Electricity

Question 1

Rule for current in a series circuit

Answer 1

It is the same everywhere

Question 2

Rule for current in a parallel circuit

Answer 2

The current from the battery is the current in each branch added up.

Question 3

Potential difference rule for a series circuit

Answer 3

pd across each component adds up to the total pd across all components.

Question 4

pd rule for parallel circuits

Answer 4

The voltage across each branch is the same as the voltage from the battery.

Question 5

Define electrical current

Answer 5

The rate of flow of electrons around a circuit

Question 6

How does a series circuit work?

Answer 6

Outer electrons in a metal are free to move. One end of the cell is negative and the other positive. The negative end pushes electrons around the circuit as they are also negative so are repelled.

Question 7

What happens if there is a break in the circuit?

Answer 7

Then the electron at the end has nowhere to go so stops. This electron repels the next so it also stops and so on until all the electrons are stopped.

Question 8

What happens when the electrons enter a bulb?

Answer 8

They hit the metal ions in the filament so they vibrate more and they get hotter. The electrons are also slowed down because of the increased number of metal ions they hit (higher resistance). All the electrons in that branch slow to the same pace as they repel each other.

Question 9

Potential difference

Answer 9

The work done per unit charge

Question 10

Emf

Answer 10

The electrical energy produced by a source of electricity per unit charge passing through the source.

Question 11

Define the resistance of a component

Answer 11

The pd across the component per the current through it

Question 12

Ohm’s law

Answer 12

The pd across a metallic conductor is directly proportional to the current through it, provided the physical conditions don’t change. (On a V I graph the line is straight and goes through 0)

Question 13

Resistivity

Answer 13

The resistance of a conductor with a cross-sectional area of 1m^2 and a length of 1m.

Question 14

Diode facts

Answer 14

Needs at least 0.6V to conduct in forwards bias. Doesn’t conduct in reverse bias (hardly).

Question 15

Define superconductor

Answer 15

A device made from a material that has zero resistivity at and below a critical temperature

Question 16

Uses of superconductors

Answer 16

Used to make high power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators. New applications include power cables that transfer electrical energy without energy dissipation.

Question 17

Semiconductor

Answer 17

The number of charge carriers increases with temperature. The resistance therefore decreases. It is due to electrons that break free from the atoms in the semiconductor. Used in thermistors.

Question 18

What is an insulator?

Answer 18

A substance that doesn’t conduct electricity because each electron is attached to an atom and cannot move away from it.

Question 19

What is a battery?

Answer 19

A combination of cells (sources of electricity)

Question 20

Characteristics of a thermistor

Answer 20

Its resistance decreases if temperature increases. It is made from an intrinsic semiconductor and has a negative temperature coefficient.

Question 21

Characteristics of a light dependent resistor

Answer 21

It’s resistance decreases as light intensity increases

Question 22

Explain how the resistance of a metal varies with temperature

Answer 22

It’s resistance increases as temperature increases because the positive ions in it vibrate more meaning the charge carriers cannot pass through the metal as easily when a pd is applied across the conductor. It has a positive temperature coefficient.

Question 23

What is the rule about the emf and any complete loop of a circuit?

Answer 23

For any complete loop of a circuit, the sum of the emfs around the loop is equal to the sum of the potential drops around the loop.

Question 24

Rule for resistors in series

Answer 24

The total resistance is equal to the sum of the individual resistances.
R = R1 + R2 + R3…

Question 25

Rule for resistors in parallel

Answer 25

The reciprocal of the total resistance is equal to the sum of the reciprocals of the resistors parallel to each other.

1/R = 1/R1 + 1/R2 +1/R3

Question 26

What is the formula for the rate of heat transfer to a component?

Answer 26

I^2 x R

Question 27

What is internal resistance?

Answer 27

The loss of potential difference per unit current in the source when current passes through the source.

Question 28

Why does the terminal voltage decrease as the current increases?

Answer 28

Because there is internal resistance in a battery. As the current flows a potential difference across the internal resistance forms which is equal to Ir. Therefore a larger current results in a larger potential difference.

Question 29

Rules for multiple cells in series

Answer 29

If they are connected in the same direction, the net emf is the sum of the individual emfs. If they are connected in opposite directions, the net emf is the difference between the emfs in each direction. The total internal resistance is the sum of the individual internal resistances.

Question 30

Rules for multiple cells in parallel

Answer 30

For a circuit with n identical cells in parallel, the current through each cell = I/n. The emf is the same with any number of identical cells if they are all in parallel.

Question 31

What is a potential divider and what are it’s uses?

Answer 31

It consists of two or more resistors in series with each other connected across a voltage source with a fixed potential difference. It splits the potential difference from a power supply. Uses: to supply a pd which is fixed at any value between 0 and the source pd, to supply a variable pd, to supply a pd that varies with physical conditions.

Question 32

Potential divider formula

Answer 32

Vout =Vin x R2 / (R1+R2)

Question 33

What happens to the Vout when R1 increases?

Answer 33

When R1 increases the potential difference across it increases. The potential difference across R1 + the potential difference across R2 = supply potential difference (which is fixed). Therefore the potential difference across R2 decreases (Vout).

Question 34

What is a sensor circuit?

Answer 34

A circuit that produces an output pd which changes as a result of a change of a physical variable.

Question 35

What is a temperature sensor made of?

Answer 35

A potential divider made using a thermistor (R2) and a variable resistor (R1).

Question 36

What is a light sensor made from?

Answer 36

A potential divider made using an LDR (R2) and a variable resistor (R1)

Question 37

What does it mean to say a component is rated at a certain power?

Answer 37

The maximum power dissipation is this power.

Question 38

How to find resistivity of a metal wire

Answer 38

Connect wire to dc voltage supply via crocodile clips and wires. Measure the length of wire between the crocodile clips. Connect an ammeter in series in the circuit. Connect a voltmeter in parallel across the wire. Vary the length between crocodile clips and note the readings of the meters and calculate resistance. Use gradient of graph of resistance against length and average cross-sectional area to get resistivity.

Question 39

How to find emf and internal resistance of a cell

Answer 39

Connect cell in series with only an ammeter and variable resistor. Connect voltmeter across variable resistor. Vary the resistance of the variable resistor, each time noting the potential difference across it and the current. Graph of potential difference (terminal voltage) against current. Gradient is -r. y-intercept is emf.