P5 - Electric Circuits Flashcards
What happens when you rub two objects together?
They become electrically charged as negative electrons
When two objects are rubbed together, they become electrically charged as electrons (which are negatively charged) are transferred from one object to the other. What happens to the charge of the objects giving and receiving the electrons?
- The object receiving the electrons becomes negatively charged
- The object giving up electrons becomes positively charged
What is the electrical charge created when you rub two objects together?
Static electricity
When two charged materials are brought together, they exert a force on each other. What is the force exerted when the two materials are of the same type of charge?
Two materials with the same type of charge repel each other.
When two charged materials are brought together, they exert a force on each other. What is the force exerted when the two materials are of different types of charge?
Two materials with different charges attract each other.
If you moved a positively charged Perspex rod near to another positively charged Perspex rod suspended on a string, what would happen to the suspended rod?
It would be repelled.
If you moved a negatively charged ebonite rod near to another positively charged suspended Perspex rod, what would happen to the suspended Perspex rod?
It would be attracted.
What is an electric current?
A flow of charge
How is an electric current measured?
In Amperes (amps)
What are Amperes (amps) used to measure?
An electric current
Talk about 3 features of an electric current.
- The components and wires are full of charges that are free to move
- The battery causes the free charges to move
- The charges aren’t used up but flow in a continuous loop.
What are the components and wires within an electric circuit full of?
Charges that are free to move
Where can charges in an electric circuit be found?
In components and wires
What causes the free charges in an electric circuit (found in components and wires) to move?
The battery
Are the charges in an electric circuit used up or flow?
Flow in a continuous loop
In metal conductors, how many charges are free to move?
Lots
In insulators how many charges are free to move?
None
What is the difference in movement of charges between metal conductors and insulators?
In metal conductors, there are lots of charges free to move. However, in insulators, there are no charges free to move.
What do metals contain in their structure which moves to create an electric current?
Free electrons
Where do metals contain free electrons that move to create an electric current?
In their structure
What do the free electrons in the structure of metals do to create an electric current?
They move
What does the movement of free electrons in the structure of metals create?
An electric current
What 2 things can supply direct current?
Cells and batteries
What is direct current?
An electrical current that always flows in the same direction
What is alternating current?
An electrical current that changes the direction of flow back and forth continuously
What can alternating current be used for?
Mains electricity
What is the mains supply of voltage to homes?
230 volts
What type of current is used for the mains supply? (HT)
Alternating current
Alternating current is used for mains supply instead of what other type of current? (HT)
Direct current
Why is alternating current used for mains supply rather than direct current? (HT) (3 things)
- It’s easier to generate
- It can be distributed more efficiently
- Only alternating current can be used in a transformer
What is potential difference another name for?
Voltage
What is the potential difference between two points in a circuit?
The work done on (or by) a given amount of charge, as it moves between these two points
What is potential difference measured in?
Volts
How is potential difference measured?
Using a voltmeter connected in parallel across the component
A bulb with 3 volts across it is taking how much energy from every unit of charge?
3 joules
A bulb with 3 volts across is taking 3 joules of energy from every unit of charge. How is this energy given of as?
Heat and light
The greater the potential difference across a component…
…the greater the current will be
When you add more batteries in series…
…the potential difference and the current increase
When you add more batteries in ______, the potential difference and the current ________
- Series
- Increase
What 2 things happen when you ad more batteries in parallel? (HT)
- The total potential difference and current remain the same
- Each battery supplies less current
Do components attract or resist the flow of charge through them?
Resist
Give some examples of components.
- Resistors
- Lamps
- Motors
The connecting wires in the circuit have some _______, but it’s so small that it’s usually ignored.
Resistance
The greater the resistance in a circuit…
…the smaller the current will be
Two lamps together in a circuit with one cell have a certain _______.
resistance
Two lamps together in a circuit with one cell have a certain resistance. What 2 things are provided if another cell is included in the circuit?
- A greater potential difference
- A greater current
How can a greater potential difference and a greater current be achieved in a circuit?
By including another cell in the circuit
What happens when you have to add resistors in series?
