Physics 1

• Energy is not something you can hold in your hand, it's just an idea
• Total energy in any interaction is always conserved, energy cannot be created or destroyed
• Energy can be turned into matter (Mass) but it's still technically true that energy cannot be created or destroyed
• Energy stores


  Different types of energy
• Kinetic energy


  Calculated using e = 1/2 mv^2
• Gravitational potential energy (GPE)


  Calculated using e = mgh
• Elastic potential energy


  Calculated using e = 1/2 ke^2
• Thermal energy


  Calculated using e = mc∆T
• Chemical potential energy is found in food and fuels, there is no equation for it
• In a closed system, no energy is lost to the surroundings and no energy comes in from the surroundings
• Gravitational potential energy at the top of a roller coaster

  Converted to kinetic energy at the bottom
• Rearranging equations to find unknown variables

  1. Divide both sides by mass to get v = sqrt(2gH)

  2. Divide both sides by charge to get V = E/Q
• If more energy is transferred to the surroundings than the measured temperature change, the calculated specific heat capacity will be higher than the true value
• Power


  The rate of energy transfer, calculated using P = E/t
• Efficiency


  Ratio of useful energy output to total energy input, calculated as useful output / total input
• Electric current


  The flow of electric charge, measured in amps (A)
• Potential difference (voltage)


  The energy transferred per unit of charge, measured in volts (V)
• Resistance


  The opposition to the flow of electric current, measured in ohms (Ω)
• Ohm's law


  1. V = IR

  2. R = V/I

  3. I = V/R
• Resistance of a resistor is constant (ohmic), but resistance of a bulb changes (non-ohmic)
• PD and current or V and I

  Directly proportional
• Graph of PD and current

  • Straight line
  • Negative values for both but still a straight line through the origin
  • Constant gradient shows a resistor has constant resistance
• Resistance


  Ohm's law: V = I * R (PD in volts = current in amps * resistance in ohms)
• Steeper gradient of the line

  Lower the resistance of the resistor
• Graph of PD and current for a bulb

  • Curved graph
  • Resistance is changing
• Resistance of metal


  Increases with higher PD and higher current
• Resistors are specially made from specific material such that their resistance stays constant even if the temperature changes
• Diode


  • Only lets current flow through in one direction
  • In one direction the resistance is very high, in the other it is very low
• LED (light emitting diode)


  Acts in the same way as a diode but emits light as well
• Measuring resistance of a metal wire

  1. Connect wire to circuit with crocodile clips

  2. Measure V and I

  3. Calculate R using Ohm's law

  4. Move one clip further up the wire to see how length affects resistance
• Series circuit


  • Total PD is shared between components
  • Current is the same for all components
  • Total resistance is the sum of all resistances
• Parallel circuit


  • PD is the same for every branch
  • Current is shared between each branch
  • Total resistance decreases as more resistors are added in parallel
• Thermistor


  Resistance decreases as temperature increases
• LDR (light dependent resistor)

  Resistance decreases as light intensity increases
• Power


  P = VI or P = I^2R
• Direct current (DC)


  Current that only flows in one direction
• Alternating current (AC)


  Current that alternates direction, resulting from an alternating PD
• In a plug, the fuse is attached to the live wire and designed to melt or blow if the current exceeds a certain number of amps
• Calculating fuse size for an appliance

  1. Use P = VI to find the current

  2. Choose next fuse size up from the calculated current
• National grid


  • Network of power stations, cables and transformers that transmit electricity across the country