Electricity and circuits

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Electricity and circuits- Current and circuits

Current= flow of electrical charge around a circuit.

Current would only flow through an electrical component if there is a potential difference across the component and if the component is complete.

When current flows, electrical work is done and energy transferred. The amount of charge passing a point in the circuit can be calculated using the equation:

charge = current × time

Q= I x T

Potential difference is the driving force that pushes the charge around it is also known as voltage

Resistance slows the flow.

Higher potential difference is the higher the current is.

Greater the resistance of component the smaller the current flows.

Electrons are negatively charged particles and they transfer energy through wires as electricity.

Charge is a property of a body which experiences a force in an electric field. Charge is measured in coulombs (C).

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Electricity and circuits- Potential difference and resistance

Potential difference

The current through a component depends on both the resistance of the component and the potential difference across the component.

Potential difference is a measure of how much energy is transferred between two points in a circuit.

Potential difference is a measure of how much energy is transferred between two points in a circuit.

Resistance

When a charge moves through a potential difference, electrical work is done and energy transferred. The potential difference can be calculated using the equation:

potential difference = current × resistance

V= I x R

potential difference (V) is measured in volts (V)

current (I) is measured in amps (A)

resistance (R) is measured in ohms (Ω)

From the equation, it can be seen that increasing the resistance for a certain potential difference will reduce the current passing through. For example, if a variable resistor is adjusted to double its initial resistance, the current passing will be halved.

Conductors have a low resistance. Insulators have large resistances.

When electrical charge flows through a component it has to do work against resistance

When doing this some energy is dissipated to the thermal energy stores of the component and to the surroundings

When current flows through resistor the resistor heats up- happens because electrons collide with the ions in the lattice that makes up the resistor as they flow through it. Giving ions energy to vibrate and heat up more ions vibrating harder it is for the electrons to get through the resistor. Meaning that given a potential difference the current decreases as the resistor heats up.

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Electricity and circuits- Investigating components

In this required practical activity, it is important to:

measure and observe current and potential difference

use appropriate apparatus and methods to measure current and potential difference for a resistor, bulb and diode

Method:

Connect the circuit as shown in the first diagram. (look in the textbook)

Adjust the variable resistor so that the potential difference is very low or zero at the start.

Record the reading on the voltmeter and ammeter- Take many readings at different resistances

Use the variable resistor to increase the potential difference.

Record the new readings on the voltmeter and ammeter.

Repeat steps three to four, each time increasing the potential difference slightly.

Reverse the power supply connections and repeat steps two to six.

Repeat the experiment but replace the fixed resistor with a bulb.

Plot a graph of current against potential difference for each component.

Circuit shouldn't get hot- this would mess up the results. Circuit starts to warm up disconnect to allow it to cool down.

For a fixed resistor, the potential difference is directly proportional to the current. Doubling the amount of energy into the resistor results in a current twice as big through the resistor. This relationship is called Ohm's Law and is true because the resistance of the resistor is fixed and does not change. A resistor is an ohmic conductor.

In a filament bulb, the current does not increase as fast as the potential difference. Doubling the amount of energy does not cause a current twice as big. The more energy that is put into the bulb, the harder it is for the current to flow - the resistance of the bulb increases. As the potential difference increases, so does the temperature of the thin wire inside the bulb, the filament. The increased vibrations of the ions in the filament because of the increased temperature make it harder for the electrons to get past.

A diode only allows current to flow in one direction. If the potential difference is arranged to try and push the current the wrong way (also called reverse-bias) no current will flow as the diode's resistance remains very large. Current will only flow if the diode is forward-biased. When forward-biased, the diode's resistance is very large at low potential differences but at higher potential differences, the resistance quickly drops and current begins to flow.

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Electricity and circuits- Series and parallel circuits

Series circuit

An electron will pass through every component on its way round the circuit. If one of the bulbs is broken then current will not be able to flow round the circuit. If one bulb goes out, they all go out.

Current in series

A series circuit is one loop; all electrons in that loop form one current. An ammeter will measure the same current wherever it is placed in the circuit:

Potential difference in series

The current will transfer energy from the power supply to the components in the circuit. Since energy has to be conserved, all of the source energy is shared between the components. Since potential difference is used to measure changes in energy, the potential difference supplied is equal to the total of the potential differences across all other components.

Resistance in series

If resistors are connected in series, the current must flow through both of them, so the resistance is increased. To find the total resistance, the resistances are added together

In series circuits:

current is the same through each component

the total potential difference of the power supply is shared between the components

the total resistance of the circuit is the sum of individual resistors

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Electricity and circuits- Series and parallel circuits

Parallel circuit

In parallel circuits, electrical components are connected alongside one another forming extra loops.

Circuit rules

An electron will not pass through every component on its way round the circuit. If one of the bulbs is broken then current will still be able to flow round the circuit through the other loop. If one bulb goes out, the other will stay on.