The battery has to push charges through more resistors, so the resistance increases.
When you add resistors in series, the battery has to push charges through more resistors, so doing what?
So the resistance increases.
How can the resistance be increased when you add resistors in series?
The battery has to push charges through more resistors
True or False?
When you add resistors in series, the battery has to push charges through more resistors, so the resistance increases.
True
When you add resistors in series, the ________ has to push charges through more __________, so the resistance _________.
- battery
- resistors
- increases
What happens when you have to add resistors in parallel?
There are more paths for the charges to flow along, so the total resistance reduces and the total current increases.
When you add resistors in parallel, there are more paths for the charges to flow along, so what happens to the total resistance and current?
The total resistance reduces and the total current increases
How can the total resistance be reduced when you add resistors in parallel?
There are more paths for the charges to flow along
How can the total current be increased when you add resistors in parallel?
There are more paths for the charges to flow along
True or False?
When you add resistors in parallel, there are more paths for the charges to flow along, so the total resistance reduces and the total current increases.
True
When you add resistors in ________, there are more paths for the charges to flow along, so the total _______ reduces and the total ______ increases.
- parallel
- resistance
- current
When an electric current flows through a component, what does it cause the component to do?
Heat up
When an electric current flows through a component, it causes the component to heat up. What is this heating effect large enough to do?
Make a lamp filament glow.
What 2 things happen as current flows? (HT)
- Moving charges collide with the vibrating ions int he wire, giving them energy
- The increase in energy causes the component to heat up
How can resistance be calculated?
Restistance (ohms) = voltage (volts) / current (amps)
A circuit has a current of 3 amps and a voltage of 6V. What is the resistance?
resistance = voltage / current = 6V / 3A = 2 ohms
How can potential difference/current be calculated? (HT)
By rearranging the resistance formula
potnetial difference = current x resistance
A circuit has a current of 0.2 amps and a bulb with a resistance of 15 ohms. What is the reading on the voltmeter? (HT)
potential difference = current x resistance = 0.2 A x 15 ohms = 3V
As long as a component’s resistance stays constant, the current through the resistor is _________ ________ to the voltage across the resistor. This is regardless of which direction the current is flowing.
Directly proportional
As long as a component’s resistance stays constant, the current through the resistor is directly proportional to the ______ across the resistor. This is regardless of which direction the current is flowing.
voltage
What does the resistance of a thermistor depend on?
Temperature
The resistance of a thermistor depends on temperature. What 2 things happen as the temperature increases?
- Its resistance decreases
- More current flows
What does the resistance of a light dependent resistor (LDR) depend on?
Light intensity
The resistance of a light dependent resistor (LDR) depends on light intensity. What 2 things happen as the light intensity increases?
- Its resistance decreases
- More current flows
What does LDR stand for?
Light dependent resistor
If a Perspex rod is rubbed with a cloth, what does it lose?
Electrons
If a Perspex rod is rubbed with a cloth, it loses electrons. What charge will the rod have now?
Positive
If a bettery of 12V is connected across a bulb and a current of 3A flows through it, what is the resistance of the bulb?
4 ohms
How could the relationship between the current and voltage across a component be described if the resistance is constant?
They’re propotional
A 12V battery is connected across a resistor of resistance 24 ohms. What current flows? (HT)
0.5A
Why does a component heat up when a current flows? (HT)
There are more collisions between the flowing electrons and the vibrating ions, giving a higher resistance. This leads to more heat being produced by the resistor
What 3 things happen when 2 components are connected in series to a battery?
- The current flowing through each component is the same
- The potential difference across the components adds up to the potential difference across the battery
- The potential difference is largest across components with the greatest resistance
a) What 3 things happen when 2 components are connected in series to a battery?
b) When does the above also apply? (HT)
a) - The current flowing through each component is the same
- The potential difference across the components adds up to the potential difference across the battery
- The potential difference is largest across components with the greatest resistance
b) When more than two components are connected in series to a battery.
How should a voltmeter be connected to measure the potential difference between any two chosen points?
In parallel across a component
What does the voltage across the battery (measured in Volts, V) provide a measure of?
