Section 6 - Electricity

Question 1

What is electric current?

Answer 1

The rate of flow of charge - amps

Question 2

What is potential difference?

Answer 2

The work done moving a unit charge between 2 points in a circuit

Question 3

What is resistance?

Answer 3

• A measure of how difficult it is for current to flow through an appliance
• A component has a resistance of 1 Ω if 1 A flows through it when a p.d. of 1 V is applied across it

Question 4

What is meant by an ohmic conductor?

Answer 4

A conductor that obey’s Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant

Question 5

How can you measure the current in a circuit?

Answer 5

An ammeter connected in series with the component

Question 6

How can you measure potential difference across a component?

Answer 6

Using a woltmeter, connected in parallel across the component being mesured

Question 7

What does the graph of a current-potential difference graph represent?

Answer 7

Rate of change of current with respect to voltage

Question 8

What does the current-voltage graph of an ohmic conductor look like?

Answer 8

The line has a constant gradient and passes through the origin - voltage is directly proportional to current

Question 9

What does a V-I graph look like?

Answer 9

A reflection in the line y=x of an I-V graph

Question 10

How can you tell that an appliance has a higher resistance than another using I-V graphs?

Answer 10

• Shallower gradient
• A higher voltage is required for the same change in current

Question 11

What does the I-V graph for a filament bulb look like?

Answer 11

An S shape - as the current increases the resistance also increases - a big increase in the voltage produces only a small increase in current

Question 12

Why does the current increasing on a filament lamp cause an increase in the resistance?

Answer 12

• As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude
• This impedes the movement of electrons through the lamp as they collide with the ions (resisitance has increased)

Question 13

What is a diode?

Answer 13

• An appliance that only allows current to flow in one direction - forward bias
• Most require a threshold voltage
• In reverse bias, the resistance of the diode is very high and the current that flows is very tiny

Question 14

Unless stated in the question, should you assume that voltmeters have zero resistance or infinite resistance? Why?

Answer 14

• Infinite resistance
• Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance

Question 15

Why should you assume that an ammeter has zero resistance unles otherwise stated?

Answer 15

There would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit

Question 16

What is a light dependent resistor?

Answer 16

• A semiconductor that is sensitive to light
• LURD - as the light intensity increases, its resistance decreases

Question 17

How does a thermistor work?

Answer 17

• TURD - as the temperature increases, resistance decreases
• They have a negative temperature coefficient

Question 18

What is resisitivity?

Answer 18

The resistance of a 1m cylinder with a cross sectional area of 1m^2 - an inherent property of a material
Measured in Ωm

Question 19

What 3 things determine resistance?

Answer 19

1. Length - the longer the wire, the more difficult it is to make current flow
2. Area - the wider the wire, the easier it will be for the electrons to pass along it
3. Resisitivity - the structure of the material may make it easy or difficult for charge to flow - depends on environmental factors as well such as temperature and light intensity

Question 20

Describe an experiment to determine the resistivity of a metal

Answer 20

1. Measure the diameter of the wire with a micrometer, then calculatet he cross-sectional area = π(d/2)^2
2. Set up the circuit
3. Vary the wire length and record the voltage and current for each length
4. Use R=V/I to work out the resistance
5. Plot a graph of resistance against wire length
6. The gradient - resistivity/cross sectional area
7. Resistivity = gradient x cross sectional area

Question 21

What is a superconductor?

Answer 21

• A material that has a resisitivity of zero at or below a critical temperature
• The critical temperature is an inherent property of the material

Question 22

How do you find the total resistance in a series circuit?

Answer 22

Add the individual resistances of each component

Question 23

How do you find the total resistance in a parallel circuit?

Answer 23

1/Rtot = 1/R1 + 1/R2 +1/R3….

Question 24

How does the current vary between each component of a series circuit?

Answer 24

The current through all of the components is the same so the current doesn’t vary

Question 25

Is the current in parallel components the same?

Answer 25

Each branch can have different currents throught them according to Kirchoff's first law

Question 26

What is Kirchoff's first law?

Answer 26

All of the current going into a junction is equal to the current leaving the junction

Question 27

What is Kirchoff's second law?

Answer 27

For any path (loop) of a circuit, the sum of all of the potential differences must equal the total emf of the circuit

Question 28

In a series circuit, if 2 cells are connected negative to negative, would their emf add up or cancel out?

Answer 28

They will cancel out εtotal = ε1 - ε2

Question 29

What is power?

Answer 29

The rate of energy transfer in Watts or joules per second

Question 30

What is the purpose of a potential divider?

Answer 30

* To provide variable potential difference * To provide a constant specific potential difference

Question 31

What is emf?

Answer 31

x amount of energy is transferred from chemical energy to electrical energy per coulomb of charge

Question 32

How can you use a graph to find the emf and internal resistance?

Answer 32

* Rearrange ε=I(R+r) into the form y=mx+c * V = -rI + ε

Question 33

What are 2 applications of superconductors?

Answer 33

1. Power cables - would reduce energy loss through heating to zero during transmission 2. Strong magnetic fields - would not require a constant power source - these could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications

Question 34

What are the emf equations?

Answer 34

ε = I(R+r) ε = V + v V = ε - v V = ε - Ir V - terminal p.d. v- lost volts R - load resistance r - internal resistance

Question 35

How can you calculate the emf in series vs parallel?

Answer 35

* Series - add the individual emfs of the cells * Parallel - every cell has the same emf

Question 36

What is a use of potential dividers?

Answer 36

To callibrate voltmeters which have a very high resistance

Question 37

What is a potentiometer?

Answer 37

* Uses a variable resistor to give a variable voltage * Used if you want to be able to continuously change a voltage, like in the volume control of a stereo

Question 38

If the question doesn't mention internal resisitance being negligible what should you probably do immediately?

Answer 38

Write the equation ε = I(R+r) and find the link to it

Question 39

Why when the voltmeter is moved across a different resistor in the circuit does the sum of the separate potential differences in the voltmeters not equal to the emf of the battery?

Answer 39

* Current in circuit changes * Because resistance in the circuit changes

Question 40

How can you make a component function for a longer time by adding more cells?

Answer 40

1. Put cells in parallel 2. More energy stored in each cell 3. Doesn't increase terminal p.d.

Question 41

Why would a fixed resisitor be used in a circuit?

Answer 41

* To limit maximum current * To prevent overheating

Question 42

What's a difficulty of using a superconductor?

Answer 42

* Must be kept below the critical temperature * Hard to maintain such low temperatures

Question 43

How do you calculate the emf and internal resistance of a power supply?

Answer 43

1. Add a voltmeter in parallel across the power supply and an ammeter in series 2. Read the ammeter and voltmeter and take measurements 3. Plot a graph of V against I 4. emf is the y intercept 5. -r is the gradient 6. Write emf and rearrange into y=mx+c to show this

Question 44

When is the maximum power delivered to the load of a circuit?

Answer 44

When load resistance = internal resistance

Question 45

What is the equation for a potential divider?

Answer 45

Vout = R1/R1+R2 x Vin