Physics Unit 2

• Fuses stop the flow of current by melting if the current is too high. So protecting sensitive components and people because if the components function at too higher temperature it can cause a fire. 
• Circuit breakers again break the circuit if current is too high. It works by interrupting the electric current when it exceeds its design limitations, therefore preventing the supply of energy to the loads, and damage to the circuit.
• Insulation and double insulation prevent people from touching exposed wires and getting shocks.
• Earthing provides a low resistance path to the earth so if some one does come into contact with a current instead of flowing through them to the earth giving them a shock it flows through the earthing wire.
• Resistance causes transfer of electrical energy to heat energy. Some components are designed to have a high resistance to make sure this happens, for example electrical heaters that have lots of resistors to ensure a high resistance so a lot of heat is produced.
• power = current x voltage. P = I x V.
• energy transferred = current x voltage x time. E = I x V x t.
• AC is constantly changing magnitude and direction. AC is how mains electricity is produced from turbines.
• DC is constant. And is produced from a battery and used in some sensitive components like in computing.
• Advantages of parallel circuits:

  • Components (e.g. bulbs) may be switched on/off independently.
  • If one component breaks, current can still flow through the other parts of the circuit.
  • Bulbs maintain a similar brightness.
• Advantages of series circuits:

  • Fewer wires, cheaper and easier to assemble.
  • Uses less power
• • As voltage increases the current also increases.
  • In general, the more components in a circuit, the lower the current.
• Wire: Current flowing through the wire is directly proportional to the voltage applied.

  • That is if, voltage across the wire is doubled, current is also doubled
• • OHMS LAW :
  • The current that flows through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant.
• Metal Filament Lamps / Thermistor: Resistance is low when filament bulb is turned on.

  • As current increases, heating effect of bulb increases. Temperature increases. Resistance of filament bulb increases.
  • Current flowing through a filament lamp is not directly proportional to voltage because resistance isn't constant.
  • Curve of the graph shows the increase of resistance as flow of current is large enough to change temperature. Current increase, resistance increase. This is because voltage isn't constant.
• Diode: Allows current to flow in only one direction.

  • When connected forward direction, it gives low resistance.
  • When connected in reverse direction, it gives high resistance.
  • Graph shows that small voltage must be applied before diode will conduct.
  • Increase of voltage, increase of current.
  • If voltage is applied in opposite direction, almost no current flows.
• Describe the qualitative effect of changing resistance on the current in a circuit:

  Since V = IR, as you increase the resistance in a circuit, the current will decrease.
• Qualitative variation of resistance of light-dependent resistors (LDRs) with illumination: As illumination increases, resistance decreases.
• Qualitative variation of thermistors with temperature: As temperature increases, resistance decreases.
• A lamp can be added to a circuit to check for a current. If current is flowing, the lamp will light up.
• Potential difference (V) = Current (A) x Resistance (Ω). V = I x R.
• Current is the rate of flow of charge.
• charge = current x time (Q = I x t)
• Electric current in solid metallic conductors is a flow of negatively charged electrons.
• Why current is conserved at a junction in a circuit: At a junction current ‘splits’ to take both paths. It comes back together when the paths meet again. I1 = I2 + I3 +I4
• Voltage across two components connected in parallel is the same. VT = V1 = V2
•  Voltages: VT = V1 + V2
• Current: IT = I1 = I2
• Resistances: RT = R1 + R2
• Voltage is the energy transferred per unit charge passed and the volt is a joule per coulomb.
• Voltage(V) = Energy Transferred (J) / Charge (C)
• Energy Transferred (J) = charge (C) x Voltage (V) 
• Sound waves are longitudinal waves which can be reflected and refracted.
• Both positive and negative electrostatic charged are only ever produced by the movement of electrons. Positive charges do not move. A positive charge is caused by electrons moving elsewhere. Material that has gained electrons has a negative charge.
• Like charges repel
• Unlike charged attract
• Photocopiers and laser printers: The paper is placed on a metal plate, with a positive charge. Light is then shined on the paper so where the light hits the metal plate, it loses its charge. The ink on the paper stops the light going through, so where there is ink, there is still charge. Toner particles, which are given a negative charge, are then attracted to the plate, where there is still charge. A piece of paper is then placed on top. It is heated to make sure the toner particles stick to the paper.
• Lightning: The water and ice particles rub in the clouds to make a negative charge. This charge is then attracted to the protons in the tall buildings/trees below, so the tall buildings/trees have an induced positive charge. This causes a surge of electricity to be attracted to the trees, so lightning is produced.
• Van Der Graff Generator: As the rubber belt moves around the plastic rollers underneath the metal dome, an electrostatic charge is built up. If you stand on an insulated chair and place your hand on the dome, the electrons will move between your body and dome, giving your body a charge. The human body conducts charge and like charges repel so the charges will spread out as much as possible throughout your body. The charge is strong enough to make your hairs repel each other and stand on end.
• Smoke Precipitator: As the smoke rise, it goes through a mesh that has a negative charge. Above the mesh are two metal plates on either side with a positive charge. The smoke particles get attracted to the mesh because they are oppositely charged. A hammer will hit the plates and the smoke will fall into small cups that collect the smoke.