Section 6 - Electricity

Question 1

Define the term current.

Answer 1

Current is the rate of flow of charge in a circuit

Question 2

How are ammeters attached to a circuit.

Answer 2

In series

Question 3

Define the term potential difference

Answer 3

The potential difference between two points is the work done in moving charge between the points

Question 4

What is meant by Resistance

Answer 4

The resistance of something is a measure of how difficult it is to get a current to flow through it, it is defined as R = V ÷ I

Question 5

What is Ohms law

Answer 5

Provided the physical conditions, such as temperature, remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.

Question 6

What happens to the resistance of an ohmic conductor if you double the potential differnece across it?

Answer 6

Nothing - the resistance of an ohmic conductor is constant

Question 7

What resistance do you normally assume a voltmeter / ammeter to have

Answer 7

Voltmeter -infinite resistance

Ammeter - 0 resistance

Question 8

Is a filament lamp an ohmic conductor

Answer 8

A filimant lamp is not an ohmic conductor. It’s characteristic IV graph is not a straight line, so therefore voltage is not proportional to current

Question 9

State the three things that the resistance of a material depends on.

Answer 9

Length Area Resistivity

Question 10

What is resistivity

Answer 10

The resistivity of a material is the resistance of a 1m length with a 1 m^2 cross sectional area. it is measured in ohm-meters

Question 11

Describe the effect of temperature on the resistance of a metal

Answer 11

As the temperature of a metal increases, its resistance will also increase

Question 12

What is a superconductor

Answer 12

A material that has zero resistivity when cooled below a critical temperature (called the transitional temperature)

Question 13

What are some disadvantages of superconducting wires

Answer 13

• It is very difficult to cool a material below its transitional temperature
• It is very expensive to keep a material below its transitional temperature

Question 14

Give some uses of superconducting wires

Answer 14

• Power Cables
• Strong Electromagnets
• Fast electronic circuits

Question 15

State the equation used to calculate the cross-sectional area of a cylindrical wire

Answer 15

Area = pie*r^2 assuming its circular

Question 16

Define the term power

Answer 16

Power (P) is the rste of transfer of energy. It’s measured in Watts (W), where 1 watt is equivalent to 1 joule per second.

Question 17

Write down three equations you could use to calculate electrical power

Answer 17

P=VI
P=V^2/R
P=(I^2)R

Question 18

Explain why batteries have an internal resistance

Answer 18

In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery and lose energy - this is the internal resistance

Question 19

What is the load resistance of a circuit

Answer 19

The load resistance is the total resistance of all the components in the external part of the circuit

Question 20

What units is electromotive force measured in

Answer 20

Volts

Question 21

Explaim what lost volts are

Answer 21

The energy wasted per coulomb overcoming an internal resistance

Question 22

What do the gradient and vertical intercept on a V-I graph for a power supply show

Answer 22

The gradient is -r (where r is internal resistance)

The verticle intercept is E (the electromotive force)

Question 23

State Kirchoff’s first law

Answer 23

The total current entering a junction = the total current leaving it

Question 24

State Kirchoff’s second law

Answer 24

The total e.m.f around a series circuit = sum of the p.d.s across each component

Question 25

What is a potential divider

Answer 25

A potential divider is a circuit containing a voltage source and a couple of resistors in series. The voltage across one of the resistors is used as an output voltage. If the resistor’s aren’t fixed, the circuit will be capable of producing a variable output voltage.

Question 26

How can you make a light sensor using a potential divider

Answer 26

By using an LDR as one of the resistors

Question 27

What is a potentiometer? Give an example of when it could be used

Answer 27

A potential divider containing a variable resistor instead of two resistors in series. Used as volume control on a stereo