The ‘push’ of the battery on the charges in the circuit
The work done on each unit of charge by the battery must equal what? (HT)
The work done by it on the circuit components
True or False?
More work is done by the charge moving through a small resistance than a large one. (HT)
False - More work is done by the charge moving through a large resistance than a small one.
A change in the resistance of one component (variable resistor, LDR or themistor) will result in what? (HT)
A change in the potential differences across all the components
What three things happen in parallel circuits with one component per parallel path?
- The current flowing through each component depends on the resistance of each component
- The total current running from (and back to) the battery is equal to the sum of the current through each of the parallel components
- The current is greatest through the component with the smallest resistance
In parallel circuits with one component per parallel path, what does the current flowing through each component depend on?
The resistance of each component
In parallel circuits with one component per parallel path, what is the total current running from (and back to) the battery equal to?
The sum of the current through each of the parallel components
In parallel circuits with one component per parallel path, where is the current greatest?
Through the component with the smallest resistance
The current through each component is the same as if it were… (HT)
…the only current present.
The current through each component is the same as if it were the only current present. If a second identical component is added in parallel, the same current… (HT)
…flows through each component
The current through each component is the same as if it were the only current present. If a second identical component is added in parallel, the total current… (HT)
…through the battery increases
The same _______ causes more current to flow through a smaller resistance than a bigger one. (HT)
voltage
The __________ ____________ across each component is equal to the potential difference of the _________.
- Potential difference
- battery
When you move a magnet into a coil of wire, what happens?
A voltage is induced between the ends of the wire.
When you move a magnet into a coil of wire, why is a voltage induced between the ends of the wire?
Because the magnetic field is being cut.
When you move a magnet into a coil of wire, what is induced between the ends of the wire?
Voltage
When you move a magnet into a coil of wire, where in the voltage induced from?
Between then ends of the wire
If the ends of the coil are connected to make a complete circuit, what will happen?
A current will be induced
Moving the magnet into the coil induces what?
A current in one direction
Moving the magnet into the coil induces a current in one direction. How can you then induce a current in the opposite direction? (2 ways)
- Moving the magnet out of the coil
- Moving the other pole of the magnet into the coil
What happens if there is no movement of the coil or magnet in electromagnetic induction?
There is no induced current
How is mains electricity produced?
By generators
What principle is used by generators to generate electricity?
Electromagnetic induction - rotation of a magnet inside a coil to generate electricity
In what ways can the size of the induced voltage can be increased? (4 things)
- Increasing the speed of rotation of the magnet
- Increasing the strength of the magnetic field, possibly by using an electromagnet
- Increasing the number of turns on the coil
- Placing an iron core inside the coil
When electric charge flows through a component or device, what is doing the work?
The power supply
When electric charge flows through a component or device, work is done by the power supply. How does this happen?
Energy is transferred from the power supply to the component and/or its surroundings
What is power a measure of?
The rate of energy transfer to an appliance or device and/or its surroundings
How is power measured?
In watts (W)
What can watts (W) be used to measure?
Power
Give the formula by which power can be calculated.
power (watts) = voltage (volts) x current (amps)
An electric motor works at a current of 3A and a voltage of 24V. What is the power of the motor?
power = voltage x current = 24V x 3A = 72W
How can potential difference be calculated by rearranging the power formula? (HT)
potential difference = power / current
A 4W light bulb works at a current of 2A. What is the potential difference? (HT)
potential difference = power / current = 4W / 2A = 2V
What are transformers used to do?
Change the voltage of an alternating current
What are transformers used to change within an alternating current?
Voltage
What are transformers used to changed voltage within?
An alternating current
What can be used to change voltage within an alternating current?
Transformers
What do transformers consist of?
2 coils of wire - the primary and secondary coils, wrapped around a soft iron core.
Within transformers, what happens when two coils of wire are close to one another?
A changing magnetic field in one coil caused by changes in the current can induce a voltage in the other
Alternating current flowing through the primary coil within a transformer creates what?
