Electricity Flashcards
What are the two types of electrical charge
positive and negative charge
What are objects with no electrical charge called
Neutral
Like charges____
opposite charges ______
Like charges will replell each other
Opposite charges attract each other
Describe an atom
The atom is made up of a central part called the nucleus, containing protons which are positively charged, and neutrons which have no charge. It is sorrounded by orbiting Electrons which are negatively charged.
What happens in an atoms when electricity flows
The electrons move
What are conductors
Conductors are materials which allow electrical current to pass through them
What are insulators
Insulators are materials that don’t allow electrical current to pass through them.
Name 3 conductors
Any metal: i.e.
Gold
Iron
Copper
Name 3 insulators
Glass
Air
Plastic
Rubber
Wood
What is electrical current
Electrical current is defined as the electric charge transferred per unit time (per second)
What is the formula for charge
Q = It
Q= charge measured in coulombs (C)
I = current measured in amps (A)
t = time measured in seconds (s)
Describe electrical fields
Charged particles have electrical fields around them. these electrical fields apply a force to charged particles. This force can make the particle move. The path it follows depends on teh electrical field
What is the radial field.
The electrical field around a point charge. The arrows on the field lines show what would happen if a positive test charge was placed in the field.
What is the function of a cell
To provide voltage
What is the function of a battery
To provide voltage
What is the function of a lamp
To produce light
What is the function of a switch
To control a circuit (on/off)
What is the function of a resistor
To control resistance (fixed)
What is the function of a variable resistor
To control resistance (variable)
What is the function of a voltmeter
to measure voltage
What is the function of an ammeter
to measure current
What is the function of a motor
to produce rotational motion
What is the function of a loud speaker
To produce sound
What is the function of a microphone
to convert sound energy into electrical energy
What are the 2 types of electrical current?
AC - Alternating current
DC - Direct current
Descrribe alternating current
In an AC circuit, the flow of electrons is
continually changing direction back and forth.
What is an example of an alternating current supply
The mains power supply is an example
of an AC supply.
Describe direct current
In a DC circuit the electrons always flow in the same direction.
What is an example of direct current supply
Examples of DC supplies include batteries and cells.
What is the identifying feature of a series circuit
A series circuit has only one loop
How does current in a series circuit work
The current in a series circuit is the same at every point
What is the identifying feature of a parallel circuit
Parallel circuits are constructed by placing each component on its own branch.
(multiple branches)
How does current in a parallel circuit work?
The currents in each of the branches add up to the total current from the supply.
What is Voltage (Potential Difference)
Voltage (also known as potential difference) is a measure of the electrical energy given to each Coulomb of charge in a circuit.
Voltage is measured in Volts (V) using a voltmeter.
How do you use a voltmeter
A voltmeter is connected in parallel across
the component being tested meaning the
circuit does not need to be broken to use
a voltmeter.
How does voltage in a series circuit work?
The voltages across all of the components in the circuit add up to the supply voltage.
How does voltage in a parallel circuit work
The voltages across each branch of a parallel circuit are the same as the voltage across the supply.
What is the voltage of the mains supply in the UK
230V
What is a fuse?
A fuse is a safety device designed to protect the flex of an appliance.
What must a fuse do?
A fuse must allow a normal running current to flow but melt/blow when a higher current flows through it.
What are the standard fuse sizes
3A, 5A and 13A
What is the general rule for appliances
Under 700W use a 3A fuse
Over 1200W use a 13A fuse
Describe resistance
Charge flowing through a wire doesn’t get it all its own way. Anything in a circuit - like bulbs and wires – will resist the current. The more it resists, the higher its resistance.
What is resistance measured in?
Resistance is measured in Ohms (Ω)
Resistance is measured using an ohmmeter
How do you find the total resistance in a series circuit?
Add up the resistances in the circuit.
How do find the resistance in parallel circuits?
1/Rt = 1/R1 + 1/R2 + ….
What does Ohm’s law state
Current in a resistor is dirrectly proportional to the potential difference across the resistor.
Ohm’s law formula?
V = IR
where:
V = voltage measured in volts (V)
I = Current measured in amperes (A)
R = resistance measured in ohms (Ω)
In a voltage current graph how can you find the resistance
Finding the gradient of the line.
What is an ohmic conductor?
An ohmic conductor is a component that obeys Ohm’s law. A graph of potential difference against current (a voltage current graph) gives a straight line with a positive gradient. The steeper the gradient, the higher the resistance.
What is a non ohmic conductor
A bulb is an example of a non ohmic conductor. its voltage current graph does not follow a straight lin. Instead it gives a curve with an increasing gradient. It shows that the resistance increase as the current increases.
