Physics—> electrical quantities

• calculate the cost of using electrical appliances where the energy unit is the kW h

  𝑐𝑜𝑠𝑡 𝑝 = 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎��𝑠𝑓𝑒𝑟𝑟𝑒𝑑 𝑘𝑊h × 𝑝𝑟𝑖𝑐𝑒 𝑝𝑒𝑟 𝑘𝑊h (𝑝Τ𝑘𝑊h)
• What is a kilowatt hour?

  A kilowatt-hour is the amount of energy that is transferred by a 1 kW device running for one hour
• Equation for electrical energy transferred

  𝐸 = 𝐼𝑉𝑡
• Electrical power equation

  P=ΔE/t

  𝑃=𝐼×𝑉
• What is electrical power?

  Electrical power is the rate at which a device transfers energy.
• How do electric circuits transfer energy?

  Energy from the chemical store of the battery is transferred by electricity to the thermal energy store of the lightbulbs and then to the thermal energy store of the surroundings by heating and by light
• From where do electric circuits transfer energy and to where ?

  Electric circuits transfer energy from a source, such as an electrical cell or mains supply, to the circuit components and then into the surroundings.
• What is the resistance of a metallic conductor inversely proportional to?

  The resistance of a metallic conductor is inversely proportional to its cross-sectional area, �� ∝ 1/𝐴
• What is the resistance of a metallic conductor directly proportional to?

  The resistance of a metallic conductor is directly proportional to its length, 𝑅 ∝ 𝑙.
• What happens when the wire is wider?

  The wider (the greater cross-sectional area of) the wire, the smaller the resistance.
• What happens when a wire is long

  The longer the wire, the greater the resistance.
• Is there a current when the PD is positive

  there is a current when the p.d. is positive as the diode has a low resistance
• Why is the current zero when the PD is negative?

  The current-voltage graph for a diode shows that the current is zero when the p.d. is negative because it has high resistance,
• Diode


  A diode is a component that only allows current to flow in one direction.

  Conventional current
• Why does the resistance of the elements lamp change in this way

  As the current increases, the temperature of the filament increases. The ions in the metal lattice vibrate more vigorously. The electrons are more likely to collide with the vigorously vibrating ions and are slowed down. Less current can flow per unit potential difference, so the resistance increases.
• Why is the Gradient decreasing?

  The gradient of the graph is decreasing as the potential difference increases in size.
• What happens to the with increasing potential difference?

  The resistance of a filament lamp increases with potential difference.
• Describe an experiment to determine the resistance of a component using an ammeter and a voltmeter

  Connect the component (under investigation) with a power supply (unit)/cell/battery.

  • Connect an DMM/ammeter in series with the component to measure the current.

  • Connect a DMM/voltmeter across/in parallel to the component to measure the potential difference.

  • Calculate resistance using 𝑅 = 𝑉. 𝐼

  • Take multiple readings of current and potential difference.
• Resistance of a variable resistor


  The resistance of a variable resistor is adjustable
• Why is the resistance of a fixed resistor constant?

  At constant temperature, the resistance of a fixed resistor is

  constant, so current is directly proportional to voltage.

  Therefore, the graph of current against voltage for a fixed

  resistor is a straight line that passes through the origin.
• Resistance of a fixed resistor


  The resistance of a fixed resistor is constant
• Resistance equation


  R=V/I
• What is resistance?

  Resistance is a material's opposition to the flow of charge. It is measured in Ohms (Ω).
• What instrument is used to measure potential difference

  E.m.f./potential difference is measured using a voltmeter connected across/in parallel to the component under investigation.
• Equation for potential difference


  V=W/Q
• What is potential difference defined as

  Potential difference is defined as the electrical work done by a unit charge passing through a component. Potential difference is measured in Volts (V).
• Equation for electromotive force

  E=W/Q
• How is electromotive force defined

  Electromotive force (e.m.f.) is defined as the electrical work done by a source in moving a unit charge around a complete circuit. E.m.f. is measured in Volts (V).
• When is the current alternating?


  When it switches between negative and positive values
• Alternating current


  A current that constantly changes direction is called alternating current (a.c.).
• Direct current


  A current that always flows in the same direction is called direct current (d.c.).
• Structure of metals

  The atoms in metals are bonded in such a way that metals are made up of a lattice (grid) of positive ions surrounded by free electrons. These electrons are free to move through the whole metal. Electric current in metals is due to the flow of these free electrons.
• Conventional current

  The opposite direction, from +ve to -ve, is called conventional current.
• Electron flow


  The direction of current from -ve to +ve is called electron flow.
• What are electrons attracted by and repelled by?

  Electrons are repelled by the negative and attracted by the positive terminal of a cell/battery.
• Equation for electric current

  𝐼 = Q/𝑡
• What is electric current defined as?

  Electrical current is defined as the charge passing a point per unit time.
• How is current measured?

  Current is measured using an ammeter connected in series with the component under investigation.
• What does electric current related to?

  Electric current is related to the flow of charge.

  It is measured in Amperes (or Amps for short), A.
• There are two types of charge, called positive and negative

  rubbing an insulator can make it become charged

  a body will become negatively charged If it gains electrons,

  a body will become positively charged If it loses electrons

  metals are examples of conductors.

  They do not become charged when rubbed