An alternating magnetic field
Alternating current flowing through the primary coil within a transformer creates an alternating magnetic field. What does this changing field then induce?
An alternating current in the secondary coil
The amount by which a transformer changes the voltage depends on what? (HT)
The number of turns on the primary and secondary coils
The amount by which a transformer changes the voltage depends on the number of turns on the primary and secondary coils. What will the changing current in the primary coil cause? (HT)
A changing magnetic field in the iron core, which in turn will cause a changing potential difference across the secondary coil.
The amount by which a transformer changes the voltage depends on the number of turns on the primary and secondary coils. What causes a changing magnetic field in the iron core, which in turn will cause a changing potential difference across the secondary coil? (HT)
The changing current in the primary coil
Complete the formula: (HT)
voltage on primary coil (V[p]) / voltage on secondary coil (V[s]) = ?
number of turns on primary coil, N[p] / number of turns on secondary coil, N[s]
Complete the formula: (HT)
? = number of turns on primary coil, N[p] / number of turns on secondary coil, N[s]
voltage on primary coil (V[p]) / voltage on secondary coil (V[s])
A transformer has 1000 turns on the primary coil and 200 turns on the secondary coil. If a voltage of 250V is applied to the primary coil, what is the voltage across the secondary coil? (HT)
250 / V[s] = 1000 / 200
250 = 5V[s] so V[s] = 250 / 5 = 50V
What does an electric motor cntain?
A coil of wire that rotates in between the opposite poles of a permanent magnet, when a current flows through the coil.
A current-carrying wire (or coil) within an electric motor can exert a force on what?
A permanent magnet or another current-carrying wire nearby
a) A current-carrying wire (or coil) within an electric motor can experience a force when?
b) Why is this?
c) When can’t the current-carrying wire experience a force?
a) If placed in a magnetic field whose lines of force are at right angles to the wire.
b) The force will be at right angles to both the current direction and the lines of force of the magnetic field.
c) If the current in a wire travels parallel to the magnetic field lines, it doesn’t experience a force.
What happens to field lines when a current flows through a coil?
It will cut the field lines in opposite directions on each side of the coil.
When a current flows through the coil, it will cut the field lines in opposite directions on each side of the coil. What does this create?
A pair of forces in opposite directions and causes the coil to rotate around its axis
What is a commutator?
A rotary switch that turns with the coil, but the brushes that touch it remain fixed.
A commutator is a rotary switch that turns with the coil, but the brushes that touch it remain fixed. What does this have the effect of?
The effect of making sure that, as the coil rotates, the current direction into the coil is switched
Where does the current ALWAYS cut field lines? (2 notes)
- Nearest the south pole of the fixed magnet in the same direction, no matter how many times the coil spins
- Nearest the north pole in the opposite direction.
The current ALWAYS cuts field lines nearest the north pole in the opposite direction. What does this therefore produce?
A pair of opposing forces whose directions remain the same, giving continuous rotation.
Give some examples of devices containing electric motors.
- Hard disk drive
- DVD player
- Electric motor vehicle
- Washing machine
- Tumble dryer
- Microwave oven
Explain how a motor is used within a hard disk drive
To rotate the hard disk at high speed under the read/write heads
Explain how a motor is used within a DVD player
To rotate the disk so that the information can be read
Explain how a motor is used within an electric motor vehicle
To provide traction via the transmission to the rear wheels.
Explain how a motor is used within a washing machine
To rotate the drum and agitate the wash so that all the clothes are washed effectively
Explain how a motor is used within a tumble dryer
To rotate the drum and provide uniform heat to all the items being dried
Explain how a motor is used within a microwave oven
To rotate the food and make sure that it’s evenly heated
What principles do generators use to generate electricity?
The principle of electromagnetic induction
What is the power of a 12V lamp in a circuit where a 0.5A current flows?
6W
In a transformer what induces a voltage across the secondary coil?
A changing magnetic field
A transformer has a primary coil of 200 turns. If the transformer changes 12V to 240V, how many turns must be on the secondary coil. (HT)
4000 turns
A 60W light bulb has a 240V power supply connected across it. What current flows?
0.25A
