Electricity & Magnetism Flashcards
State how charges are carried in:
a) Metals
b) Liquids
a) Charges are carried by electrons
b) Charges are carried by ions
Define electric current
The amount of charge (coulombs) which flows by a point per second
State the formula, with the correct SI units, for calculating electrical current
I = Q/t
Distinguish between direct (electron) current and alternating (conventional) current
Direct current only flows in one direction, namely from negative to positive. Alternating current changes direction of flow about 50 times per second
Define potential difference
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
State the formula used to calculate potential difference
V = E/Q
State the voltage of a lightbulb when 48 joules of energy is converted into light ennergy with every 4 passing coulombs
12V
Define resistance
The opposition of a conducter to the flow of charge
Name three factors which can increase resistance
- Increase in the length length of wire
- Decrease in the cross-sectional area of the wire
- Increase in temperature
State the formula used to calculate resistance
R = V/I
State Ohm’s law
The current through a metallic conductor is directly proportional to the the potential difference across its ends, given that the temperature stays constant
Name a conductor which obeys ohm’s law
Ohmic conductor
Give an example of a non-ohmic conductor
Filament bulb
Give two formulae, with the correct SI units, used to calculate electrical power
- P = W (energy converted)/t
- P = V x I
Discuss the importance of the earth wire
Prevents the outer casing of an appliance from becoming live by taking excess current into the earth
Describe a ferrous material
Material which can be magnetised. The main ferro-magnetic materials are cobalt, nickel, steel and iron
Explain the domain theory of megnetism
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
Distinguish between soft and hard magnetic materials
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.
Suggest two ways in which a solenoid can be strengthened
- Increase in the number of turns of the coil
- Increase in current
State the two main advantages of an electromagnet
- Can be switched on and off
- Its magnetic field can be adjusted to change the strength of the magnet or to invert polarity
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
a) Np/Ns = Vp/Vs
b) VpIp = VsIs
