Physics - Voltage and Resistance

Question 1

Where does energy enter the circuit?

Answer 1

The power supply

Question 2

Where does energy leave the circuit?

Answer 2

The components

Question 3

What is voltage?

Answer 3

The electrical energy transferred [electrical working] to the component per unit charge passed

Question 4

What is the relationship between voltage, energy and charge?

Answer 4

Voltage = Energy/Charge

Question 5

What is a volt equal to?

Answer 5

1 J/C

Question 6

How many Volts are in the mains vs in a cell?

Answer 6

230V - 1.5V (Though it can vary)

Question 7

How is a Voltmeter connected to a component?

Answer 7

It is connected in parallel with the component

Question 8

In a series circuit, how is the voltage divided if there are two components of differing resistance?

Answer 8

The voltage divides in proportion to the ratio of the other resistances. For example, in a circuit with 6V and with two components (one with 6ohms and another with 12 ohms) the voltage for the first component will be 2V and for the second it will be 4V.

Question 9

In a series circuit, how is the voltage divided if there are two components of the same resistance?

Answer 9

The voltage divides by the number of them. For example, if there are 6V in a circuit with 3 components of the same resistance, the voltage for each will be 2V.

Question 10

In a series circuit, will the voltage change if the resistance of all of the components doubles?

Answer 10

No - provided they are in the same ratio, the resistance will remain the same. However, the current would half as it is affected by the total resistance in a circuit.

Question 11

In a parallel circuit, does the number of branches affect the voltage that each branch receives?

Answer 11

No - each branch will receive the same amount that the cell provides (e.g. 1.5V). This however draws more voltage from the cell so it will run out more quickly.

Question 12

If you attach 4 1.5V cells correctly, what Voltage would they supply a series circuit with?

Answer 12

6V

Question 13

If you attach 3 1.5V cells correctly, but attach 1 1.5V cell wrongly (wrong poles attaches), what voltage would be supplied to a series circuit?

Answer 13

3V - each incorrectly attached one makes another useless so effectively there are only two cells

Question 14

What is the equation linking resistance, voltage and current?

Answer 14

Resistance (ohms) = voltage (volts)/current (amps)

R = V/I

Question 15

What is charge measured in?

Answer 15

Coulombs - C

Question 16

Give four ways that resistance is decreased in a wire

Answer 16

Make it out of a different material (e.g. copper), make it wider, make it shorter and make it cooler

Question 17

What is Ohm’s law?

Answer 17

V = IR

Voltage = Current x resistance

Question 18

What is Ohmic behaviour?

Answer 18

When something gives constant resistance regardless of its voltage or current (e.g. resistor or wire)

Question 19

Describe a circuit used to investigate the I-V behaviour of a metal wire

Answer 19

Power bank connected to an ammeter, then a coil of wire, with a voltmeter connected in parallel

Question 20

Describe a circuit used to investigate the I-V behaviour of a filament bulb

Answer 20

Power bank connected to an ammeter, then a filament bulb, with a voltmeter connected in parallel

Question 21

Describe a circuit used to investigate the I-V behaviour of a diode

Answer 21

Power bank connected to an ammeter, then a diode, with a voltmeter connected in parallel

Question 22

What does the I-V graph of a wire look like? What conclusion can you make?

Answer 22

A straight line - it obeys ohmic behaviour (it does have a linear relationship between voltage and current), and the current is directly proportional to the voltage. The resistance is constant

Question 23

What does the I-V graph of a filament bulb look like? What conclusion can you make?

Answer 23

A straight line at the bottom, but then it curves to level off. It obeys ohmic behaviour at the start but then it curves and doesn’t - it does not have a linear relationship between voltage and current

The current isn’t proportional to the voltage because the temperature of the filament increases so the resistance increases

Question 24

What does the I-V graph of a diode look like? What conclusion can you make?

Answer 24

It starts off as nothing (it’s just a horizontal line), but then suddenly (at about 0.7V) it rises rapidly to a very steep line.

It doesn’t obey Ohm’s Law, as it does not have a linear relationship between voltage and current. The resistance is extremely high until it reaches 0.7V and the resistance drops rapidly

Question 25

Write down the relationship between charge, current and time in words and symbols

Answer 25

Charge = Current × Time

Q=It

Question 26

Use a microscopic model to explain why metals resist the flow of current

Answer 26

Metals resist the flow of current because they are made of ionic lattices, and delocalised electrons. When the electrons flow through the lattices, they may collide with the metal ions, and transfer some of their kinetic energy in the process, thus slowing down and the current reduces.

Question 27

Use a microscopic model to explain why the resistance of a metal increases as its temperature increases

Answer 27

As the temperature increases, the metal ions vibrate more, because temperature is linked to kinetic energy (temperature is kinetic energy amongst all of the particles). Therefore, the delocalised electrons collide with the ions more often and transfer their kinetic energy to the ions, slowing them down and the current decreases as a result.

Question 28

Use a microscopic model to explain why metals heat up when a current flows

Answer 28

Current is the flow of a charged particle, and through wire it is often electrons that flow. When they flow they may collide with the fixed ions that make up metal lattices - and in the process transfer kinetic energy. Kinetic energy on a microscopic scale is often thermal energy on a macroscopic scale, and thus the metal heats up.

Question 29

Write down the equation that give the combined resistance of three different resistors in series

Answer 29

R_T=R_1+R_2+R_3

Total resistance = Resistance 1 + Resistance 2 + Resistance 3

Question 30

Write down the equation that give the combined resistance of three different resistors in parallel

Answer 30

1/R_T =1/R_1 +1/R_2 +1/R_3

1/Total Resistance = 1/Resistance 1 + 1/Resistance 2 + 1/Resistance 3

Question 31

Describe the basic structure of the metallic state.

Answer 31

Fixed ions and delocalised electrons

Question 32

Describe the breakdown of insulators and semiconductors so they can conduct

Answer 32

Enough voltage causes the electrons in an insulator to delocalise and conduct through the material

Question 33

What is a semiconductor?

Answer 33

A material that conducts better than an insulator, but worse than a metal. Conduction is due to excited electrons in the material, therefore its resistance decreases as temperature increases

Question 34

Describe and explain how resistance changes with length

Answer 34

Resistance increases in proportion to length due to the additional material the current must flow through. This is similar to putting resistors in series

Question 35

Describe and explain how resistance changes with cross sectional area

Answer 35

Resistance decreases as cross sectional area increases. This is due to the additional paths that the current can flow through. This is similar to putting resistors in series.