P4.2 Electrical quantities

Question 1

Statements about electrical charge

Answer 1

• There are positive and negative
• Positive charges repel other positive charges; negative charges repel other negative charges
• Positive charges attract negative charges

Question 2

Describe electrostatic charging by friction

Answer 2

Only insulators can be charged by friction, involve the rubbing of two insulators against each other makes the electrons move from one body to another.

Question 3

Simple methods to determine if an object is charged ?

Answer 3

1. Hang a rod with a known charge from a string so it van move freely
2. Bring another rod near to the suspended rod
3. • If the suspended swings away, the new rod has the same charge (repulsion)
     + If the suspended rod swings towards it, the new rod has the opposite charge (attraction)
     + If there is no movement => New rod is not charged (neutral)

Question 4

Define electrical conductors and electrical insulators, and giving typical examples

Answer 4

• Electrical conductors are substances that allow the flow of electric charges passing through it; e.g copper, silver, gold, steel, iron,…
• Electrical insulators are substances that inhibit the flow of electric charges passing through it; e.g glass, plastic, wood, ceramic,…

Question 5

Definition of charge

Answer 5

A property that particles have if they experience a force when placed in an electric field; Symbol is Q ; unit is coulomb (C)

Question 6

Definition of an electric field

Answer 6

A region in which an electric charge experiences a force

Question 7

Definition of direction of electric field

Answer 7

The direction of an electric field at a point is the direction of the force on a positive charge at that point

Question 8

Definition of electric current

Answer 8

The charge passing a point per unit time; this is related to the flow of charge; in metal, this is the flow of electrons

Question 9

Equation for electric current

Answer 9

I = Q/t I: electric current (Amp); Q: Charge (Coulomb); t: time (second)

Question 10

Difference between direct current (d.c) and alternating current (a.c)

Answer 10

• Direct current is a current where electric current flows in the same direction all the time
• Alternating current is a current where electric current flows first one way then the other in a circuit.

Question 11

What is a conventional current

Answer 11

The direction positive charges would flow in a complete circuit, from positive to negative terminals of a cell.

Question 12

What is an electron flow

Answer 12

The flow of electrons from negative to positive terminals of a cell.

Question 13

Definition of ammeter and the use of ammeter with different ranges

Answer 13

• A meter for measuring electric current
• Analogue is used to measure over a large range (0.1 - 5 amps)
• Digital measures over much smaller range (milliamps/microamps)

Question 14

Describe electrical conduction in metals in terms of movement of delocalised electrons

Answer 14

Metals can have an electric current flow through (conduct electricity) as their electrons are free to move and carry the charge through the material.

Question 15

Definition of electromotive force

Answer 15

The electrical work done by a source in moving/driving a unit charge around a complete circuit; measured in volts (V)

Question 16

Definition of potential difference

Answer 16

The work done by a unit charge passing between two points in a circuit, measured in volts (V)

Question 17

Definition of voltmeters and their use in different ranges

Answer 17

• A meter for measuring the potential difference between two points
• Analogue measures over a large range (0.5 - 5 V)
• Digital measures over a smaller range (millivolts/microvolts)

Question 18

Equation for potential difference across 1 component

Answer 18

V = E/Q V: potential difference (Volts); E: Electrical energy (Joules); Q: The amount of charge (Coulomb)

Question 19

Overview of voltage

Answer 19

• Voltage of the source is the cause of current in the circuit
• Voltage of the source is shared between the components in a series circuit.

Question 20

Definition of resistance

Answer 20

The potential difference across a component divided by the current through it

Question 21

Equation for resistance

Answer 21

R = V/I R: resistance (Ohm); V: potential difference across the component in circuit (Volt); I: the current through the component (Amp)

R = pL/A R: resistance (Ohm); p: resistivity (Ohm-meter); L: length of wire (m); A: cross-sectional area of the wire (m^2)

Question 22

Relationship of resistance with metallic electrical conductor

Answer 22

1. Resistance is directly proportional to length
2. Resistance inversely proportional to cross-sectional area

Question 23

Describe an experiment to determine resistance using voltmeter and ammeter and do the appropritate calculations

Answer 23

1. Connect the batter, ammeter, resistor and switch in a series, connect the voltmeter in parallel across the resistor
2. Close the switch to allow the current to flow, records the voltage and current from the meters
3. Calculate the resistance using the equation R = V/I

Question 24

Explain the current-voltage graph of a resistor of constant resistance

Answer 24

This is a straight line graph through the origin, therefore current is directly proportional to the voltage assuming a constant temperature and resistance

Question 25

Knowledge of electric circuits

Answer 25

Electric circuits transfer energy from a source of electrical energy, such as an electrical cell or mains supply, to the circuit components and then into the surroundings.

Question 26

Equation for electrical power

Answer 26

P = IV P: electrical power (Watt); I: current (amps); V: potential difference (Volts)

Question 27

Equation for electrical energy supplied

Answer 27

E= VIt E: electrical energy (joule); V: potential difference (Volt); I: current (Amp); t: time (second)

Question 28

Equation for electrical energy used by a home device for hours

Answer 28

E = Pt E: energy used by home device (Kilowatt-hour kWh); P: power used by home device (kilowatt kW); t: time taken (hour)

Question 29

Define kilowatt-hour (kWh) and calculate the cost of using electrical appliances where the energy unit is kWh

Answer 29

+ 1 kilowatt-hour is the energy used when a 1kW device is used for 1 hour. => this is easier to measure the energy used by a home device that is often on for hours + Equation: Cost of using electrical device (unit currency) = price (unit currency) x energy used (kWh)