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Electricity
| Question | Answer |
|---|---|
| Define Current | The rate of flow of charge |
| Define Ampere | When 1 coulomb passes a point in a circuit in 1 second |
| What is the equation for Charge | Q=It |
| What is Kirschoff's first law | -Current is not used up by components in a circuit -Current is the same wherever you measure it in a series circuit -What goes into a junction must come out |
| Which way does conventional current flow | Positive------>negative |
| Which way do electrons flow | Negative -------->positive |
| Explain how a fuse works | -If I is too high -No of collisions between electrons and ions increases -Energy is transferred to the fuse wire so it heats up -Fuse wire melts -Circuit is now incomplete and safe |
| Describe a IV graph for a lamp | At ohmic section: -low V -V is proportional to I -R is constant Non ohmic section -V is high -As V increases by increments I increases by ever decreasing amounts |
| Explain the ohmic section of an IV graph for a lamp | -I is low -Low no of collisions -Low energy transferred to ions -Internal energy stays low -R is constant |
| Explain the non ohmic section of an IV graph for a lamp | -I is high -High no of collisions -High energy transferred to ions -Ions vibrate more R increases so m (gradient) decreases |
| What is ohm's law | I is directly proportional to V in an idealised conductor with no significant heating effects. |
| What is the equation for ohm's law | V=IR |
| What are the 4 factors that affect resistance | -Thermal energy -Length of thin wire -Cross sectional area -No of electrons per square metre |
| How does thermal energy affect resistance | -TE increases -Collisions increases -E transferred to ions increases -Resistance increases |
| How does length of a thin wire affect resistance | -Extending wire is the same as adding resistors in series -Length increases -No if collisions increases -E transferred to ions -R increases |
| What is the analogy for increasing cross sectional area of wire | Same as adding resistors in parallel it creates multi-path routes |
| Give two uses of diodes | -Protects from low V -Protects from reverse flows |
| Which way does current flow easily in a diode | Forward bias |
| What is the threshold voltage | The voltage in which current can flow in the forward bias (1.6V) |
| When can current flow in the reverse bias | At a very high V known as the breakdown voltage |
| Define voltage | The energy carried per unit charge |
| Define volt | When one joule is carried by one coulomb |
| What is Kirstoff's 2nd law | The p.ds in any fixed loop add up to the power supply voltage |
| Is a voltmeter an infinitely high resistor or an infinitely low resistor | Infinitely high |
| Is an ammeter an infinitely high resistor or an infinitely low resistor | Infinitely low |
| What is the equation for resistors in series | Rtot=R1+R2…+Rn |
| What is the equation for resistors in parallel | 1/Rtot=1/R1+1/R2...+1/Rn |
| What is the potential divider equation | V1=(R1/(R1+R2))*Vout |
| How does a negative temperature coefficient thermistor work | As temperature increases resistance decreases |
| How can thermistors be used for a greenhouse heater | -If temperature is too low then current across a potential divider is low -Therefore resistance in the thermistor and the heater is low so it comes on -When temperature is high the resistance drops -Therefore heater switches off |
| Define resistivity | The material specific constant that relates the resistance to the unit volume of a material, with perfectly conductive contacts on opposite faces |
| What is the equation for resistivity | ρ=RA/L |
| What are the units of resistivity | Ohm-metres |
| Define EMF | -An idealised voltage supplied if no current flows. -Produced inside a source of electrical energy. -The energy carried per unit charge. |
| Define ohmic conductor | The current through an ohmic conductor (at a constant temperature) is directly proportional to the voltage across it. Therefore the resistance remains constant |
| Define resistance | The ratio of voltage across a component to the current flowing through it |
| What is the equation for emf | Ɛ=V+Ir |
Created by:
Liam P
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