Current Electricity

Question 1

What is electric current? State it’s units

Answer 1

•The rate of flow of charge
•It is measured in amperes

Question 2

What is potential difference?

Answer 2

•The work done moving a unit charge between 2 points in a circuit.
•V = W / Q

Question 3

What is resistance?

Answer 3

•How difficult it is for current to flow through an appliance
• R = V / I

Question 4

What is meant by an ohmic conductor?

Answer 4

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

Question 5

How can you measure the current in a circuit?

Answer 5

By connecting an ammeter in series with the component

Question 6

How do you measure potential difference across a component?

Answer 6

By connecting a voltmeter in parallel across the component

Question 7

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

Answer 7

Rate of change of current with respect to voltage.

Question 8

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

Answer 8

•Current flows through the lamp
•Electrical energy is converted to heat energy so the metal ions vibrate with increasing amplitude
•this obstructs the movement of electrons as they collide with the ions

Question 9

What is a diode?

Answer 9

A diode is a appliance that only allows current to flow in one direction

Question 10

Should you assume voltmeters have zero resistance or infinite resistance?

Answer 10

You should assume they have infinite resistance as current will not flow through it

Question 11

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

Answer 11

This means that there would be 0 potential difference across the ammeter and no energy is lost across it. It does not affect the circuit.

Question 12

What is a Light Dependent Resistor (LDR)?

Answer 12

•A semiconductor that is sensitive to light
•As the light intensity increases, it’s resistance decreases

Question 13

How does a thermistor work?

Answer 13

•As the temperature increases, the resistance decreases.
•This means thermistors have a negative temperature coefficient

Question 14

What is resistivity?

Answer 14

•A property that describes the extent to which a material opposes the flow of electric current through it
•p = RA / L

Question 15

Describe an experiment to determine the resistivity of a metal

Answer 15

1. Measure the diameter of the wire with a micrometer. Calculate the cross-sectional area with πd^2 / 4
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 graph of resistance against wire length
6. The gradient = resistivity / cross-sectional area
7. So resistivity = gradient x cross-sectional area

Question 16

What is a superconductor?

Answer 16

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

Question 17

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

Answer 17

•R total = R1 + R2 + R3 + …
•Add the individual resistances of each component

Question 18

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

Answer 18

The current through all of the components is same

Question 19

Is the current in parallel components the same?

Answer 19

No, each branch of a parallel circuit can have a different current through them according to Kirchoffs’s first law

Question 20

What is Kirchhoff’s first law?

Answer 20

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

Question 21

What is Kirchhoff’s second law?

Answer 21

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

Question 22

What is power?

Answer 22

The rate of energy transfer

Question 23

Give an equation for power in terms flow current and voltage

Answer 23

P = IV

Question 24

What is the purpose of a potential divider?

Answer 24

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

Question 25

What is emf?

Answer 25

•Electromotive force
•The electrical energy transferred by a power supply per unit charge

Question 26

Rearrange the equation E = I(R + r) into the form y = mx + c

Answer 26

1. E = IR + Ir
2. E = V + Ir
   So for a graph of V against I
   V = -rl + E
   Gradient = -r and y-intercept = E

Question 27

What are the advantages of superconductors?

Answer 27

Low power dissipation, high speed operation, and high sensitivity

Question 28

State Ohm’s law

Answer 28

Ohm’s law states that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R) when the temperature remains constant

Question 29

Examples of superconductor use

Answer 29

•MRI scans
•Power cables - to reduce energy loss
through heating