Transformers

• A transformer is a device used to change the value of an alternating potential difference or current

  • This is achieved using the generator effect
• A basic transformer consists of:

  • A primary coil
  • A secondary coil
  • A soft iron core
• Iron is used because it is easily magnetised
• Structure of a transformer

  A) primary coil

  B) iron core

  C) magnetic

  D) iron core

  E) secondary coil

  
• An alternating current is supplied to the primary coil of the transformer

  • the current is continually changing direction - it will produce a changing magnetic field around the primary coil
• The iron core in the transformer is easily magnetised, so the changing magnetic field passes through it

  • as a result of the AC and iron core, there is now a changing magnetic field inside the secondary coil
  • this changing field cuts through the secondary coil and induces a potential difference
• As the magnetic field is continually changing the potential difference induced will be alternating

  • The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil
• If the secondary coil is part of a complete circuit it will cause an alternating current to flow
• A transformer consists of a primary and secondary coil

  • The primary coil is the first coil
  • The second coil is the second coil
• A step-up transformer increases the potential difference of a power source

  • A step-up transformer has more turns on the secondary coil than on the primary coil (Ns > Np)
• A step-down transformer decreases the potential difference of a power source

  • A step-down transformer has fewer turns on the secondary coil than on the primary coil (Ns < Np)
• The output potential difference (voltage) of a transformer depends on:

  • The number of turns on the primary and secondary coils
  • The input potential difference (voltage)
• The equation for voltage of a transformer:

  A) potential difference

  B) primary coil

  C) potential difference

  D) secondary coil

  E) number of turns

  F) primary coil

  G) number of turns

  H) secondary coil

  
• • Symbol equation for voltage of a transformer: Vp/Vs = Np/Ns
  • Vp = potential difference (voltage) across the primary coil in volts (V)
  • Vs = potential difference (voltage) across the secondary coil in volts (V)
  • np = number of turns on primary coil
  • ns = number of turns on secondary coil
• When you are using the transformer equation make sure you have used the same letter(p or s) in the numerators (top line) of the fraction and the same letter (p or s) in the denominators (bottom line) of the fraction

  • There will be less rearranging to do in a calculation if the variable which you are trying to find is on the numerator (top line) of the fraction
  • The individual loops of wire going around each side of the transformer should be referred to as turns and not coils
• An ideal transformer would be 100% efficient

  • Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output
• If a transformer is 100% efficient:

  • Input power = Output power
• The equation to calculate electrical power is: P = V × I

  • P = power in Watts (W)
  • V = potential difference in volts (V)
  • I = current in amps (A)
• If a transformer is 100% efficient then: Vp × Ip = Vs × Is

  • Vp = potential difference across primary coil in volts (V)
  • Ip = current through primary coil in Amps (A)
  • Vs = potential difference across secondary coil in volts (V)
  • Is = current through secondary coil in Amps (A)
• The equation of the power of the transformer could also be written as: Ps = Vp × Ip

  • Ps = output power (power produced in secondary coil) in Watts (W)
• Transformers have a number of roles:

  • They are used to increase the potential difference of electricity before it is transmitted across the national grid
  • They are used to lower the high voltage electricity used in power lines to the lower voltages used in houses
  • They are used in adapters to lower mains voltage to the lower voltages used by many electronic devices
• When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
• To transmit the same amount of power as the input power the potential difference at which the electricity is transmitted should be increased

  • This will result in a smaller current being transmitted through the power lines
  • This is because P = IV, so if V increases, I must decrease to transmit the same power
• A smaller current flowing through the power lines results in less heat being produced in the wire

  • This will reduce the energy loss in the power lines
• Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables

  A) power

  B) step-up

  C) high

  D) low

  E) step-down

  F) home

  
• When a current passes through a wire, the current creates a heating effect which means the wires warm up

  • This means they lose electrical energy as heat which reduced the efficiency of the transformer
  • This is due to electrical resistance which is present in all wires
• The power (energy per second) lost in the wire is given by the following equation P = I^2R

  • P = power in watts (W)
  • I = current in amps (A)
  • R = resistance in ohms (Ω)
• Since the power is the energy lost per second, the total energy lost in a time t will be: E = P × t

  • E = energy in joules (J)
  • t  = time in seconds (s)
• A step-up transformer may be used to increase the voltage of a power supply from the power station to the transmission wires
• The number of turns and voltage for the transformer is related by the following equation: Vs/Vp = Ns/Np

  • Vp = potential difference (voltage) across the primary coil in volts (V)
  • Vs = potential difference (voltage) across the secondary coil in volts (V)
  • np = number of turns on the primary coil
  • ns = number of turns on the secondary coil
• A step-up transformer has more turns on the secondary coil, Ns, than on the primary coil, Np
• Since a transformer cannot output more power than is put into, increasing the voltage must result in the current being lowered: IpVp = IsVs

  • Ip = current in the primary coil in amps (A)
  • Is = current in the secondary coil in amps (A)
• Lower current results in less power and energy loss in the cables

  • This makes the transfer of electrical energy through the wires more efficient
• If you forget the equation P = I^2R just remember 'Twinkle twinkle little star, power equals I squared R''
• Symbol for transformer

  
• Change number of turns = change the voltage in the secondary wire

  • if you increase number of turns on secondary side, the voltage induced in the second side, the transformer will increase in proportion
  • applies in step-up
• AC current on a magnet induces a magnetic field on the secondary wire
• Factors that affect transformers

  • number of turns used altogether
  • size and type of wire
  • type of metal used for the core
• Output voltage/input voltage = output turns/input turns
• Input voltage x input current = output voltage x output current