Electricity-physics

• static electricity is when charge ''builds up'' on an object and then stays static. How the charge builds up depends on what materials are used.
• when an atom loses electrons it becomes positive, when it gains electrons it becomes negative
• how is static energy made?

  Static energy is generated when insulators are rubbed together. Friction moves electrons from one insulator to another making them both charged. This charge is called electrostatic charge. The charge remains on the object because the electrons can not move freely.
• Objects with opposite charges attract each other. Objects with the same charges repel each other.
• Region in which a charge will experience a force. The direction of the field is defined by the direction a positive charge will move in the field. As you get further away from the charged object the field strength becomes weaker.
• Electric current is a flow of electrical charge. The size of the electrical current is the rate of flow of electrical charge.

  Charge flow(C, Coulombs) = current(A, Ampere) x time(S, Seconds)
• An electric current is the flow of electrons, these are called charge

  carriers. Electrons flow in the metal because they are free or delocalised . The source of potential difference ‘pushes’ them around the circuit.
• The potential difference is a measure of the work done or energy transferred to the component by each coulomb of charge that passes through it.

  Energy transferred = charge x potential difference
• The potential difference (Voltage) across a component is meausred using a voltmeter placed in a parallel.
• Electric resistance describes how hard it is for an electric current to flow through the component.

  Resistance(Ohm's) = Potential difference(Volts) /current(Amps)
• A component that follows Ohm’s law has voltage proportional to current. The constant of proportionality for this is resistance. So to follow Ohm’s law a component must have constant resistance when the external conditions are constant, such as temperature.
• The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.
• The resistance of components such as lamps, diodes, thermistors and

  LDRs is not constant; it changes with the current through the component.
• In the parallel cicuits resistance increases as you add more resistors. The total resistance is the sum of the individual resistors.
• Ohmic resistor

  Obey Ohm's law

  Potential difference is directly proportional to the current provided the temperature is constant.
• The resistance of a filament lamp increases as the temperature of the filament increases. A filament lamp contains a thin coil of wire. This heats up when an electric current passes through it, and produces light as a result. The resistance of the lamp increases as the temperature of its filament increases. The current flowing through a filament lamp is not directly proportional to the voltage across it.
• The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
• series circuits

  The total resistance is the sum of the resistance of each component in the circuit. Total resistance (Rtotal) = R1 + R2. The current is the same at every point in the circuit. The voltage is shared between each component in the circuit.

  – Total voltage (Vtotal) = V1 + V2
• Parallel circuits

  The voltage is the same across each branch

  – Vtotal = V1 = V2

  The total current through the circuit is the sum of the current through each component

  – Total current (Itotal)= I1 + I2

  Current can be different on different branches.

  • When resistors are added in parallel the total resistance goes down.
• When we have resistors in parallel circuits the total resistance goes down.
• The UK power supply

  In the UK our mains supply is alternating potential difference. It provides 230V with a frequency of 50Hz. This 50Hz means the electrons go forward and back to the same place 50 times per second.
• The live wire (brown) carries the alternating potential difference from the supply. The neutral wire (blue) completes the circuit. The earth wire (green and yellow stripes) is a safety wire to stop the appliance becoming live. The potential difference between the live wire and earth (0V) is about 230V. The neutral wire is at or close to earth potential (0V) The earth wire is at 0V, it only carries a current if there is a fault.
• Electric shocks

  Our bodies are at earth potential (0V), it is called this as the Earth has a potential of zero volts. If we were to touch a live wire (230V),

  there would be a big difference in potential. This causes a current to flow through our bodies, resulting in an electric shock and in severe cases death.
• A fuse is a small safety device that contains a length of wire that is

  designed to melt if the current in the circuit gets too high.
• Appliances with metal cases are usually earthed. If a fault occurs a large current flows from the live wire to earth. This melts the fuse and

  disconnects the live wire. Some appliances are double insulated, and therefore have no earth connection, this means that their casing is made of an insulator.
• When there is a difference in charge sparks can occur. The greater the charge on an isolated object the greater the potential difference between the object and earth. If the potential difference becomes high

  enough a spark (electrons) may jump across the gap between the object and any earthed conductor which is bought near it. This is why we get electric shocks, the human body has earth potential.
• Electric field

  A charged object creates an electric field around itself, which causes a non- contact force on any other charged object in the field.
• A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field. A second charged object placed in the field experiences a force. The force gets stronger as the distance

  between the objects decreases. The field lines also known as lines of force show the effect the charge will have on a positive charge.
• A direct current (DC) is defined as a current that is steady constantly flowing in the same direction in a circuit, from positive to negative.
• An alternating current (AC) is defined as a current that continuously changes its direction, going back and forth around the circuit.
• Electrical power describes how much electrical energy is converted every second by device. Can be converted into other useful sorts of energy.

  Power is measured in Watts(W)

  P=E/T or E=PXT