Deescribe a variable resistor
Variable resistors are components in which the resistance can be changed.
This is often done by moving the position of a slider or turning a dial.
They are used in dimmer switches and volume controls.
Describe a Light dependent resistor
Light dependent resistors (LDRs) will vary resistance with varying light levels.
As the light level increase, the resistance of the LDR will decrease.
Light Up Resistance Down LURD
Describe a thermistor
Thermistors will vary resistance with varying temperatures
As the temperature increase, the resistance of the thermistor will decrease.
Temperature Up Resistance Down TURD
What is an electronic circuit
Electronic circuits are electrical circuits that are used to do something.
What are the 3 main sections of an electronic circuit
Input –> Process —> Output
What is a photovoltaic cell
A photovoltaic cell is often just called a solar cell. It converts light energy to electrical energy.
What is the diode?
The diode is a device that only allows current to flow in one direction.
It will only work when the cathode (-) of the diode is connected to the cathode (-) terminal of the power supply (anode connected to anode)
What is the LED
LED stands for Light Emitting Diode.
Like a diode, it is a low current device. It is destroyed by high currents. This means we need to fit a series resistor to LEDs to reduce the current to a safe level.
How do you workout the value of a resistor for a LED
Frist work out the voltage across the resistor, Vr = Vs - Vled (MAX)
The resistor will need to have a maximum of ____ volts and ____ amps through it. Use Ohms law to find resistance.
What is a capacitor
A capacitor consists of two metal plates separated by an insulator. It’s a device which can store electric charge.
What is the relay
The relay is a switch that is operated by a magnetic field.
When the first circuit is complete, the current in the coil causes it to become magnetised. This pulls shut the switch in the second circuit.
Describe a potential divider
A potential divider is a way of
dividing up the voltage (potential)
from a supply. This means we can
get exactly the voltage we need to
operate a circuit.
In a potential divider circuit, the voltage across a resistor is related to its resistance. The higher the resistance of a resistor, the higher the voltage across it.
Because a potential divider circuit is a series circuit, the current passing through both resistors is the same.
What are the two potential divider equations
V2 = (R2/R1+R2)Vs
V1/V2 = R1/R2
Where:
Vs = voltage across the supply
V1 = voltage across resistor 1
V2 = voltage across resistor 2
R1 = resistance of resistor 1
R2 = resistance of resistor 2
What are tansistor circuits?
Transistors can be used as electronic switches in circuits.
What are the 2 types of trnasistor circuits
NPN transistors
MOSFETs
Describe NPN transistors
If a high enough voltage is applied to the base of the transistor, then current flows between the collector and emitter.
The ‘switch-on’ voltage for an NPN transistor is about 0.7 V.
Below 0.7 V, the transistor will be OFF
Above 0.7 V, the transistor will be ON
Why are variable resistors used in transistor circuits
Many switching circuits involving transistors have a variable resistor as one of the components in the potential divider.
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Adjusting the variable resistor allows you adjust when the circuit switches on and off.
What does MOSFET stand for
Metal Oxide Semiconductor field transistors
Describe MOSFETs
they are operate the same way as NPN transistors
They ‘switch on’ at a higher voltage – about 2 V.
They can handle bigger output current than NPN transistors, so can run bulbs and motors.
What is power?
Power is a measure of the energy transferred per second.
Electrical devices are labelled with their power ratings.
What is the Power relationship
P = E/t
where:
P = power measured in watts (W)
E= energy measured in joules (J)
t = time measured in seconds (s)
What is electrical power?
Electrical power is the rate at which electrical energy is transferred.
It depends on both the current and the potential difference (voltage).
What is the relations ship for electrical power
P = IV
where:
P = power measured in watts (W)
I = current measured in amperes (A)
V = Voltage measured in volts (V)
What are the alternative power relations ships
P = I^2 R
P = V^2/R
Describe how a fuse works
Plugs have fuses fitted in them.
If an appliance – like a TV – starts to draw too high a current, the thin wire in the fuse will melt and cut off the current.
This stops the flex from overheating and possibly causing a fire.
How do we know what fuse to use?
We can use P= I V to work out which fuse to use by calculating the operating current of the appliance.
What are input devices?
Input devices convert one form of energy into electrical energy. This allows an external signal to be converted into an electrical one for use in a circuit.
What is an analogue system?
An analogue system has a signal which can exist in any state between its maximum and minimum value. It is often described as a wave.
What is a digital system
A digital system is one in which the signal can only exist at one of two states, off and on, also often described as L and R.
What is the formula for energy efficiency?
Energy efficiency =
useful energy out * 100/Total energy in
