Physics

• Energy stores + systems

  Energy is transfered between stores.
• Energy stores + systems:

  . Thermal

  . Kinetic

  . Gravitational potential

  . Elastic potential

  . Chemical

  . Magnetic

  . Electrostatic

  . Nuclear
• Energy stores +systems:

  Energy is transferred: mechanically, by doing work, electrically, by heating or by radiation.
• Energy stores + systems:

  System - fancy word for a single object or a group of objects that you are interested in.
• Energy stores + systems:

  When a system changes, energy is transferred.

  It can be transferred into or away from the system.

  Closed system - where neither matter nor energy can enter or leave. Net change is always 0.
• Energy stores + systems:

  Heating:

  Boiling water in a kettle - the water is the system. Energy is transferred to the water by heating, into the waters thermal energy store.
• Energy stores + systems:

  Work done = energy transferred.

  Work can be done when current flows or by a force moving an object.
• Energy stores: kinetic

  Movement means energy in an objects kinetic energy store.

  Energy is transferred to this store when an object speed's up.

  Energy depends on objects mass and speed.

  Formula: Ek=0.5mv2

  Measured in Joules
• Energy stores: gravitational potential

  Raised objects store energy in gravitational potential.

  Lifting an object requires work, the higher the object is lifted the more energy is transferred to this store.

  Amount of energy depends on the objects mass, height and strength of the gravitational field.

  Formula: Ep=mgh
• Energy stores: elastic potential

  Stretching or squashing an object can transfer energy to its elastic potential energy store. As long as the limit of proportionality hasn't been exceeded, energy of a stretched spring can be found using:

  Formula: Ee=0.5ke2
• Conservation of energy:

  Energy is always conserved. It can never be created or destroyed.
• Conservation of energy: dissipated energy

  When transferred between stores, not all of energy is transferred usefully into the store you want it to go into. Some energy is always dissipated.

  Its also called wasted energy because the energy is being stored in a way that's not useful.
• Power: rate of doing work

  Power is the rate of energy transfer. It is measured in watts.

  1 watt = 1 joule of energy transferred per second

  Power: P = E÷t
• Electricity: current

  Current - flow of electrical charge. The unit is ampere 'A'

  Current has the same value everywhere in the circuit
• Electricity: potential difference

  Potential difference - (voltage) the driving force that pushes the charge around. Its unit is volt "V".
• Electricity: resistance

  Resistance - anything that slows down the flow. Unit is ohm.

  The current flowing through a component depends on the potential difference and the Resistance of the component
• Electricity: total charge

  Total charge through a circuit depends on current and time. Size of the current is the rate of flow of charge.

  Charge flow = current × time

  Q = I×t
• Electricity: circuit diagram symbols

  Learn these:

  
• Electricity: resistance

  Potential difference = current × resistance

  Resistance depends on whether components are in series or parallel or the length of wire.

  Ammeter - measures the current (in amps). It must always be placed in series

  Voltmeter - measures potential difference. It must be placed in parallel
• Electricity: RP

  Steps:

  1. Attached crocodile clip to wire level with 0cm on the ruler.
  2. Attached 2nd clip to wire e.g. 10cm away
  3. Record the current through the wire and pd across it
  4. Move the clip another 10cm away and record the new length, current and pd
  5. Repeat
  6. Calculate the resistance for each length of wire using R = V÷I
  7. Plot a graph of resistance
  8. Graph should be a straight line through the origin
• Electricity: I-V graphs

  Resistance of ohmic conductors

  (wire or a resistor) doesn't change with the current. At a constant temperature, the current is the same as the potential difference across it.

  Filament lamp - when an electrical charge flows through it, it transfers some energy to the thermal energy store which is designed to heat up. Resistance increases with temperature so as the current increases the Resistance increases in the lamp.

  Diodes - the Resistance depends on the direction of the current. They have a very high Resistance if the current flow is reversed.
• Electricity: LDR

  LDR is short for Light Dependant Resistor.

  It is a resistor that is dependant on the intensity of light.

  In bright light the resistance falls and in darkness the resistance is highest.
• Electricity: thermistor

  Thermistor is a a temperature Dependant resistor.

  In hot conditions the resistance drops and in cool conditions the resistance goes up.
• Series circuits


  The different components are connects in a line, end to end, between the +ve and -ve of the power supply.
• What happens if you disconnect one component from a series circuit?

  The circuit is broken and they all stop.
• How are voltmeters connected in a series circuit?

  Always in parallel
• Potential difference in series

  The total pd of the supply is shared between the various components. The pd round a series circuit always add up to equal the source pd.

  V = V1 + V2 +...
• Is current the same through all components?

  Yes
• What is the size of current in series?

  The size is determined by the total pd of the cells and the total resistance of the circuit. I = V ÷ R
• What is potential difference represented by?

  V
• What is current represented by?

  I
• What is resistance represented by?

  R
• What is the total resistance of 2 components?

  The sum of their resistances
• Resistance in series


  The potential difference across each resistor is lower, so the current through each resistor is also lower. When a resistor is added to a series circuit the total current is reduced. This means the total resistance of the circuit increases.
• Do cell potential differences add up?

  Yes
• Which is more common? Parral circuits or series circuits?

  Parallel circuits
• How are ammeters always connected?

  In series
• Parallel circuits


  Each component is separately connected to the +ve and -ve of the supply. If you move or disconnect one of them it will hardly effect the others.so most things are connected in parallel.
• What is the potential difference like in parallel?

  It is the same across all components.
• Current in parallel circuits

  The total current flowing around the circuit is equal to the total of all the currents through the separate components.

  There are junctions the current goes through and rhe same amount that goes in has to come out.