Physics-ELECTRICITY

• Current, I


  (amperes, A) The charge transferred per second
• Conductor


  • Contains free electrons
• Insulator


  • Very few free electrons
• Alternating current


  Changes direction every fraction of a second
• Direct current


  Travels in one direction
• Electric field


  The area where an electric charge experiences a force
• Potential difference, V


  (volts, V) The energy given to each coulomb of charge
• Resistance


  (ohms, Ω) The opposition to the flow of charge in a circuit
• Series circuit


  • Made up of only one loop
• Parallel circuit


  • Made up of more than one loop
• Potential Divider


  A circuit used to split the supply voltage
• Power


  (watts, W) The energy transferred in one second
• Transistor


  An automatic switch
• Charge


  There are two types of charge, positive and negative. Like charges repel (move away from) each other and different charges attract (move towards). Charge can be detected using a gold-leaf electroscope or a coulomb meter.
• Current


  There are two types of current alternating current (AC) and direct current (DC). The difference between the two can be seen by connecting them to an oscilloscope. Current is measured in amperes, A, using an ammeter, symbol .

  
• Electric Fields


  • In an electric field a charged particle or object, experiences a force. If the forces acting on any object are unbalanced, it will cause the object to accelerate. We use lines of force to show the strength and direction of the force. The closer the field lines the stronger the force. Field lines are continuous - they start on positive charge and finish on negative charge.
• Potential Difference


  Voltage is the term used to describe the energy given to the charges by the supply (cell, battery or power source). It is defined as the energy given to each coulomb of charge in a circuit. It is measured in volts using a voltmeter, symbol . 1 V is equivalent to 1 JC-1. Potential difference refers to the difference in energy levels of the charges across a component and so describes the energy changed to other forms as the charges pass through the component.

  
• Resistance


  Resistance is the opposition to the flow of charges in a circuit. Resistance is measured in ohms, Ω , using an ohmmeter, symbol . For a given resistor the ratio V/I is constant even when the current changes. This is known as Ohm's law. A component that obeys Ohm's law is said to be ohmic. This will give a straight line through the origin when a graph of potential difference against current is plotted. A component that does not obey Ohm's law is said to be non-ohmic. It will produce a curve when a graph of potential difference against current is plotted. This shows that the resistance is changing.
• Series and Parallel Circuits

  • Series: Current same at all points, Voltage adds up to the supply voltage, Resistance RS = R1 + R2 + R3

  Parallel: Current splits across each branch, Voltage same across each branch, Resistance 1/RP = 1/R1 + 1/R2 +1/R3
• Potential Divider


  A potential divider is a series circuit so the supply voltage is split across the resistors. The voltage across each resistor is dependent on the resistance of the resistor. The bigger the resistance, the larger the voltage across it will be.
• Power


  Electrical power is the rate at which electrical energy is converted into other forms. The energy transferred per second is known as the power and is measured in watts, W. 1 W is equivalent to 1 Js-1.
• Power Transmission


  Electrical energy is carried by long transmission lines between the power station and our homes and industries. Long transmission lines have resistance which means there is power loss in the lines. We can calculate the power loss using the relationship P = I2R.
• Electronic Systems


  • All electronic systems have 3 parts: An input device converts heat, light, sound or movement to electrical energy. The processor changes the electrical signal in some way. The output device changes electrical energy to heat, light, sound or movement.
• The LED


  A LED (Light Emitting Diode) converts electrical energy into light, but it will only do so when is connected the correct way round. A resistor is always placed in series with a LED: to prevent it being damaged by too large a current passing through it, to prevent it being damaged by too large a voltage passing across it, to protect it.
• Calculating the Resistance of the Series Resistor for an LED

  VR = VS – VLED

  = 5 – 1.5

  = 3.5 V

  R = V/I

  = 3.5/10 x 10-3

  = 350 Ω
• Input Resistance


  • There are two types of input device: Energy Changers (convert some form of energy into an electrical signal, e.g. microphones, thermocouples and solar cells) and Resistance Changers (their resistance depends on some external factor, e.g. thermistors, LDRs and variable resistors).
• The Thermistor

  The resistance of a thermistor depends on temperature. As temperature increases resistance decreases. (Temperature Up Resistance Down)
• The LDR (Light Dependent Resistor)

  The resistance of an LDR depends on light intensity. As light intensity increases resistance decreases. (Light Up Resistance Down)
• Capacitor


  The unit of capacitance is the farad. The voltage across a capacitor increases with time. Capacitors are usually connected in series with a resistor. This allows the time for the capacitor to charge to be controlled. The time taken for the capacitor to charge can be increased by increasing the capacitance of the capacitor or increasing the resistance of the resistor.
• Transistors

  • There are two types of transistor: N-channel enhancement MOSFET and NPN Transistor. Both of these transistors can be used as an automatic switch. An NPN transistor switches on when the voltage across the base and the emitter reaches 0.7V.
• Light Controlled Circuit


  As light level decreases resistance of LDR increases, Voltage across LDR increases, Voltage at base of transmitter increases above 0.7 V, Transistor switches on, LED lights.
• Temperature Controlled Circuit


  As temperature increases resistance of thermistor decreases, Voltage across thermistor decreases & voltage across variable resistor increases, Voltage across base of transmitter increases above 0.7 V, Transistor switches on, LED lights.
• Time Delay Circuit


  Switch closed: Voltage across capacitor is 0 V, Voltage at base of transistor is 0 V, Transistor is off, LED is off. Switch closed: Voltage across capacitor increases, Voltage at base of transistor increases above 0.7 V, Transistor switches on, LED lights. Since the capacitor takes time to charge there is a delay between the switch being opened and the LED lighting.