge. V = E/Q.

1V = kgm^2s^-3A^-1

Question 6

Define emf

Answer 6

the supply voltage to a circuit

Question 7

Define electronvolt

Answer 7

The amount of energy an electron gains by passing through voltage of 1V.
1eV = 1.6 x 10^19 J

Question 8

Define Resistance

Answer 8

Opposition to the flow of current. R = V/I

Question 9

State Ohms Law

Answer 9

Current through a component is directly proportional to voltage, providing temperature is constant.

Question 10

Define Resistivity

Answer 10

The property of a material to resist the flow of electric current.

Question 11

State the equation fro resistivity

Answer 11

P = RA/l

Question 12

Resistance is the result of

Answer 12

collisions between charge carriers and atoms in the currents path.

Question 13

Define Drift Velocity

Answer 13

The slow overall movement of electrons. I =nAqv

Question 14

Higher temperature in metal causes

Answer 14

atoms to vibrate more, more collisions - slowing drift velocity of electrons

Question 15

Higher current causes

Answer 15

more collisions, as electrons move faster, atoms vibrate more- temp increases

Question 16

Define I = nAqv

Answer 16

n- density of charge carriers
A- cross sectional area
q - charge on each carrier
v - mean drift velocity

Question 17

What is a NTC

Answer 17

Negative Temperature Coefficient - as temp increases, resistivity falls.

Question 18

Semiconductor

Answer 18

Materials with a small number of delocalised electrons that are free to conduct

Question 19

LDR

Answer 19

Light Dependant Resistor.

Question 20

Series Circuit

Answer 20

Current is constant. emf is split between components.

Question 21

Parallel Circuit

Answer 21

Current is split at junctions. pd is same at each component.

Question 22

Conservation of Electric Charge

Answer 22

Total current in = total current out

Question 23

Electric Voltage Rules

Answer 23

in order to conserve electrical energy around any closed loop, sum of emfs is equal to sum of pds around that loop.

Question 24

What is a Potential Divider

Answer 24

A circuit designed to provide specific voltage by splitting emf across two resistors.

Question 25

Potential Divider Equation

Answer 25

Vout = Vin x R2/R1 + R2

Question 26

Potential Difference is

Answer 26

The change in electric potential energy per coulomb between two points

Question 27

why the filament is more likely to fail when being | switched on rather than at other times.

Answer 27

Maximum heating is when lamp is switched on / when current is highest Filament breaks due to melting caused by temperature rise

Question 28

Explain how temp and increased atomic vibrations effect lamp and the thermistor.

Answer 28

For the lamp: Increased atomic vibrations reduce the movement of electrons, resistance of lamp increases with temp. For the thermistor: Increased atomic vibrations reduce movement of electrons, increase in temperature leads to a large increase in n, r decreases as temp increases.

Question 29

Practical use for a diode

Answer 29

- Protect components. - Rectification/ AC to DC - Prevent earth leakage - Voltage controlled switch

Question 30

A thermistor has a NTC. Explain with reference to I = nAQv what happens to its resistance when its temp increases.

Answer 30

temp increases, resistance decreases. Decrease in v. Large increase in n. A and Q remain constant

Question 31

Explain why the resistance of a tungsten filament increases with temperature

Answer 31

Resistance due to collisions between electrons, as particles have more energy and vibrate faster. Increased frequency of collision - impedes flow of electrons.

Question 32

A student has a metal conductor and a plastic insulator (same dimensions). He applies same pd across each. Explain how the relative values of n for the metal conductor and plastic insulator affect the current in each.

Answer 32

n is greater in conductors so there is a larger current flow in conductors

Question 33

equation for internal resistance

Answer 33

``` ε = I(R+r) V = ε -Ir ```

Question 34

Describe the I-V characteristic for an NTC thermistor.

Answer 34

As voltage increases, current increases Increased current causes an increase in temperature This means resistance decreases because NTC thermistors decrease in resistance with temperature so gradient of graph increases

Question 35

Explain the difference between resistance and resistivity

Answer 35

Resistivity is a constant for the material. Resistance depends on resistivity and dimensions

Question 36

Resistors in series act as

Answer 36

Potential dividers. Vout= Vin x R1/R1+R2

Question 37

The temperature of the wire increases. Explain what happens to the drift velocity of the electrons if the potential difference remains constant.

Answer 37

Increased lattice/ions/atoms vibrations causing resistance to increase/ increased electron collisions with ions. This leads to a reduction in the drift velocity

Question 38

Explain the variation of resistance with potential difference for the filament bulb in terms of particle behaviour.

Answer 38

As V increases, electron energy increases. Greater energy transfer in collisions with lattice, causing temperature to increase. Amplitude of vibrations increase. Collision rate between ions and electrons increases. Due to increase in resistance, current doesn't increase in proportion to pd.

Question 39

With reference to charge carriers, explain why the resistance of the thermistor RT, changes with temperature.

Answer 39

Number of charge carriers increases with temperature, this lowers resistance, despite increase in lattice vibrations.

Question 40

Explain why the resistance of a metal sample increases with an increase of temperature.

Answer 40

Rise in temp causes amplitude of vibrating ions to increase. This causes the number of collisions per second between ions and moving electrons to increase. So rate of flow of electrons decreases, causing resistance to increase.