Electricity & Magnetism

Question 1

State how charges are carried in:

a) Metals
b) Liquids

Answer 1

a) Charges are carried by electrons

b) Charges are carried by ions

Question 2

Define electric current

Answer 2

The amount of charge (coulombs) which flows by a point per second

Question 3

State the formula, with the correct SI units, for calculating electrical current

Answer 3

I = Q/t

Question 4

Distinguish between direct (electron) current and alternating (conventional) current

Answer 4

Direct current only flows in one direction, namely from negative to positive. Alternating current changes direction of flow about 50 times per second

Question 5

Define potential difference

Answer 5

A battery tranforms chemical energy into electrical energy. In a circuit, there is a build-up of electrons at the negative terminal of a battery, and a shortage of electrons at the positive terminal. This creates a potential difference, which drives charge around the circuit. Potential difference is thus the ability to drive charge around a circuit

Question 6

State the formula used to calculate potential difference

Answer 6

V = E/Q

Question 7

State the voltage of a lightbulb when 48 joules of energy is converted into light ennergy with every 4 passing coulombs

Answer 7

12V

Question 8

Define resistance

Answer 8

The opposition of a conducter to the flow of charge

Question 9

Name three factors which can increase resistance

Answer 9

• Increase in the length length of wire
• Decrease in the cross-sectional area of the wire
• Increase in temperature

Question 10

State the formula used to calculate resistance

Answer 10

R = V/I

Question 11

State Ohm’s law

Answer 11

The current through a metallic conductor is directly proportional to the the potential difference across its ends, given that the temperature stays constant

Question 12

Name a conductor which obeys ohm’s law

Answer 12

Ohmic conductor

Question 13

Give an example of a non-ohmic conductor

Answer 13

Filament bulb

Question 14

Give two formulae, with the correct SI units, used to calculate electrical power

Answer 14

• P = W (energy converted)/t

- P = V x I

Question 15

Discuss the importance of the earth wire

Answer 15

Prevents the outer casing of an appliance from becoming live by taking excess current into the earth

Question 16

Describe a ferrous material

Answer 16

Material which can be magnetised. The main ferro-magnetic materials are cobalt, nickel, steel and iron

Question 17

Explain the domain theory of megnetism

Answer 17

The theory suggests that ferrous materials like steel contain within its structure tiny magnetised cells called domains. In a magnetised material, all domains are lined up and their magnetic effects reinforce each other, while in an unmagnetised material, the domains point in different directions and therefore cancel each other out

Question 18

Distinguish between soft and hard magnetic materials

Answer 18

Soft magnets, like iron, are easy to magnetise, but are only permanent magnets. Hard magnets, like steel, are more difficult to magnetise but once magnetised, they are permanent magnets.

Question 19

Suggest two ways in which a solenoid can be strengthened

Answer 19

• Increase in the number of turns of the coil

- Increase in current

Question 20

State the two main advantages of an electromagnet

Answer 20

• Can be switched on and off

- Its magnetic field can be adjusted to change the strength of the magnet or to invert polarity

Question 21

State the formula used to calculate voltage and amount of turns in transformers when a) using the amount of turns on a coil and b) when using current to determine voltage

Answer 21

a) Np/Ns = Vp/Vs

b) VpIp = VsIs