Current in parallel

Since there are different loops, the current will split as it leaves the cell and pass through one or other of the loops. An ammeter placed in different parts of the circuit will show how the current splits

Potential difference in parallel

Since energy has to be conserved, the energy transferred around the circuit by the electrons is the same whichever path the electrons follow. Since potential difference is used to measure changes in energy, the potential difference supplied is equal to the potential differences across each of the parallel components

Resistance in parallel

If resistors are connected in parallel so that the current will flow through either one or the other, but not both, then the overall resistance is reduced.

In parallel circuits:

the total current supplied is split between the components on different loops

potential difference is the same across each loop

the total resistance of the circuit is reduced as the current can follow multiple paths

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Electricity and circuits- Investigating series and parallel circuits

REFER TO BBC BITESIZE FOR THE FIGURES

Method:

Set up the circuit as shown in figure 1, turn the power supply on and close the switch.

Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, V is potential difference and I is current.

Change the resistor and repeat step two to find the resistance of a second resistor.

Arrange the two resistors in series as shown in figure 2 and close the switch.

Record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I.

Arrange the two resistors in parallel as shown in figure 3 and close the switch.

Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel.

Placing the resistors in series causes the resistance to be double that of a single resistor because there is only one path for the electrons to follow - the supply must drive current through one resistor and then the other.

Placing the resistors in parallel causes the resistance to be half that of a single resistor.

The outcomes of this experiment are the same using filament lamps, or combinations of resistors and lamps, as the lamps act as resistors.

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Electricity and circuits- Energy and power in electric circuits

Heating up wires

As electrons flow through wires, they collide with the ions in the wire which causes the ions to vibrate more. This increased vibration of the ions increases the temperature of the wire. Energy has been transferred from the chemical energy store of the battery into the internal energy store of the wire.

The amount of energy transferred each second (power) between the energy stores can be calculated using the equation:

power = current × potential difference

P=I×V

power (P) is measured in watts (W)

current (I) is measured in amps (A)

potential difference (V) is measured in volts (V)

One watt is equal to one joule per second (J/s).

Power dissipated in a resistance can also be written as:

power = current2 × resistance

P=I^2×R

power (P) is measured in watts (W)

current (I) is measured in amps (A)

resistance (R) is measured in ohms (Ω)

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Electricity and circuits- Energy and power in electric circuits

The power of an electrical component (or appliance) is given by the equation:

P = IV

Power(W)= Current(A) x Potential difference(V)

The total energy transferred by an appliance depends on how long the appliance is on for and its power

Power of an appliance is the energy that is transferred per second. More energy it transfers in a given time the higher the power

Power can be find by using;

Power (W)= Energy transferred (J) / Time (S)

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Electricity and circuits- Questions from spec

Q. Describe the differences between series and parallel circuits

A. A series circuit is a circuit in which two components share a common node and the same current flows through them. However, in a parallel circuit, components share two common nodes.

Q. Why is a voltmeter connected in a circuit?

A. It is connected in parallel with a component to measure the potential difference (voltage), in volt, across it.

Q. What is potential difference?

A. It is the energy transferred per unit charge passed and hence that the volt is a joule per coulomb

Q. Describe the formula for energy transfer?

A. energy transferred (joule, J) = charge moved (coulomb, C) × potential difference (volt, V) E = Q×V

Q. What is an ammeter and where is it connected?

A. It is connected in series with a component to measure the current, in amp, in the component

Q. What is electric current?

A. It is the rate of flow of charge and the current in metals is a flow of electrons

Q. Describe the formula for charge

A. charge (coulomb, C) = current (ampere, A) × time (second, s) Q = I ×t

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Electricity and circuits- Questions from spec

Q. What happens in a closed circuit?

A. It includes a source of potential difference there will be a current in the circuit

Q. Where is current conserved?

A. Current is conserved at a junction in a circuit

Q. Describe the formula for potential difference.

A. potential difference (volt, V) = current (ampere, A) × resistance (ohm, Ω) V = I × R

Q. Why is it that in a series circuit is there is 2 resistors the net resistance increases?

A. In a series circuit the total resistance across all of the components (the 'net resistance') increases as more components are added. The two resistors have the same current flowing through them. The potential difference across them will be different if they have different resistances.

Q. Why is it that in a parallel circuit is there is 2 resistors the net resistance decreases?

A. In a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through. The two resistors have the same potential difference across them. The total current in the circuit is the sum of the currents through each branch.

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Electricity and circuits- Questions from spec

CORE PRACTICALS

1) Investigate the relationship between potential difference, current and resistance for a resistor and a filament lamp

Connect the circuit as shown in the first diagram.

Ensure that the power supply is set to zero at the start.

Record the reading on the voltmeter and ammeter.

Use the variable resistor to alter the potential difference.

Record the new readings on the voltmeter and ammeter.

Repeat steps three to four, each time increasing the potential difference slightly.

Reverse the power supply connections and repeat steps two to six.

Plot a graph of current against potential difference for each component.

Repeat the experiment but replace the fixed resistor with a lamp.

2) Test series and parallel circuits using resistors and filament lamps

Set up the circuit as shown with the fixed resistor

Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter

For each voltage, record the value of the current from the ammeter 3 times and calculate the average current

Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3

Make sure to switch off the circuit in between readings to prevent heating of the component and wires

Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)

Replace the fixed resistor with the filament lamp and repeat the experiment from step 1