BMAT Section 2: Physics - Electricity Flashcards
How can insulators be charged?
- Via friction (rubbing against each other)
- As electrons are transferred from one object to the other
- Forming +ve charge on one object and -ve charge on the other
What is charging caused by?
The gain or loss of electrons
Direct current
The flow of electric charge in only one direction
Alternating current
The flow of electric charge that periodically reverses direction
Difference between conductors and insulators
Insulators:
- Do not conduct electricity
- As their electrons cannot flow throughout the material, they are fixed
Conductors:
- Can conduct electricity
- As their electrons can flow, and are not fixed (they are delocalised)
3 examples of conductors:
- Metals
- Aqueous solutions of salts
- Graphite
- The human body
3 examples of insulators:
- Plastics
- Paper
- Styrofoam
- Rubber
- Glass
Symbol, unit and definition of current
- I
- Amperes (A)
- The rate of flow of electric charge
Symbol, unit and definition of charge
- Q
- Coulombs (C)
- A physical property of a material, the fundamental quantity of electricity
Equation for charge
- Charge = current x time
- Q = I x t
Ammeter
- Measures the current (in amps) flowing through a wire
- Must always be placed in series with whatever is being investigated
Voltmeter
- Measures the potential difference (in volts) across the test wire
- Must always be placed in parallel with whatever is being investigated
Equation for voltage
- Voltage = Current x resistance
- V = I x R
Explain the voltage-current graphs for filament lamps
- The resistance is not constant so it changes with the current through the component
- The resistance of a filament lamp increases as the temperature of the filament increases and
the current through a diode flows in one direction only, so it has a higher resistance in the
reverse direction
Negative temperature coefficient thermistors (NTC thermistors)
- As temperature increases, resistance decreases
- E.g. thermostats - the heating automatically turns on/off at a certain temperature
Light dependent resistors (LDR)
- As light intensity increases, resistance decreases
- E.g. automatic night lights - a light automatically turns on as it gets dark)
Ideal diode
- A semiconductor which only allows current flow in one direction
- When positive voltage is applied across the anode to the cathode, the diode conducts forward current instantly
Current, potential difference and resistance in a series circuit
- Same current everywhere
- Potential difference is shared between various components
- Total resistance is the resistance of each component combined
Current, potential difference and resistance in a parallel circuit
- Current is shared between each branch
- Potential difference is the same in all components
- Thee total resistance for two resistors in parallel is less than the resistance of any individual resistor
Equation for voltage
- Voltage = Energy / Charge
- V = E / Q
Equation for power
- Power = current x voltage
- P = IV or P = I^2R
Equation for energy transfer
- Energy transfer = power x time
- E = V x I x T
Symbol and units for power
- P
- Watts (W)
Symbol and units for energy transfer
- E
- Joules (J)
