DC electricity

• Define current


  The rate of flow of charge in a wire or component.
• What charge carrier are in metals? What happens?

  Conduction electrons, they repeatably collide and each other with the positive ions in the metal
• Define coloumb

  the amount of charge that passes in 1 second if the current is 1 ampere.
• Define voltage

  is the work done per unit charge moved.
• What rule does potential difference follow in parallel circuits?

  Potential difference is the same across all branches.
• Definition of A VOLT.

  potential difference across component when 1 volt is converted into 1 joule of energy when moving 1 coulomb of charge through a component.
• What is the resistance of voltmeters and ammeters

  voltmeter- infinite Ammeter-none
• What is an Ohmic conductor?

  A conductor that obeys ohms law- current is directly proportional to potential difference across it when physical conditions remain constant.
• What does the gradient show on an V/R graph or any divided Voltage Current point

  resistance
• On an I/V graph what does a steep gradient show

  low resistance BUT high resistance on V/I graph
• What is the shape of a filament light graph

  
• What happens when current runs through a filament bulb?

  resistance to current flow increases its temperature
• Why does gradient change on filament bulb graph?

  As currents heats metal coil, resistance increases as vibrating ions and electrons make it harder for charge carriers to pass through so therefore gradient gets shallower to signify increasing resistance
• What is a semiconductor? what are they used for?

  a conductor that when energy is supplied releases charge carriers. Excellent sensors for changes in surroundings
• What is a NTC thermistor?

  Resistance decreases as temperature goes up (opposite to convention)
• Explain an I/V graph of NTC Thermistor

  Increase voltage and current increases causing rise in temperature causing release of charge particles causing decrease in Resistance and hence graph gets steeper.

  
• Why does increase in temperature release more charge carriers?

  Thermal energy gives electrons more energy to escape from their atoms so more are available to carry charge.
• What do diodes do?

  Only allow current to flow in forward bias direction
• What is the threshold voltage of most diodes?

  0.6V
• Why doesn't it flow in reverse direction?

  resistance if very high and current is very small.
• What 3 factors determine resistance?

  resistivity, cross sectional area, length.
• What makes a current easier to flow?

  shorter length, wider areas, lower resistivity material.
• How do you determine resistivity of a wire?

  1) measure wire diameter at 3 places using a micrometer.2) clamp test wire to rule with circuit attached to the wire at zero end of the ruler.3) attach flying lead at different points along the wire and measure p.d. and current to calculate resistance.4) repeat measurements at different lengths along the wire from 0.1m to 1.0m.5) Plot results on graph of resistance against length and draw line of best fit.6) Gradient =resistivity/ areas so times gradient by cross sectional area to give resistivty.
• What are super conductors?

  A conductor who's resistance disappears below a certain temperature.
• Temperature at which resistance disappears?


  Transition temperature
• Why are super conductors useful?

  No heat is wasted through resistance so we transfer energy via a circuit with no losses.
• At what temperature is transition temperature? Why is this difficult?

  10 Kelvin (-263), expensive to reach this temperature
• What are the uses of super conductors?


  Power cables, strong electromagnets, electronic circuits.
• Define power? units? unit equivalence?


  The rate or transfer of energy. Watt. 1 Watt is equivalent to 1 joule/sec
• Equation for total energy?

  Multiply power by time.
• Where does resistance come from?

  Electrons colliding with atoms and losing energy.
• What is internal resistance? what does it do?

  Chemical energy that causes electrons to move and collide with atoms inside the battery. Causes battery to heat up.
• What is EMF? What is its unit?


  The electrical energy the battery produces for each coulomb of charge.Volts
• What is load resistance/ external resistance?

  the total resistance of the components in the external circuit
• What is terminal p.d? (V)

  the energy transfer when one coulomb of charge flows through loaded resistance.
• What would happen if there was no internal resistance?

  Terminal p.d would be equal to emf. 100% efficiency for a battery
• What is lost volts? (v)

  The energy wasted per coulomb of charge to overcome internal resistance.
• What is the rule for totaling emf's in a series circuit?

  add them all up
• What is the rule for totalling emf's in parallel circuit?

  emf total is equal to a single emf if and only if all components have same emf.
• How do you calculate Internal resistance and EMF in a circuit?

  1) Vary current in circuit by changing load resistance using variable resistance.2) Measure p.d and for several currents.3) Record V and I in a table, plot on graph.4) Rearrange V=-rI+emf, vertical axis intercept is emf.5) gradient = -r